Parallel & Distributed Systems - Anuradha Bhatia · Parallel & Distributed Systems Anuradha Bhatia...

Parallel Distributed Systems

Parallel &

Distributed

Systems

B.E. COMPUTER ENGINEERING (CPC803)

2015-2016

ANURADHA BHATIA

CBGS

M.E. Computer Engineering

MU

Parallel & Distributed Systems

Anuradha Bhatia

Table of Contents

1. Introduction .................................................................................................................................................... 1

2. Pipeline Processing ................................................................................................................................... 36

3. Synchronous Parallel Processing ......................................................................................................... 52

4. Introduction to Distributed Systems .................................................................................................. 65

5. Communication .......................................................................................................................................... 88

6. Resource and Process Management.................................................................................................. 124

7. Synchronization ....................................................................................................................................... 153

8. Consistency and Replication ................................................................................................................ 177


Anuradha Bhatia

Disclaimer

The content of the book is the copyright property of the author, to be used by the students for

the reference for the subject “Parallel and Distributed Systems”, CPC803, Eighth Semester, for

the Final Year Computer Engineering, Mumbai University.

The complete set of the e-book is available on the author’s website www.anuradhabhatia.com,

and students are allowed to download it free of charge from the same.

The author does not gain any monetary benefit from the same, it is only developed and designed

to help the teaching and the student feternity to enhance their knowledge with respect to the

curriculum prescribed by Mumbai University.

http://www.anuradhabhatia.com/


Anuradha Bhatia

1. Introduction CONTENTS

1.1 Parallel Computing.

1.2 Parallel Architecture.

1.3 Architectural Classification

1.4 Scheme, Performance of Parallel Computers

1.5 Performance Metrics for Processors

1.6 Parallel Programming Models, Parallel Algorithms.

Parallel & Distributed Systems 1. Introduction

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1.1 Basics of Parallel Distributed Systems and Parallel Computing

1. What is Parallel Computing?

i. Traditionally, software has been written for serial computation:

To be run on a single computer having a single Central Processing Unit (CPU);

A problem is broken into a discrete series of instructions.

Instructions are executed one after another.

Only one instruction may execute at any moment in time.

Figure 1.1: Parallel Computing

ii. In the simplest sense, parallel computing is the simultaneous use of multiple

compute resources to solve a computational problem:

To be run using multiple CPUs

A problem is broken into discrete parts that can be solved concurrently

Each part is further broken down to a series of instructions

Instructions from each part execute simultaneously on different CPUs


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Figure 1.2: Multiple Compute

iii. The compute resources might be:

A single computer with multiple processors;

An arbitrary number of computers connected by a network;

A combination of both.

iv. The computational problem should be able to:

Be broken apart into discrete pieces of work that can be solved

simultaneously;

Execute multiple program instructions at any moment in time;

Be solved in less time with multiple compute resources than with a single

compute resource.

1.2 The Universe is Parallel

i. Parallel computing is an evolution of serial computing that attempts to emulate

what has always been the state of affairs in the natural world: many complex,

interrelated events happening at the same time, yet within a temporal sequence.

For example:


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Figure 1.3: Universe of Parallel Computing

1.3 Uses for Parallel Computing

i. Science and Engineering: Historically, parallel computing has been considered to

be "the high end of computing", and has been used to model difficult problems

in many areas of science and engineering:


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o Atmosphere, Earth, Environment

o Physics - applied, nuclear, particle,

condensed matter, high pressure, fusion,

photonics

o Bioscience, Biotechnology, Genetics

o Chemistry, Molecular Sciences

o Geology, Seismology

o Mechanical Engineering - from

prosthetics to spacecraft

o Electrical Engineering, Circuit

Design, Microelectronics

o Computer Science,

Mathematics

Table 1.1

ii. Industrial and Commercial: Today, commercial applications provide an equal or

greater driving force in the development of faster computers. These applications

require the processing of large amounts of data in sophisticated ways. For

example:

o Databases, data mining

o Oil exploration

o Web search engines, web

based business services

o Medical imaging and

diagnosis

o Pharmaceutical design

o Financial and economic modelling

o Management of national and multi-national

corporations

o Advanced graphics and virtual reality,

particularly in the entertainment industry

o Networked video and multi-media

technologies

o Collaborative work environments

Table 1.2

1.4 Why Use Parallel Computing?

i. Save time and/or money: In theory, throwing more resources at a task will

shorten it’s time to completion, with potential cost savings. Parallel computers

can be built from cheap, commodity components.


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ii. Solve larger problems: Many problems are so large and/or complex that it is

impractical or impossible to solve them on a single computer, especially given

limited computer memory.

iii. Provide concurrency: A single compute resource can only do one thing at a time.

Multiple computing resources can be doing many things simultaneously.

iv. Use of non-local resources: Using compute resources on a wide area network, or

even the Internet when local compute resources are scarce. For example:

v. Limits to serial computing: Both physical and practical reasons pose significant

constraints to simply building ever faster serial computers:

Transmission speeds - the speed of a serial computer is directly dependent

upon how fast data can move through hardware.

Absolute limits are the speed of light (30 cm/nanosecond) and the

transmission limit of copper wire (9 cm/nanosecond).

Increasing speeds necessitate increasing proximity of processing elements.

Limits to miniaturization - processor technology is allowing an increasing

number of transistors to be placed on a chip. However, even with

molecular or atomic-level components, a limit will be reached on how

small components can be.

Economic limitations - it is increasingly expensive to make a single

processor faster. Using a larger number of moderately fast commodity

processors to achieve the same (or better) performance is less expensive.

Current computer architectures are increasingly relying upon hardware

level parallelism to improve performance:

Multiple execution units

Pipelined instructions

Multi-core


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Figure 1.4: Core Layout

1.5 Concepts and Terminology

1. von Neumann Architecture

i. Named after the Hungarian mathematician John von Neumann who first

authored the general requirements for an electronic computer in his 1945

papers.

ii. Since then, virtually all computers have followed this basic design, differing

from earlier computers which were programmed through "hard wiring".

Figure 1.5: von Neumann


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iii. Comprised of four main components:

Memory

Control Unit

Arithmetic Logic Unit

Input/output

iv. Read/write, random access memory is used to store both program

instructions and data

Program instructions are coded data which tell the computer to

do something

Data is simply information to be used by the program

v. Control unit fetches instructions/data from memory, decodes the

instructions and then sequentially coordinates operations to accomplish

the programmed task.

vi. Arithmetic Unit performs basic arithmetic operations

vii. Input/output is the interface to the human operator.

2. Flynn's Classical Taxonomy

i. One of the more widely used classifications, in use since 1966, is called

Flynn's Taxonomy.

ii. Flynn's taxonomy distinguishes multi-processor computer architectures

according to how they can be classified along the two independent

dimensions of Instruction and Data. Each of these dimensions can have

only one of two possible states: Single or Multiple.

iii. The matrix below defines the 4 possible classifications according to Flynn:


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S I S D

Single Instruction, Single Data

S I M D

Single Instruction, Multiple Data

M I S D

Multiple Instruction, Single Data

M I M D

Multiple Instruction, Multiple Data

Figure 1.6: Flynn’s Classical Taxonomy

A. Single Instruction, Single Data (SISD)

A serial (non-parallel) computer

Single Instruction: Only one instruction stream is being acted on by

the CPU during any one clock cycle

Single Data: Only one data stream is being used as input during any

one clock cycle

Deterministic execution

This is the oldest and even today, the most common type of

computer

Examples: older generation mainframes, minicomputers and

workstations; most modern day PC’s.

Figure 1.7: SISD

B. Single Instruction, Multiple Data (SIMD)

Single Instruction: All processing units execute the same instruction

at any given clock cycle


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Multiple Data: Each processing unit can operate on a different data

element

Best suited for specialized problems characterized by a high degree

of regularity, such as graphics/image processing.

Synchronous (lockstep) and deterministic execution

Two varieties: Processor Arrays and Vector Pipelines

Examples:

Processor Arrays: Connection Machine CM-2, MasPar MP-1

& MP-2, ILLIAC IV

Vector Pipelines: IBM 9000, Cray X-MP, Y-MP & C90, Fujitsu

VP, NEC SX-2, Hitachi S820, ETA10

Most modern computers, particularly those with graphics

processor units (GPUs) employ SIMD instructions and execution

units.

Figure 1.8: SIMD

C. Multiple Instruction, Single Data (MISD)

Multiple Instruction: Each processing unit operates on the data

independently via separate instruction streams.

Single Data: A single data stream is fed into multiple processing

units.


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Few actual examples of this class of parallel computer have ever

existed. One is the experimental Carnegie-Mellon C.mmp

computer (1971).

Some conceivable uses might

multiple frequency filters operating on a single signal

stream

Multiple cryptography algorithms attempting to crack a

single coded message.

Figure 1.9: MISD

D. Multiple Instruction, Multiple Data (MIMD)

Multiple Instruction: Every processor may be executing a different

instruction stream

Multiple Data: Every processor may be working with a different

data stream

Execution can be synchronous or asynchronous, deterministic or

non-deterministic

Currently, the most common type of parallel computer - most

modern supercomputers fall into this category.

Examples: most current supercomputers, networked parallel

computer clusters and "grids", multi-processor SMP computers,

multi-core PCs.


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Note: many MIMD architectures also include SIMD execution sub-

components.

Figure 1.10: MIMD

1.6 General Parallel Terminology

1. Supercomputing / High Performance Computing (HPC): Using the world's fastest and

largest computers to solve large problems.

2. Node: A standalone "computer in a box". Usually comprised of multiple

CPUs/processors/cores. Nodes are networked together to comprise a

supercomputer.

3. CPU / Socket / Processor / Core: This varies, depending upon who you talk to. In the

past, a CPU (Central Processing Unit) was a singular execution component for a

computer. Then, multiple CPUs were incorporated into a node. Then, individual CPUs

were subdivided into multiple "cores", each being a unique execution unit. CPUs with

multiple cores are sometimes called "sockets" - vendor dependent. The result is a

node with multiple CPUs, each containing multiple cores. The nomenclature is

confused at times.

Figure 1.11: CPU


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4. Task: A logically discrete section of computational work. A task is typically a program

or program-like set of instructions that is executed by a processor. A parallel program

consists of multiple tasks running on multiple processors.

5. Pipelining: Breaking a task into steps performed by different processor units, with

inputs streaming through, much like an assembly line; a type of parallel computing.

6. Shared Memory: From a strictly hardware point of view, describes a computer

architecture where all processors have direct (usually bus based) access to common

physical memory. In a programming sense, it describes a model where parallel tasks

all have the same "picture" of memory and can directly address and access the same

logical memory locations regardless of where the physical memory actually exists.

7. Symmetric Multi-Processor (SMP): Hardware architecture where multiple processors

share a single address space and access to all resources; shared memory computing.

8. Distributed Memory: In hardware, refers to network based memory access for

physical memory that is not common. As a programming model, tasks can only

logically "see" local machine memory and must use communications to access

memory on other machines where other tasks are executing.

9. Communications: Parallel tasks typically need to exchange data. There are several

ways this can be accomplished, such as through a shared memory bus or over a

network, however the actual event of data exchange is commonly referred to as

communications regardless of the method employed.

10. Synchronization: The coordination of parallel tasks in real time, very often associated

with communications. Often implemented by establishing a synchronization point

within an application where a task may not proceed further until another task(s)

reaches the same or logically equivalent point. Synchronization usually involves

waiting by at least one task, and can therefore cause a parallel application's wall clock

execution time to increase.

11. Granularity: In parallel computing, granularity is a qualitative measure of the ratio of

computation to communication.


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12. Coarse: Relatively large amounts of computational work are done between

communication events

13. Fine: Relatively small amounts of computational work are done between

communication events

14. Observed Speedup: Observed speedup of a code which has been parallelized, defined

as:

wall-clock time of serial execution

-----------------------------------

wall-clock time of parallel execution

One of the simplest and most widely used indicators for a parallel program's

performance.

15. Parallel Overhead: The amount of time required to coordinate parallel tasks, as

opposed to doing useful work. Parallel overhead can include factors such as:

Task start-up time

Synchronizations

Data communications

Software overhead imposed by parallel compilers, libraries, tools,

operating system, etc.

Task termination time

16. Massively Parallel: Refers to the hardware that comprises a given parallel system -

having many processors. The meaning of "many" keeps increasing, but currently, the

largest parallel computers can be comprised of processors numbering in the hundreds

of thousands.

17. Embarrassingly Parallel: Solving many similar, but independent tasks simultaneously;

little to no need for coordination between the tasks.

18. Scalability: Refers to a parallel system's (hardware and/or software) ability to

demonstrate a proportionate increase in parallel speedup with the addition of more

processors. Factors that contribute to scalability include:


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Hardware - particularly memory-cpu bandwidths and network

communications

Application algorithm

Parallel overhead related

Characteristics of your specific application and coding

1.7 Performance attributes

1. Performance of a system depends upon

i. Hardware technology

ii. Architectural features

iii. Efficient resource management

iv. Algorithm design

v. Data structures

vi. Language efficiency

vii. Programmer skill

viii. Compiler technology

2. Performance of computer system we would describe how quickly a given system can

execute a program or programs. Thus we are interested in knowing the turnaround time.

Turnaround time depends on:

i. Disk and memory accesses

ii. Input and output

iii. Compilation time

iv. Operating system overhead

v. Cpu time

3. An ideal performance of a computer system means a perfect match between the

machine capability and program behavior.

4. The machine capability can be improved by using better hardware technology and

efficient resource management.


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5. But as far as program behavior is concerned it depends on code used, compiler

used and other run time conditions. Also a machine performance may vary from

program to program.

6. Because there are too many programs and it is impractical to test a CPU's speed

on all of them, benchmarks were developed. Computer architects have come up

with a variety of metrics to describe the computer performance.

i. Clock rate and CPI / IPC: Since I/O and system overhead frequently overlaps

processing by other programs, it is fair to consider only the CPU time used

by a program, and the user CPU time is the most important factor. CPU is

driven by a clock with a constant cycle time (usually measured in

nanoseconds, which controls the rate of internal operations in the CPU.

The clock mostly has the constant cycle time (t in nanoseconds). The

inverse of the cycle time is the clock rate (f = 1/τ, measured in megahertz).

A shorter clock cycle time, or equivalently a larger number of cycles per

second, implies more operations can be performed per unit time. The size

of the program is determined by the instruction count (Ic). The size of a

program is determined by its instruction count, Ic, the number of machine

instructions to be executed by the program. Different machine instructions

require different numbers of clock cycles to execute. CPI (cycles per

instruction) is thus an important parameter.

ii. MIPS: The millions of instructions per second, this is calculated by dividing

the number of instructions executed in a running program by time

required to run the program. The MIPS rate is directly proportional to the

clock rate and inversely proportion to the CPI. All four systems attributes

(instruction set, compiler, processor, and memory technologies) affect the

MIPS rate, which varies also from program to program. MIPS does not

proved to be effective as it does not account for the fact that different

systems often require different number of instruction to implement the

program. It does not inform about how many instructions are required to


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perform a given task. With the variation in instruction styles, internal

organization, and number of processors per system it is almost

meaningless for comparing two systems.

iii. MFLOPS (pronounced ``megaflops'') stands for ``millions of floating point

operations per second.'' This is often used as a ``bottom-line'' figure. If one

know ahead of time how many operations a program needs to perform,

one can divide the number of operations by the execution time to come

up with a MFLOPS rating. For example, the standard algorithm for

multiplying n*n matrices requires 2n3 – n operations (n2 inner products,

with n multiplications and n-1additions in each product). Suppose you

compute the product of two 100 *100 matrices in 0.35 seconds. Then the

computer achieves

(2(100)3 – 100)/0.35 = 5,714,000 ops/sec = 5.714 MFLOPS

iv. Throughput rate: Another important factor on which system’s performance

is measured is throughput of the system which is basically how many

programs a system can execute per unit time Ws. In multiprogramming the

system throughput is often lower than the CPU throughput Wp which is

defined as

Wp = f/(Ic * CPI)

Unit of Wp is programs/second.

v. Speed or Throughput (W/Tn) - the execution rate on an n processor system,

measured in FLOPs/unit-time or instructions/unit-time.

vi. Speedup (Sn = T1/Tn) - how much faster in an actual machine, n processors

compared to

asymptotic speedup.

vii. Efficiency (En = Sn/n) - fraction of the theoretical maximum speedup

achieved by n processors.

viii. Degree of Parallelism (DOP) - for a given piece of the workload, the

number of processors that can be kept busy sharing that piece of


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computation equally. Neglecting overhead, we assume that if k processors

work together on any workload, the workload gets done k times as fast as

a sequential execution.

ix. Scalability - The attributes of a computer system which allow it to be

gracefully and linearly scaled up or down in size, to handle smaller or larger

workloads, or to obtain proportional decreases or increase in speed on a

given application. The applications run on a scalable machine may not

scale well. Good scalability requires the algorithm and the machine to have

the right properties

Thus in general there are five performance factors (Ic, p, m, k, t) which are

influenced by four system attributes:

Instruction-set architecture (affects Ic and p)

Compiler technology (affects Ic and p and m)

CPU implementation and control (affects p *t ) cache and memory

hierarchy (affects memory access latency, k ´t)

Total CPU time can be used as a basis in estimating the execution rate

of a processor.

1.8 Parallel Computing Algorithms

i. Parallel algorithms are designed to improve the computation speed of a computer. For

analyzing a Parallel Algorithm, we normally consider the following parameters −

Time complexity (Execution Time),

Total number of processors used, and

Total cost.

Time Complexity

i. The main reason behind developing parallel algorithms was to reduce the

computation time of an algorithm. Thus, evaluating the execution time of an algorithm

is extremely important in analyzing its efficiency.


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ii. Execution time is measured on the basis of the time taken by the algorithm to solve a

problem. The total execution time is calculated from the moment when the algorithm

starts executing to the moment it stops. If all the processors do not start or end

execution at the same time, then the total execution time of the algorithm is the

moment when the first processor started its execution to the moment when the last

processor stops its execution.

iii. Time complexity of an algorithm can be classified into three categories−

Worst-case complexity − When the amount of time required by an algorithm

for a given input is maximum.

Average-case complexity − When the amount of time required by an algorithm

for a given input is average.

Best-case complexity − When the amount of time required by an algorithm for

a given input is minimum.

Asymptotic Analysis

i. The complexity or efficiency of an algorithm is the number of steps executed by the

algorithm to get the desired output. Asymptotic analysis is done to calculate the

complexity of an algorithm in its theoretical analysis. In asymptotic analysis, a large

length of input is used to calculate the complexity function of the algorithm.

ii. Note − Asymptotic is a condition where a line tends to meet a curve, but they do not

intersect. Here the line and the curve is asymptotic to each other.

iii. Asymptotic notation is the easiest way to describe the fastest and slowest possible

execution time for an algorithm using high bounds and low bounds on speed. For this,

we use the following notations −

Big O notation

Omega notation

Theta notation


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Big O notation

In mathematics, Big O notation is used to represent the asymptotic characteristics of

functions. It represents the behavior of a function for large inputs in a simple and

accurate method. It is a method of representing the upper bound of an algorithm’s

execution time. It represents the longest amount of time that the algorithm could take

to complete its execution. The function −

f(n) = O(g(n))

if there exists positive constants c and n0 such that f(n) ≤ c * g(n) for all n where n ≥

n0.

Omega notation

Omega notation is a method of representing the lower bound of an algorithm’s

execution time. The function −

f(n) = Ω (g(n))

if there exists positive constants c and n0 such that f(n) ≥ c * g(n) for all n where n ≥

n0.

Theta Notation

Theta notation is a method of representing both the lower bound and the upper bound

of an algorithm’s execution time. The function −

f(n) = θ(g(n))

if there exists positive constants c1, c2, and n0 such that c1 * g(n) ≤ f(n) ≤ c2 * g(n) for

all n where n ≥ n0.

1.9 Parallel Computing Algorithms Models

1. The Data-Parallel Model

i. The data-parallel model is one of the simplest algorithm models. In this

model, the tasks are statically or semi-statically mapped onto processes

and each task performs similar operations on different data.

ii. This type of parallelism that is a result of identical operations being applied

concurrently on different data items is called data parallelism.


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iii. The work may be done in phases and the data operated upon in different

phases may be different.

iv. Typically, data-parallel computation phases are interspersed with

interactions to synchronize the tasks or to get fresh data to the tasks.

v. Since all tasks perform similar computations, the decomposition of the

problem into tasks is usually based on data partitioning because a uniform

partitioning of data followed by a static mapping is sufficient to guarantee

load balance.

vi. Data-parallel algorithms can be implemented in both shared-address-

space and message-passing paradigms.

vii. The partitioned address-space in a message-passing paradigm may allow

better control of placement, and thus may offer a better handle on locality.

viii. On the other hand, shared-address space can ease the programming

effort, especially if the distribution of data is different in different phases

of the algorithm.

ix. Interaction overheads in the data-parallel model can be minimized by

choosing a locality preserving decomposition and, if applicable, by

overlapping computation and interaction and by using optimized collective

interaction routines.

x. A key characteristic of data-parallel problems is that for most problems,

the degree of data parallelism increases with the size of the problem,

making it possible to use more processes to effectively solve larger

problems.

2. The Task Graph Model

i. The computations in any parallel algorithm can be viewed as a task-

dependency graph.

ii. The task-dependency graph may be either trivial, as in the case of matrix

multiplication, or nontrivial However, in certain parallel algorithms, the

task-dependency graph is explicitly used in mapping. In the task graph


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model, the interrelationships among the tasks are utilized to promote

locality or to reduce interaction costs.

iii. This model is typically employed to solve problems in which the amount of

data associated with the tasks is large relative to the amount of

computation associated with them.

iv. Usually, tasks are mapped statically to help optimize the cost of data

movement among tasks.

v. Sometimes a decentralized dynamic mapping may be used, but even then,

the mapping uses the information about the task-dependency graph

structure and the interaction pattern of tasks to minimize interaction

overhead.

vi. Work is more easily shared in paradigms with globally addressable space,

but mechanisms are available to share work in disjoint address space.

vii. Typical interaction-reducing techniques applicable to this model include

reducing the volume and frequency of interaction by promoting locality

while mapping the tasks based on the interaction pattern of tasks, and

using asynchronous interaction methods to overlap the interaction with

computation.

viii. Examples of algorithms based on the task graph model include parallel

quicksort sparse matrix factorization, and many parallel algorithms derived

via divide-and-conquer decomposition.

ix. This type of parallelism that is naturally expressed by independent tasks in

a task-dependency graph is called task parallelism.

3. The Work Pool Model

i. The work pool or the task pool model is characterized by a dynamic

mapping of tasks onto processes for load balancing in which any task may

potentially be performed by any process.

ii. There is no desired premapping of tasks onto processes. The mapping may

be centralized or decentralized. Pointers to the tasks may be stored in a


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physically shared list, priority queue, hash table, or tree, or they could be

stored in a physically distributed data structure.

iii. The work may be statically available in the beginning, or could be

dynamically generated; i.e., the processes may generate work and add it

to the global (possibly distributed) work pool.

iv. If the work is generated dynamically and a decentralized mapping is used,

then a termination detection algorithm (In the message-passing paradigm,

the work pool model is typically used when the amount of data associated

with tasks is relatively small compared to the computation associated with

the tasks. As a result, tasks can be readily moved around without causing

too much data interaction overhead.

v. The granularity of the tasks can be adjusted to attain the desired level of

tradeoff between load-imbalance and the overhead of accessing the work

pool for adding and extracting tasks.

vi. Parallelization of loops by chunk scheduling or related methods is an

example of the use of the work pool model with centralized mapping when

the tasks are statically available.

vii. Parallel tree search where the work is represented by a centralized or

distributed data structure is an example of the use of the work pool model

where the tasks are generated dynamically.

4. The Master-Slave Model

i. In the master-slave or the manager-worker model, one or more master

processes generate work and allocate it to worker processes.

ii. The tasks may be allocated a priori if the manager can estimate the size of

the tasks or if a random mapping can do an adequate job of load balancing.

In another scenario, workers are assigned smaller pieces of work at

different times.


Anuradha Bhatia 24

iii. The latter scheme is preferred if it is time consuming for the master to

generate work and hence it is not desirable to make all workers wait until

the master has generated all work pieces.

iv. In some cases, work may need to be performed in phases, and work in

each phase must finish before work in the next phases can be generated.

In this case, the manager may cause all workers to synchronize after each

phase.

v. Usually, there is no desired premapping of work to processes, and any

worker can do any job assigned to it. The manager-worker model can be

generalized to the hierarchical or multi-level manager-worker model in

which the top-level manager feeds large chunks of tasks to second-level

managers, who further subdivide the tasks among their own workers and

may perform part of the work themselves.

vi. This model is generally equally suitable to shared-address-space or

message-passing paradigms since the interaction is naturally two-way; i.e.,

the manager knows that it needs to give out work and workers know that

they need to get work from the manager.

vii. While using the master-slave model, care should be taken to ensure that

the master does not become a bottleneck, which may happen if the tasks

are too small (or the workers are relatively fast).

viii. The granularity of tasks should be chosen such that the cost of doing work

dominates the cost of transferring work and the cost of synchronization.

ix. Asynchronous interaction may help overlap interaction and the

computation associated with work generation by the master. It may also

reduce waiting times if the nature of requests from workers is non-

deterministic.

5. The Pipeline or Producer-Consumer Model

i. In the pipeline model, a stream of data is passed on through a succession

of processes, each of which perform some task on it.


Anuradha Bhatia 25

ii. This simultaneous execution of different programs on a data stream is

called stream parallelism.

iii. With the exception of the process initiating the pipeline, the arrival of new

data triggers the execution of a new task by a process in the pipeline. The

processes could form such pipelines in the shape of linear or

multidimensional arrays, trees, or general graphs with or without cycles.

iv. A pipeline is a chain of producers and consumers. Each process in the

pipeline can be viewed as a consumer of a sequence of data items for the

process preceding it in the pipeline and as a producer of data for the

process following it in the pipeline.

v. The pipeline does not need to be a linear chain; it can be a directed graph.

The pipeline model usually involves a static mapping of tasks onto

processes.

vi. Load balancing is a function of task granularity. The larger the granularity,

the longer it takes to fill up the pipeline, i.e. for the trigger produced by the

first process in the chain to propagate to the last process, thereby keeping

some of the processes waiting.

vii. However, too fine a granularity may increase interaction overheads

because processes will need to interact to receive fresh data after smaller

pieces of computation. The most common interaction reduction technique

applicable to this model is overlapping interaction with computation.

6. Hybrid Models

i. In some cases, more than one model may be applicable to the problem at

hand, resulting in a hybrid algorithm model.

ii. A hybrid model may be composed either of multiple models applied

hierarchically or multiple models applied sequentially to different phases

of a parallel algorithm. In some cases, an algorithm formulation may have

characteristics of more than one algorithm model.


Anuradha Bhatia 26

iii. For instance, data may flow in a pipelined manner in a pattern guided by a

task-dependency graph. In another scenario, the major computation may

be described by a task-dependency graph, but each node of the graph may

represent a super task comprising multiple subtasks that may be suitable

for data-parallel or pipelined parallelism.

1.10 Parallel Programming Models

i. There are several parallel programming models in common use:

Shared Memory (without threads)

Threads

Distributed Memory / Message Passing

Data Parallel

Hybrid

Single Program Multiple Data (SPMD)

Multiple Program Multiple Data (MPMD)

ii. Parallel programming models exist as an abstraction above hardware and

memory architectures.

iii. Although it might not seem apparent, these models are NOT specific to a

particular type of machine or memory architecture. In fact, any of these

models can (theoretically) be implemented on any underlying hardware.

Two examples from the past are discussed below.

iv. SHARED memory model on a DISTRIBUTED memory machine: Kendall

Square Research (KSR) ALLCACHE approach.

v. Machine memory was physically distributed across networked machines,

but appeared to the user as a single shared memory (global address space).

Generically, this approach is referred to as "virtual shared memory".

vi. DISTRIBUTED memory model on a SHARED memory machine: Message

Passing Interface (MPI) on SGI Origin 2000.

vii. The SGI Origin 2000 employed the CC-NUMA type of shared memory

architecture, where every task has direct access to global address space


Anuradha Bhatia 27

spread across all machines. However, the ability to send and receive

messages using MPI, as is commonly done over a network of distributed

memory machines, was implemented and commonly used.

1. Shared Memory Model (without threads)

i. In this programming model, tasks share a common address space, which

they read and write to asynchronously.

ii. Various mechanisms such as locks / semaphores may be used to control

access to the shared memory.

iii. An advantage of this model from the programmer's point of view is that

the notion of data "ownership" is lacking, so there is no need to specify

explicitly the communication of data between tasks. Program

development can often be simplified.

iv. An important disadvantage in terms of performance is that it becomes

more difficult to understand and manage data locality.

Keeping data local to the processor that works on it conserves memory

accesses, cache refreshes and bus traffic that occurs when multiple

processors use the same data.

Unfortunately, controlling data locality is hard to understand and

beyond the control of the average user.

v. Implementation: Native compilers and/or hardware translate user

program variables into actual memory addresses, which are global. On

stand-alone SMP machines, this is straightforward.

vi. On distributed shared memory machines, such as the SGI Origin, memory

is physically distributed across a network of machines, but made global

through specialized hardware and software.

2. Threads Model

i. This programming model is a type of shared memory programming.

ii. In the threads model of parallel programming, a single process can have

multiple, concurrent execution paths.


Anuradha Bhatia 28

iii. Perhaps the most simple analogy that can be used to describe threads is

the concept of a single program that includes a number of subroutines:

Figure 1.12: Thread Model

The main program a.out is scheduled to run by the native operating

system. a.out loads and acquires all of the necessary system and user

resources to run.

a.out performs some serial work, and then creates a number of tasks

(threads) that can be scheduled and run by the operating system

concurrently.

Each thread has local data, but also, shares the entire resources of a.out.

This saves the overhead associated with replicating a program's resources

for each thread. Each thread also benefits from a global memory view

because it shares the memory space of a.out.

A thread's work may best be described as a subroutine within the main

program. Any thread can execute any subroutine at the same time as other

threads.


Anuradha Bhatia 29

Threads communicate with each other through global memory (updating

address locations). This requires synchronization constructs to ensure that

more than one thread is not updating the same global address at any time.

Threads can come and go, but a.out remains present to provide the

necessary shared resources until the application has completed.

iv. Implementation: From a programming perspective, threads

implementations commonly comprise:

v. A library of subroutines that are called from within parallel source code

vi. A set of compiler directives imbedded in either serial or parallel source

code

vii. Threaded implementations are not new in computing. Historically,

hardware vendors have implemented their own proprietary versions of

threads. These implementations differed substantially from each other

making it difficult for programmers to develop portable threaded

applications.

viii. Unrelated standardization efforts have resulted in two very different

implementations of threads: POSIX Threads and OpenMP.

ix. POSIX Threads

Library based; requires parallel coding

Specified by the IEEE POSIX 1003.1c standard (1995).

C Language only

Commonly referred to as Pthreads.

Most hardware vendors now offer Pthreads in addition to their

proprietary threads implementations.

Very explicit parallelism; requires significant programmer attention

to detail.

x. OpenMP

Compiler directive based; can use serial code


Anuradha Bhatia 30

Jointly defined and endorsed by a group of major computer

hardware and software vendors. The OpenMP Fortran API was

released October 28, 1997. The C/C++ API was released in late

1998.

Portable / multi-platform, including Unix and Windows NT

platforms

Available in C/C++ and Fortran implementations

Can be very easy and simple to use - provides for "incremental

parallelism"

Microsoft has its own implementation for threads, which is not

related to the UNIX POSIX standard or OpenMP.

3. Distributed Memory / Message Passing Model

i. This model demonstrates the following characteristics:

ii. A set of tasks that use their own local memory during computation.

Multiple tasks can reside on the same physical machine and/or across an

arbitrary number of machines.

Tasks exchange data through communications by sending and

receiving messages.

Data transfer usually requires cooperative operations to be

performed by each process. For example, a send operation must

have a matching receive operation.


Anuradha Bhatia 31

Figure 1.13: Message Passing Model

Tasks exchange data through communications by sending and

receiving messages.

Data transfer usually requires cooperative operations to be

performed by each process. For example, a send operation must

have a matching receive operation.

iii. From a programming perspective, message passing implementations

usually comprise a library of subroutines. Calls to these subroutines are

imbedded in source code. The programmer is responsible for determining

all parallelism.

iv. Historically, a variety of message passing libraries have been available

since the 1980s. These implementations differed substantially from each

other making it difficult for programmers to develop portable applications.

v. In 1992, the MPI Forum was formed with the primary goal of establishing

a standard interface for message passing implementations.

vi. Part 1 of the Message Passing Interface (MPI) was released in 1994. Part

2 (MPI-2) was released in 1996.


Anuradha Bhatia 32

vii. MPI is now the "de facto" industry standard for message passing, replacing

virtually all other message passing implementations used for production

work. MPI implementations exist for virtually all popular parallel

computing platforms. Not all implementations include everything in both

MPI1 and MPI2.

4. Data Parallel Model

i. The data parallel model demonstrates the following characteristics:

Most of the parallel work focuses on performing operations on a

data set. The data set is typically organized into a common

structure, such as an array or cube.

A set of tasks work collectively on the same data structure,

however, each task works on a different partition of the same data

structure.

Tasks perform the same operation on their partition of work, for

example, "add 4 to every array element".

ii. On shared memory architectures, all tasks may have access to the data

structure through global memory.

iii. On distributed memory architectures the data structure is split up and

resides as "chunks" in the local memory of each task.


Anuradha Bhatia 33

Figure 1.14: Data Parallel Model

iv. Implementations:

Programming with the data parallel model is usually accomplished by

writing a program with data parallel constructs. The constructs can be

calls to a data parallel subroutine library or, compiler directives

recognized by a data parallel compiler.

FORTRAN 90 and 95 (F90, F95): ISO/ANSI standard extensions to Fortran

77.

Contains everything that is in Fortran 77

New source code format; additions to character set

Additions to program structure and commands

Variable additions - methods and arguments

Pointers and dynamic memory allocation added


Anuradha Bhatia 34

Array processing (arrays treated as objects) added

Recursive and new intrinsic functions added

Many other new features

High Performance FORTRAN (HPF): Extensions to Fortran 90 to support

data parallel programming.

Contains everything in Fortran 90

Directives to tell compiler how to distribute data added

Assertions that can improve optimization of generated code

added

Data parallel constructs added (now part of Fortran 95)

HPF compilers were relatively common in the 1990s, but are no longer

commonly implemented.

Compiler Directives: Allow the programmer to specify the distribution

and alignment of data. FORTRAN implementations are available for most

common parallel platforms.

Distributed memory implementations of this model usually require the

compiler to produce object code with calls to a message passing library

Figure 1.14: Directives


Anuradha Bhatia 35

(MPI) for data distribution. All message passing is done invisibly to the

programmer.

5. Hybrid Model

i. A hybrid model combines more than one of the previously described

programming models.

ii. Currently, a common example of a hybrid model is the combination of the

message passing model (MPI) with the threads model (OpenMP).

Threads perform computationally intensive kernels using local, on-

node data

Communications between processes on different nodes occurs over

the network using MPI

iii. This hybrid model lends itself well to the increasingly common hardware

environment of clustered multi/many-core machines.

iv. Another similar and increasingly popular example of a hybrid model is

using MPI with GPU (Graphics Processing Unit) programming.

GPUs perform computationally intensive kernels using local, on-node

data

Communications between processes on different nodes occurs over

sthe network using MPI


Anuradha Bhatia

2. Pipeline Processing

CONTENTS

2.1 Introduction

2.2 Pipeline Performance

2.3 Arithmetic Pipelines

2.4 Pipelined Instruction Processing

2.5 Pipeline Stage Design

2.6 Hazards, Dynamic

2.7 Instruction Scheduling

Parallel & Distributed Systems 2. Pipeline Processing

Anuradha Bhatia 37

2.1 Introduction

1. Pipelining is one way of improving the overall processing performance of a processor.

2. This architectural approach allows the simultaneous execution of several instructions.

3. Pipelining is transparent to the programmer; it exploits parallelism at the instruction

level by overlapping the execution process of instructions.

4. It is analogous to an assembly line where workers perform a specific task and pass the

partially completed product to the next worker.

2.2 Pipeline Structure

1. The pipeline design technique decomposes a sequential process into several sub

processes, called stages or segments.

2. A stage performs a particular function and produces an intermediate result. It consists

of an input latch, also called a register or buffer, followed by a processing circuit.

3. The processing circuit of a given stage is connected to the input latch of the next stage

A clock signal is connected to each input latch.

Figure 2.1: Pipeline Structure

4. At each clock pulse, every stage transfers its intermediate result to the input latch of

the next stage. In this way, the final result is produced after the input data have passed

through the entire pipeline, completing one stage per clock pulse.

5. The period of the clock pulse should be large enough to provide sufficient time for a

signal to traverse through the slowest stage, which is called the bottleneck (i.e., the

stage needing the longest amount of time to complete).


Anuradha Bhatia 38

6. In addition, there should be enough time for a latch to store its input signals. If the

clock's period, P, is expressed as P = tb + tl, then tb should be greater than the

maximum delay of the bottleneck stage, and tl should be sufficient for storing data

into a latch.

2.3 Pipeline Performance Measure

1. The ability to overlap stages of a sequential process for different input tasks (data or

operations) results in an overall theoretical completion time of

where n is the number of input tasks, m is the number of stages in the pipeline, and P

is the clock period.

2. The term m*P is the time required for the first input task to get through the pipeline,

and the term (n-1)*P is the time required for the remaining tasks. After the pipeline

has been filled, it generates an output on each clock cycle. In other words, after the

pipeline is loaded, it will generate output only as fast as its slowest stage.

3. Even with this limitation, the pipeline will greatly outperform no pipelined techniques,

which require each task to complete before another task’s execution sequence

begins.

4. To be more specific, when n is large, a pipelined processor can produce output

approximately m times faster than a no pipelined processor. On the other hand, in a

no pipelined processor, the above sequential process requires a completion time of

where i is the delay of each stage.

For the ideal case when all stages have equal delay T seq can be rewritten as

2.4 Types of Pipeline

1. Pipelines are usually divided into two classes: instruction pipelines and arithmetic

pipelines.


Anuradha Bhatia 39

2. A pipeline in each of these classes can be designed in two ways: static or dynamic.

3. A static pipeline can perform only one operation (such as addition or

multiplication) at a time.

4. The operation of a static pipeline can only be changed after the pipeline has been

drained. (A pipeline is said to be drained when the last input data leave the

pipeline.)

5. For example, consider a static pipeline that is able to perform addition and

multiplication.

6. Each time that the pipeline switches from a multiplication operation to an addition

operation, it must be drained and set for the new operation.

Figure 2.2: General Pipeline Structure


Anuradha Bhatia 40

2.5 Arithmetic Pipelining

1. The pipeline structures used for instruction pipelining may be applied in some cases

to other processing tasks.

2. If pipelining is to be useful, however, we must be faced with the need to perform a

long sequence of essentially similar tasks.

3. Large numerical applications often make use of repeated arithmetic operations for

processing the elements of vectors and arrays.

4. Architectures specialized for applications if this type often provide pipelines to speed

processing of floating-point arithmetic sequences.

5. This type of pipelining is called arithmetic pipelining.

Figure 2.3: A pipelined Floating point adder

6. Arithmetic pipelines differ from instruction pipelines in some important ways. They

are generally synchronous.

7. This means that each stage executes in a fixed number of clock cycles. In a

synchronous pipeline, moreover, no buffering between stages is provided.

8. Each stage must be ready to accept the data passed from a previous stage when that

data is produced.


Anuradha Bhatia 41

2.6 Instruction pipelining

1. In order to speed up the operation of a computer system beyond what is possible with

sequential execution, methods must be found to perform more than one task at a

time.

2. One method for gaining significant speedup with modest hardware cost is the

technique of pipelining.

3. In this technique, A task is broken down into multiple steps, and independent

processing units are assigned to each step. Once a task has completed its initial step,

another task may enter that step while the original task moves on to the following

step.

4. The process is much like an assembly line, with a different task in progress at each

stage. In theory, a pipeline which breaks a process into N steps could achieve an N-

fold increase in processing speed. Due to various practical problems, the actual gain

may be significantly less.

5. The concept of pipelines can be extended to various structures of interconnected

processing elements, including those in which data flows from more than one source

or to more than one destination, or may be fed back into an earlier stage.

6. We will limit our attention to linear sequential pipelines in which all data flows

through the stages in the same sequence, and data remains in the same order in which

it originally entered.

7. Pipelining is most suited for tasks in which essentially the same sequence of steps

must be repeated many times for different data.


Anuradha Bhatia 42

Figure 2.4: Stages of Instruction Pipeline

8. An instruction pipeline increases the performance of a processor by overlapping the

processing of several different instructions. Often, this is done by dividing the

instruction execution process into several stages.

9. As shown in stages of instruction pipeline above diagram, an instruction pipeline often

consists of five stages, as follows:

i. Instruction fetch (IF). Retrieval of instructions from cache (or main

memory).

ii. Instruction decoding (ID). Identification of the operation to be

performed.

iii. Operand fetch (OF). Decoding and retrieval of any required operands.

iv. Execution (EX). Performing the operation on the operands.

v. Write-back (WB). Updating the destination operands.

o


Anuradha Bhatia 43

2.7 Instruction Processing

1. The first step in applying pipelining techniques to instruction processing is to divide

the task into steps that may be performed with independent hardware.

2. The most obvious division is between the FETCH cycle (fetch and interpret

instructions) and the EXECUTE cycle (access operands and perform operation).

3. If these two activities are to run simultaneously, they must use independent registers

and processing circuits, including independent access to memory (separate MAR and

MBR).

4. It is possible to further divide FETCH into fetching and interpreting, but since

interpreting is very fast this is not generally done.

5. To gain the benefits of pipelining it is desirable that each stage take a comparable

amount of time.

6. The result of each stage is passed on to the next stage.

b

Figure 2.5: Execution cycle for four consecutive instructions

2.8 Problems in Instruction Pipelining

Several difficulties prevent instruction pipelining from being as simple as the above description

suggests. The principal problems are:

1. Timing variations: Not all stages take the same amount of time. This means that

the speed gain of a pipeline will be determined by its slowest stage. This problem

is particularly acute in instruction processing, since different instructions have

different operand requirements and sometimes vastly different processing time.

Moreover, synchronization mechanisms are required to ensure that data is passed

from stage to stage only when both stages are ready.


Anuradha Bhatia 44

2. Data hazards: When several instructions are in partial execution, a problem arises

if they reference the same data. We must ensure that a later instruction does not

attempt to access data sooner than a preceding instruction, if this will lead to

incorrect results. For example, instruction N+1 must not be permitted to fetch an

operand that is yet to be stored into by instruction N.

3. Branching: In order to fetch the "next" instruction, we must know which one is

required. If the present instruction is a conditional branch, the next instruction

may not be known until the current one is processed.

4. Interrupts: Interrupts insert unplanned "extra" instructions into the instruction

stream. The interrupt must take effect between instructions, that is, when one

instruction has completed and the next has not yet begun. With pipelining, the

next instruction has usually begun before the current one has completed.

Possible solutions to the problems described above include the following strategies:

1. Timing Variations: To maximize the speed gain, stages must first be chosen to be as

uniform as possible in timing requirements. However, a timing mechanism is needed.

A synchronous method could be used, in which a stage is assumed to be complete in

a definite number of clock cycles. However, asynchronous techniques are generally

more efficient. A flag bit or signal line is passed forward to the next stage indicating

when valid data is available. A signal must also be passed back from the next stage

when the data has been accepted. In all cases there must be a buffer register between

stages to hold the data; sometimes this buffer is expanded to a memory which can

hold several data items. Each stage must take care not to accept input data until it is

valid, and not to produce output data until there is room in its output buffer.

2. Data Hazards: To guard against data hazards it is necessary for each stage to be aware

of the operands in use by stages further down the pipeline. The type of use must also

be known, since two successive reads do not conflict and should not be cause to slow

the pipeline. Only when writing is involved is there a possible conflict. The pipeline is

typically equipped with a small associative check memory which can store the address

and operation type (read or write) for each instruction currently in the pipe. The


Anuradha Bhatia 45

concept of "address" must be extended to identify registers as well. Each instruction

can affect only a small number of operands, but indirect effects of addressing must

not be neglected.

3. Branching: The problem in branching is that the pipeline may be slowed down by a

branch instruction because we do not know which branch to follow. In the absence of

any special help in this area, it would be necessary to delay processing of further

instructions until the branch destination is resolved. Since branches are extremely

frequent, this delay would be unacceptable. One solution which is widely used,

especially in RISC architectures, is deferred branching. In this method, the instruction

set is designed so that after a conditional branch instruction, the next instruction in

sequence is always executed, and then the branch is taken. Thus every branch must

be followed by one instruction which logically precedes it and is to be executed in all

cases. This gives the pipeline some breathing room. If necessary this instruction can

be a no-op, but frequent use of no-ops would destroy the speed benefit. Use of this

technique requires a coding method which is confusing for programmers but not too

difficult for compiler code generators. Most other techniques involve some type of

speculative execution, in which instructions are processed which are not known with

certainty to be correct. It must be possible to discard or "back out" from the results

of this execution if necessary. The usual solution is to follow the "obvious" branch,

that is, the next sequential instruction, taking care to perform no irreversible action.

Operands may be fetched and processed, but no results may be stored until the

branch is decoded. If the choice was wrong, it can be abandoned and the alternate

branch can be processed. This method works reasonably well if the obvious branch is

usually right. When coding for such pipelined CPU's, care should be taken to code

branches (especially error transfers) so that the "straight through" path is the one

usually taken. Of course, unnecessary branching should be avoided. Another

possibility is to restructure programs so that fewer branches are present, such as by

"unrolling" certain types of loops. This can be done by optimizing compilers or, in

some cases, by the hardware itself. A widely-used strategy in many current


Anuradha Bhatia 46

architectures is some type of branch prediction. This may be based on information

provided by the compiler or on statistics collected by the hardware. The goal in any

case is to make the best guess as to whether or not a particular branch will be taken,

and to use this guess to continue the pipeline. A more costly solution occasionally

used is to split the pipeline and begin processing both branches. This idea is receiving

new attention in some of the newest processors.

4. Interrupts: The fastest but most costly solution to the interrupt problem would be to

include as part of the saved "hardware state" of the CPU the complete contents of the

pipeline, so that all instructions may be restored to their original state in the pipeline.

This strategy is too expensive in other ways and is not practical. The simplest solution

is to wait until all instructions in the pipeline complete, that is, flush the pipeline from

the starting point, before admitting the interrupt sequence. If interrupts are frequent,

this would greatly slow down the pipeline; moreover, critical interrupts would be

delayed. A compromise solution identifies a "point of no return," the point in the pipe

at which instructions may first perform an irreversible action such as storing operands.

Instructions which have passed this point are allowed to complete, while instructions

that have not reached this point are canceled.

2.9 Non-linear Pipelines

1. More sophisticated instruction pipelines can sometimes be nonlinear or no

sequential.

2. One example is branch processing in which the pipeline has two forks to process two

possible paths at once.

3. Sequential processing can be relaxed by a pipeline which allows a later instruction to

enter when a previous one is stalled by a data conflict. This, of course, introduces

much more difficult timing and consistency problems.

4. Pipelines for arithmetic processing often are extended to two-dimensional structures

in which input data comes from several other stages and output may be passed to

more than oe destination.


Anuradha Bhatia 47

5. Feedback to previous stages can also occur. For such pipelines special algorithms are

devised, called systolic algorithms, to effectively use the available stages in a

synchronized fashion.

Example: Prove that K stage pipeline processes are k times faster

than nonlinear pipelines

Clock cycle of the pipeline:

Latch delay: d

= max {m}

+ d

Pipeline frequency: f = 1 /

Speed up and Efficiency

K-stage pipeline processes n tasks in k + (n-1) clock cycles: k cycles

for the first task and n-1 cycles for the remaining n-1 tasks

Total time to process n tasks Tk = [k + (n-1)]

For the non-pipelined processor T1 = n k

Speedup factor

As the K stage reaches n and the value of n is approximated to infinity.

The speedup factor is equivalent to K


Anuradha Bhatia 48

2.10 Pipeline control: scheduling

1. Controlling the sequence of tasks presented to a pipeline for execution is extremely

important for maximizing its utilization.

2. If two tasks are initiated requiring the same stage of the pipeline at the same time, a

collision occurs, which temporarily disrupts execution.

i. Reservation table. There are two types of pipelines: static and

dynamic. A static pipeline can perform only one function at a time,

whereas a dynamic pipeline can perform more than one function at a

time. A pipeline reservation table shows when stages of a pipeline are

in use for a particular function. Each stage ofthe pipeline is represented

by a row in the reservation table.

Figure 2.6: A static pipeline and its corresponding Reservation Table

ii. Latency. The delay, or number of time units separating two initiations,

is called latency. A collision will occur if two pieces of input data are

initiated with a latency equal to the distance between two X's in a

reservation table. For example, the table in Figure 3.15 has two X's with

a distance of 1 in the second row. Therefore, if a second piece of data

is passed to the pipeline one time unit after the first, a collision will

occur in stage 2.

2.11 Scheduling Static Pipelines

i. Forbidden list. Every reservation table with two or more X's in any given

row has one or more forbidden latencies, which, if not prohibited, would


Anuradha Bhatia 49

allow two data to collide or arrive at the same stage of the pipeline at the

same time. The forbidden list F is simply a list of integers corresponding to

these prohibited latencies.

ii. Collision vectors. A collision vector is a string of binary digits of length N+1,

where N is the largest forbidden latency in the forbidden list. The initial

collision vector, C, is created from the forbidden list in the following way:

each component ci of C, for i=0 to N, is 1 if i is an element of the forbidden

list. Otherwise, ci is zero. Zeros in the collision vector indicate allowable

latencies, or times when initiations are allowed into the pipeline.

iii. State diagram. State diagrams can be used to show the different states of

a pipeline for a given time slice. Once a state diagram is created, it is easier

to derive schedules of input data for the pipeline that have no collisions.

Figure 2.7: State Diagram for Static Pipeline

2.12 Scheduling Dynamic Pipelines

1. When scheduling a static pipeline, only collisions between different input data for a

particular function had to be avoided. With a dynamic pipeline, it is possible for

different input data requiring different functions to be present in the pipeline at the

same time. Therefore, collisions between these data must be considered as well. As

with the static pipeline, however, dynamic pipeline scheduling begins with the

compilation of a set of forbidden lists from function reservation tables. Next the

collision vectors are obtained, and finally the sate diagram is drawn.

i. Forbidden lists. With a dynamic pipeline, the number of forbidden lists is

the square of the number of functions sharing the pipeline. In Figure 3.17


Anuradha Bhatia 50

the number of functions equals 2, A and B; therefore, the number of

forbidden lists equals 4, denoted as AA, AB, BA, and BB. For example, if the

forbidden list AB contains integer d, then a datum requiring function B

cannot be initiated to the pipeline at some later time t+d, where t

represents the time at which a datum requiring function A was initiated.

Figure 2.8: Dynamic Pipeline and its reservation table

ii. Collision vectors and collision matrices. The collision vectors are

determined in the same manner as for a static pipeline; 0 indicates a

permissible latency and a 1 indicates a forbidden latency. For the

preceding example, the collision vectors are

The collision vectors for the A function form the collision matrix MA, that

is,

The collision vectors for the B function form the collision matrix MB

For the above collision vectors, the collision matrices are


Anuradha Bhatia 51

iii. State diagram. The state diagram for the dynamic pipeline is developed in

the same way as for the static pipeline. The resulting state diagram is much

more complicated than a static pipeline state diagram due to the larger

number of potential collisions.

Figure 2.9: State diagram for dynamic pipeline


Anuradha Bhatia

3. Synchronous Parallel Processing

CONTENTS

3.1 Introduction, Example-SIMD Architecture and Programming Principles

3.2 SIMD Parallel Algorithms

3.3 Data Mapping and memory in array processors

3.4 Case studies of SIMD parallel Processors

Parallel & Distributed Systems 3. Synchronous Parallel Processing

Anuradha Bhatia 53

3.1 Principle of SIMD

1. The Synchronous parallel architectures coordinate Concurrent operations in

lockstep through global clocks, central control units, or vector unit controllers.

2. A synchronous array of parallel processors is called an array processor. These

processors are composed of N identical processing elements (PES) under the

supervision of a one control unit (CU) This Control unit is a computer with high

speed registers, local memory and arithmetic logic unit.

3. An array processor is basically a single instruction and multiple data (SIMD)

computers. There are N data streams; one per processor, so different data can be

used in each processor. The figure below show a typical SIMD or array processor

Figure 3.1: Principle of SIMD Processor

4. A SIMD processor has a single control unit reading instructions pointed to by a

single program counter, decoding them and sending control signal to PE’s.

5. Data are to be supplied to and driven from

Figure 3.2: SIMD


Anuradha Bhatia 54

3.2 Example of SIMD

1. ILLIAC-IV

i. The ILLIAC-IV project was started in 1966 at the University of Illinois.

ii. A system with 256 processors controlled by a CP was envisioned.

iii. The set of processors was divided into four quadrants of 64 processors.

iv. The PE array is arranged as an 8x8 torus.

Figure 3.2: ILLIAC -IV

2. CM-2

i. The CM-2, introduced in 1987, is a massively parallel SIMD machine.

ii. Figure 5.23 shows the architecture of CM-2.

3. MasPar MP

i. The MasPar MP-1 is a data parallel SIMD with basic configuration

consisting of the data parallel unit (DDP) and a host workstation.

ii. The DDP consists of from 1,024 to 16,384 processing elements.

iii. The programming environment is UNIX-based. Programming

languages are MDF(MasPar FORTRAN), MPL(MasPar Programming

Language)


Anuradha Bhatia 55

Figure 3.3: MasPar MP

4. Arithmetic Example

Figure 3.4: Arithmetic Example

3.3 Parallel Computer Memory Architectures

1. Shared Memory

i. Shared memory parallel computers vary widely, but generally have in

common the ability for all processors to access all memory as global

address space.

ii. Multiple processors can operate independently but share the same

memory resources.

iii. Changes in a memory location effected by one processor are visible to all

other processors.


Anuradha Bhatia 56

iv. Shared memory machines can be divided into two main classes based upon

memory access times: UMA and NUMA.

a. Uniform Memory Access (UMA)

i. Most commonly represented today by Symmetric Multiprocessor (SMP)

machines

ii. Identical processors

iii. Equal access and access times to memory

iv. Sometimes called CC-UMA - Cache Coherent UMA. Cache coherent means

if one processor updates a location in shared memory, all the other

processors know about the update. Cache coherency is accomplished at

the hardware level.

Figure 3.5: UMA

b. Non-Uniform Memory Access (NUMA)

i. Often made by physically linking two or more SMPs

ii. One SMP can directly access memory of another SMP

iii. Not all processors have equal access time to all memories

iv. Memory access across link is slower


Anuradha Bhatia 57

v. If cache coherency is maintained, then may also be called CC-NUMA -

Cache Coherent NUMA.

Figure 3.6: NUMA

c. Advantages

i. Global address space provides a user-friendly programming perspective to

memory

ii. Data sharing between tasks is both fast and uniform due to the proximity

of memory to CPUs

d. Disadvantages

i. Primary disadvantage is the lack of scalability between memory and CPUs.

Adding more CPUs can geometrically increases traffic on the shared

memory-CPU path, and for cache coherent systems, geometrically

increase traffic associated with cache/memory management.

ii. Programmer responsibility for synchronization constructs that ensure

"correct" access of global memory.

iii. Expense: it becomes increasingly difficult and expensive to design and

produce shared memory machines with ever increasing numbers of

processors.


Anuradha Bhatia 58

2. Distributed Memory

i. Like shared memory systems, distributed memory systems vary widely

but share a common characteristic.

ii. Distributed memory systems require a communication network to

connect inter-processor memory.

iii. Processors have their own local memory. Memory addresses in one

processor do not map to another processor, so there is no concept of

global address space across all processors.

iv. Because each processor has its own local memory, it operates

independently. Changes it makes to its local memory have no effect on the

memory of other processors. Hence, the concept of cache coherency does

not apply.

v. When a processor needs access to data in another processor, it is usually

the task of the programmer to explicitly define how and when data is

communicated. Synchronization between tasks is likewise the

programmer's responsibility.

vi. The network "fabric" used for data transfer varies widely, though it can can

be as simple as Ethernet.

Figure 3.7: Distributed Memory


Anuradha Bhatia 59

a. Advantages

i. Memory is scalable with the number of processors. Increase the number

of processors and the size of memory increases proportionately.

ii. Each processor can rapidly access its own memory without interference

and without the overhead incurred with trying to maintain cache

coherency.

iii. Cost effectiveness: can use commodity, off-the-shelf processors and

networking.

b. Disadvantages

i. The programmer is responsible for many of the details associated with

data communication between processors.

ii. It may be difficult to map existing data structures, based on global

memory, to this memory organization.

iii. Non-uniform memory access (NUMA) times

3. Hybrid Distributed-Shared Memory

i. The largest and fastest computers in the world today employ both shared

and distributed memory architectures.

ii. The shared memory component can be a cache coherent SMP machine

and/or graphics processing units (GPU).

iii. The distributed memory component is the networking of multiple

SMP/GPU machines, which know only about their own memory - not the

memory on another machine. Therefore, network communications are

required to move data from one SMP/GPU to another.

iv. Current trends seem to indicate that this type of memory architecture will

continue to prevail and increase at the high end of computing for the

foreseeable future.


Anuradha Bhatia 60

Figure 3.8: Hybrid Distributed Shared Memory

3.4 SIMD Parallel Algorithms

1. Doubling Algorithms: The program for adding up n numbers in O(lg n) time is an

example of a general class of parallel algorithms known by several different

names:

Parallel-prefix Operations.

Doubling Algorithms.

In each case a single operation is applied to a large amount of data in such a way

that the amount of relevant data is halved in each step. The term “Doubling

Algorithms” is somewhat more general than “Parallel Prefix Operations”. The

latter term is most often used to refer to generalizations of our algorithm for

adding

2. The Brent Scheduling Principle: One other general principle in the design of

parallel algorithm is the Brent Scheduling Principle. It is a very simple and

ingenious idea that often makes it possible to reduce the number of processors

used in parallel algorithms, without increasing the asymptotic execution time. In

general, the execution time increases somewhat when the number of processors

is reduced, but not by an amount that increases the asymptotic time. In other

words, if an algorithm has an execution time of O (lgk n), then the execution-time

might increase by a constant factor.

3. Pipelining: This is another technique used in parallel algorithm design. Pipelining

can be used in situations where we want to perform several operations in


Anuradha Bhatia 61

sequence {P1.…..Pn}, where these operations have the property that some steps

of Pi+1 can be carried out before operation Pi is finished. In a parallel algorithm, it

is often possible to overlap these steps and decrease total execution-time.

Although this technique is most often used in MIMD algorithms, many SIMD

algorithms are also able to take advantage of it.

4. Divide and Conquer: This is the technique of splitting a problem into small

independent components and solving them in parallel. This uses FFT, Parallel

prefix and minimal spanning tree algorithms.

3.5 SIMD Architecture

1. A type of parallel computer

2. Single instruction: All processing units execute the same instruction issued by the

control unit at any given clock cycle where there are multiple processor executing

instruction given by one control unit.

3. Multiple data: Each processing unit can operate on a different data element as

shown if figure below the processor are connected to or interconnection network

providing multiple data to processing unit shared memory.

Figure 3.9: SIMD Architecture

4. This type of machine typically has an instruction dispatcher, a very high bandwidth

internal network, and a very large array of very small-capacity instruction units.

5. Thus single instruction is executed by different processing unit on different set of

data as shown above.


Anuradha Bhatia 62

6. Best suited for specialized problems characterized by a high degree of regularity,

such as image processing and vector computation.

7. Synchronous (lockstep) and deterministic execution

8. Two varieties: Processor Arrays e.g., Connection Machine CM-2, Maspar MP-1,

MP-2 and Vector Pipelines processor e.g., IBM 9000, Cray C90, Fujitsu VP, NEC SX-

2, Hitachi S820

Figure 3.10: Processor Array

3.6 SIMD Arrays and Mapping

1. Array processor performs a single instruction in multiple execution units in the

same clock cycle

2. The different execution units have same.

3. Use of parallel execution units for processing different vectors of the arrays.

4. Use of memory interleaving, and memory address registers and n memory data

registers in c


Anuradha Bhatia 63

Figure 3.11: SIMD Array

5. Data level parallelism in array processor, for example, the multiplier unit pipelines

are in parallel Computing x[i] × y[i] in number of parallel units

6. It multifunctional units simultaneously perform the actions

7. Second type attached array processor- The attached array processor has an

input/output interface to a common processor and another interface with a local

memory the local memory interconnects main memory.

7. The interleaved array processor is as shown below


Anuradha Bhatia 64

Figure 3.12: Interleaved Array Processor

Figure 3.13: Features of SIMD


Anuradha Bhatia

4. Introduction to Distributed Systems

CONTENTS

4.1 Definition

4.2 Issues, Goals

4.3 Types of distributed systems

4.4 Distributed System Models

4.5 Hardware concepts

4.6 Software Concept

4.7 Models of Middleware

4.8 Services offered by middleware

4.9 Client Server model.

Parallel & Distributed Systems 4. Introduction to Distributed Systems

Anuradha Bhatia 66

4.1 Definition

1. A distributed system is a collection of independent computers that appears to its

users as a single coherent system.

2. A distributed system consists of components (i.e., computers) that are

autonomous.

3. The major aspect is that users (be they people or programs) think they are dealing

with a single system.

4. One way or the other the autonomous components need to collaborate. How to

establish this collaboration lies at the heart of developing distributed systems.

5. Distributed systems should also be relatively easy to expand or scale

6. This characteristic is a direct consequence of having independent computers, but

at the same time, hiding how these computers actually take part in the system as

a whole.

7. A distributed system will normally be continuously available, although perhaps

some parts may be temporarily out of order.

8. Users and applications should not notice that parts are being replaced or fixed, or

that new parts are added to serve more users or applications

9. Distributed systems should also be relatively easy to expand or scale.

10. Figure 4.1 shows four networked computers and three applications, of which

application B is distributed across computers 2 and 3.

11. Each application is offered the same interface. The distributed system provides

the means for components of a single distributed application to communicate

with each other, but also to let different applications communicate.

12. At the same time, it hides, as best and reasonable as possible, the differences in

hardware and operating systems from each application.


Anuradha Bhatia 67

Figure 4.1: A distributed system organized as middleware.

4.2 Issues

1. Distributed systems differ from traditional software because components are

dispersed across a network.

2. Widely varying modes of use: The component parts of systems are subject to wide

variations in workload – for example, some web pages are accessed several million

times a day. Some parts of a system may be disconnected, or poorly connected

some of the time – for example, when mobile computers are included in a system.

Some applications have special requirements for high communication bandwidth

and low latency – for example, multimedia applications.

3. Wide range of system environments: A distributed system must accommodate

heterogeneous hardware, operating systems and networks. The networks may

differ widely in performance – wireless networks operate at a fraction of the speed

of local networks. Systems of widely differing scales, ranging from tens of

computers to millions of computers, must be supported.

4. Internal problems: Non-synchronized clocks, conflicting data updates and many

modes of hardware and software failure involving the individual system

components.

5. External threats: Attacks on data integrity and secrecy, denial of service attacks.

A physical model is a representation of the underlying hardware elements of a

distributed system that abstracts away from specific details of the computer and

networking technologies employed.


Anuradha Bhatia 68

6. Not taking this dispersion into account during design time is what makes so many

systems needlessly complex and results in mistakes that need to be patched later

on. Peter Deutsch, then at Sun Microsystems, formulated these mistakes as the

following false assumptions that everyone makes when developing a distributed

application for the first time:

i. The network is reliable.

ii. The network is secure.

iii. The network is homogeneous.

iv. The topology does not change.

v. Latency is zero.

vi. Bandwidth is infinite.

vii. Transport cost is zero.

viii. There is one administrator.

4.3 Goals

1. Four important goals that should be met to make building a distributed system

worth the effort.

2. A distributed system should make resources easily accessible; it should reasonably

hide the fact that resources are distributed across a network; it should be open;

and it should be scalable.

i. Making Resources Accessible: The main goal of a distributed system is to

make it easy for the users (and applications) to access remote resources,

and to share them in a controlled and efficient way. Resources can be just

about anything, but typical examples include things like printers,

computers, storage facilities, data, files, Web pages, and networks, to

name just a few. There are many reasons for wanting to share resources.

One obvious reason is that of economics. For example, it is cheaper to let

a printer be shared by several users in a small office than having to buy and

maintain a separate printer for each user. Likewise, it makes economic


Anuradha Bhatia 69

sense to share costly resources such as supercomputers, high-

performance storage systems, image setters, and other expensive

peripherals. Connecting users and resources also makes it easier to

collaborate and exchange information, as is clearly illustrated by the

success of the Internet with its simple protocols for exchanging files, mail,

documents, audio, and video. The connectivity of the Internet is now

leading to numerous virtual organizations in which geographically widely-

dispersed groups of people work together by means of groupware, that is,

software for collaborative editing, teleconferencing, and so on. Likewise,

the Internet connectivity has enabled electronic commerce allowing us to

buy and sell all kinds of goods without actually having to go to a store or

even leave home.

ii. Distribution Transparency: An important goal of a distributed system is to

hide the fact that its processes and resources are physically distributed

across multiple computers. A distributed system that is able to present

itself to users and applications as if it were only a single computer system

is said to be transparent. Let us first take a look at what kinds of

transparency exist in distributed systems. After that we will address the

more general question whether transparency is always required. Access

transparency deals with hiding differences in data representation and the

way that resources can be accessed by users. At a basic level, we wish to

hide differences in machine architectures, but more important is that we

reach agreement on how data is to be represented by different machines

and operating systems. For example, a distributed system may have

computer systems that run different operating systems, each having their

own file-naming conventions. Differences in naming conventions, as well

as how files can be manipulated, should all be hidden from users and

applications. Another example is where we need to guarantee that several


Anuradha Bhatia 70

replicas, located on different continents, need to be consistent all the time.

In other words, if one copy is changed, that change should be propagated

to all copies before allowing any other operation. It is clear that a single

update operation may now even take seconds to complete, something

that cannot be hidden from users.

Table 4.1: Different forms of transparency in a distributed system (ISO, 1995).

iii. Openness: An open distributed system is a system that offers services

according to standard rules that describe the syntax and semantics of

those services. For example, in computer networks, standard rules govern

the format, contents, and meaning of messages sent and received. Such

rules are formalized in protocols. In distributed systems, services are

generally specified through interfaces, which are often described in an

Interface Definition Language (IDL). Interface definitions written in an IDL

nearly always capture only the syntax of services. In other words, they

specify precisely the names of the functions that are available together

with types of the parameters, return values, possible exceptions that can

be raised, and so on. The hard part is specifying precisely what those

services do, that is, the semantics of interfaces. In practice, such

specifications are always given in an informal way by means of natural

language. If properly specified, an interface definition allows an arbitrary

process that needs a certain interface to talk to another process that

provides that interface. It also allows two independent parties to build


Anuradha Bhatia 71

completely different implementations of those interfaces, leading to two

separate distributed systems that operate in exactly the same way.

iv. Scalability: Scalability of a system can be measured along at least three

different dimensions. First, a system can be scalable with respect to its size,

meaning that we can easily add more users and resources to the system.

Second, a geographically scalable system is one in which the users and

resources may lie far apart. Third, a system can be administratively

scalable that it can still be easy to manage even if it spans many

independent administrative organizations. Unfortunately, a system that is

scalable in one or more of these dimensions often exhibits some loss of

performance as the system scales up.

Scalability Problems When a system needs to scale, very different types of

problems need to be solved. Let us first consider scaling with respect to

size. If more users or resources need to be supported, we are often

confronted with the limitations of centralized services, data, and

algorithms as shown in Figure 4.2. For example, many services are

centralized in the sense that they are implemented by means of only a

single server running on a specific machine in the distributed system. The

problem with this scheme is obvious: the server can become a bottleneck

as the number of users and applications grows. Even if we have virtually

unlimited processing and storage capacity, communication with that

server will eventually prohibit further growth.

Table 4.2: Examples of scalability limitations.

Unfortunately, using only a single server is sometimes unavoidable.

Imagine that we have a service for managing highly confidential


Anuradha Bhatia 72

information such as medical records, bank accounts and so on. In such

cases, it may be best to implement that service by means of a single server

in a highly secured separate room, and protected from other parts of the

distributed system through special network components. Copying the

server to several locations to enhance performance maybe out of the

question as it would make the service less secure. Finally, centralized

algorithms are also a bad idea. In a large distributed system, an enormous

number of messages have to be routed over many lines. From a theoretical

point of view, the optimal way to do this is collect complete information

about the load on all machines and lines, and then run an algorithm to

compute all the optimal routes. This information can then be spread

around the system to improve the routing. These algorithms generally

have the following characteristics, which distinguish them from centralized

algorithms:

1. No machine has complete information about the system state.

2. Machines make decisions based only on local information,

3. Failure of one machine does not ruin the algorithm.

4. There is no implicit assumption that a global clock exists.

Scaling Techniques Hiding communication latencies is important to

achieve geographical scalability. The basic idea is simple: try to avoid

waiting for responses to remote (and potentially distant) service requests

as much as possible. For example, when a service has been requested at a

remote machine, an alternative to waiting for a reply from the server is to

do other useful work at the requester's side. Essentially, what this means

is constructing the requesting application in such a way that it uses only

asynchronous communication. When a reply comes in, the application is

interrupted and a special handler is called to complete the previously-

issued request. Asynchronous communication can often be used in batch-

processing systems and parallel applications, in which more or less


Anuradha Bhatia 73

independent tasks can be scheduled for execution while another task is

waiting for communication to complete. Alternatively, a new thread of

control can be started to perform the request. Although it blocks waiting

for the reply, other threads in the process can continue.

Figure 4.2: The difference between letting (a) a server or (b) a client check forms as they are being

filled.

4.4 Types of distributed systems

The various types of distributed systems to be studied are

1. Distributed Computing Systems

i. An important class of distributed systems is the one used for high-

performance computing tasks.

ii. A distinction between two subgroups in cluster computing, underlying

hardware consists of a collection of similar workstations or PCs, closely

connected by means of a high-speed local-area network, each node runs

the same operating system.


Anuradha Bhatia 74

iii. Situation becomes quite different in the case of grid computing. This

subgroup consists of distributed systems that are often constructed as a

federation of computer systems, where each system may fall under a

different administrative domain, and may be very different when it comes

to hardware, software, and deployed network technology.

A. Cluster Computing Systems: Cluster computing systems became

popular when the price/performance ratio of personal computers

and workstations improved. At a certain point, it became financially

and technically attractive to build a supercomputer using off-the-

shelf technology by simply hooking up a collection of relatively

simple computers in a high-speed network. In virtually all cases,

cluster computing is used for parallel programming in which a

single (compute intensive) program is run in parallel on multiple

machines. An important part of this middleware is formed by the

libraries for executing parallel programs. Many of these libraries

effectively provide only advanced message-based communication

facilities, but are not capable of handling faulty processes, security,

etc.

Figure 4.3: Cluster Computing System

B. Grid Computing Systems: A characteristic feature of cluster

computing is its homogeneity. In most cases, the computers in a


Anuradha Bhatia 75

cluster are largely the same, they all have the same operating

system, and are all connected through the same network. In

contrast, grid computing systems have a high degree of

heterogeneity: no assumptions are made concerning hardware,

operating systems, networks, administrative domains, security

policies, etc. A key issue in a grid computing system is that

resources from different organizations are brought together to

allow the collaboration of a group of people or institutions. Such a

collaboration is realized in the form of a virtual organization. The

people belonging to the same virtual organization have access

rights to the resources that are provided to that organization.

Typically, resources consist of compute servers (including

supercomputers, possibly implemented as cluster computers),

storage facilities, and databases. In addition, special networked

devices such as telescopes, sensors, etc., can be provided as well.

The architecture consists of four layers. The lowest fabric layer

provides interfaces to local resources at a specific site. Note that

these interfaces are tailored to allow sharing of resources within a

virtual organization. Typically, they will provide functions for

querying the state and capabilities of a resource, along with

functions for actual resource management (e.g., locking resources).

The connectivity layer consists of communication protocols for

supporting grid transactions that span the usage of multiple

resources. For example, protocols are needed to transfer data

between resources, or to simply access a resource from a remote

location. In addition, the connectivity layer will contain security

protocols to authenticate users and resources. Note that in many

cases human users are not authenticated; instead, programs acting

on behalf of the users are authenticated. In this sense, delegating


Anuradha Bhatia 76

rights from a user to programs is an important function that needs

to be supported in the connectivity layer. We return extensively to

delegation when discussing security in distributed systems.

The resource layer is responsible for managing a single resource. It

uses the functions provided by the connectivity layer and calls

directly the interfaces made available by the fabric layer. For

example, this layer will offer functions for obtaining configuration

information on a specific resource, or, in general, to perform

specific operations such as creating a process or reading data. The

resource layer is thus seen to be responsible for access control, and

hence will rely on the authentication performed as part of the

connectivity layer.

The next layer in the hierarchy is the collective layer. It deals with

handling access to multiple resources and typically consists of

services for resource discovery, allocation and scheduling of tasks

onto multiple resources, data replication, and so on. Unlike the

connectivity and resource layer, which consist of a relatively small,

standard collection of protocols, the collective layer may consist of

many different protocols for many different purposes, reflecting

the broad spectrum of services it may offer to a virtual

organization.

Figure 4.4: Layer Structure


Anuradha Bhatia 77

2. Distributed information Systems

Another important class of distributed systems is found in organizations that were

confronted with a wealth of networked applications, but for which

interoperability turned out to be a painful experience. Many of the existing

middleware solutions are the result of working with an infrastructure in which it

was easier to integrate applications into an enterprise-wide information system.

As applications became more sophisticated and were gradually separated into

independent components, it became clear that integration should also take place

by letting applications communicate directly with each other.

A. Transaction Processing Systems: practice, operations on a

database are usually carried out in the form of transactions.

Programming using transactions requires special primitives that

must either be supplied by the underlying distributed system or by

the language runtime system. Typical examples of transaction

primitives. The exact list of primitives depends on what kinds of

objects are being used in the transaction. Typical examples of

transaction primitives are shown in Table 4.3.

Table 4.1: Primitive and description

The exact list of primitives depends on what kinds of objects are

being used in the transaction (Gray and Reuter, 1993). In a mail

system, there might be primitives to send, receive, and forward

mail. In an accounting system, they might be quite different. READ


Anuradha Bhatia 78

and WRITE are typical examples, however. Ordinary statements,

procedure calls, and so on, are also allowed inside a transaction.

Figure 4.5: The role of a TP monitor in distributed systems

BEGIN_ TRANSACTION and END_TRANSACTION are used to delimit the scope of a

transaction. The operations between them form the body of the transaction. The

characteristic feature of a transaction is either all of these operations are executed

or none are executed. These may be system calls, library procedures, or bracketing

statements in a language, depending on the implementation. This all-or-nothing

property of transactions is one of the four characteristic properties that

transactions have. More specifically, transactions are:

1. Atomic: To the outside world, the transaction happens indivisibly.

2. Consistent: The transaction does not violate system invariants.

3. Isolated: Concurrent transactions do not interfere with each other.

4. Durable: Once a transaction commits, the changes are permanent.

These properties are often referred to by their initial letters: ACID.

The first key property exhibited by all transactions is that they are atomic. This

property ensures that each transaction either happens completely, or not at all,

and if it happens, it happens in a single indivisible, instantaneous action. While a

transaction is in progress, other processes (whether or not they are themselves

involved in transactions) cannot see any of the intermediate states. The second

property says that they are consistent. What this means is that if the system has

certain invariants that must always hold, if they held before the transaction, they


Anuradha Bhatia 79

will hold afterward too. For example. In a banking system, a key invariant is the

law of conservation of money. After every internal transfer, the amount of money

in the bank must be the same as it was before the transfer, but for a brief moment

during the transaction, this invariant may be violated. The violation is not visible

outside the transaction, however. The third property says that transactions are

isolated or serializable. What it means is that if two or more transactions are

running at the same time, to each of them and to other processes, the final result

looks as though all transactions in

Sequentially in some (system dependent) order. The fourth property says that

transactions are durable. It refers to the fact that once a transaction commits, no

matter what happens, the transaction goes forward and the results become

permanent. No failure after the commit can undo the results or cause them to be

lost.

Figure 4.6: A nested transaction.

This need for inter application communication led to many different communication

models, which we will discuss in detail in this book (and for which reason we shall keep it

brief for now). The main idea was that existing applications could directly exchange

information, as shown in Figure 4.5.


Anuradha Bhatia 80

Figure 4.7: Middleware as a communication facilitator in enterprise application integration.

B. Enterprise Application Integration: The more applications became decoupled

from the databases they were built upon, the more evident it became that

facilities were needed to integrate applications independent from their

databases. In particular, application components should be able to

communicate directly with each other and not merely by means of the

request/reply behavior that was supported by transaction processing systems.

This need for inter application communication led to many different

communication models, which we will discuss in detail in this book (and for

which reason we shall keep it brief for now). The main idea was that existing

applications could directly exchange information.


Anuradha Bhatia 81

Figure 4.8: Communication Middleware

3. Distributed Embedded Systems: The distributed systems we have been discussing

so far are largely characterized by their stability: nodes are fixed and have a more

or less permanent and high-quality connection to a network. To a certain extent,

this stability has been realized through the various techniques that are discussed

in this book and which aim at achieving distribution transparency. For example,

the wealth of techniques for masking failures and recovery will give the impression

that only occasionally things may go wrong.

4.5 Distributed System Models

Systems that are intended for use in real-world environments should be designed to

function correctly in the widest possible range of circumstances and in the face of many

possible difficulties and threats. Each type of model is intended to provide an abstract,

simplified but consistent description of a relevant aspect of distributed system design:

i. Physical models are the most explicit way in which to describe a system; they

capture the hardware composition of a system in terms of the computers (and

other devices, such as mobile phones) and their interconnecting networks.

ii. Architectural models describe a system in terms of the computational and

communication tasks performed by its computational elements; the


Anuradha Bhatia 82

computational elements being individual computers or aggregates of them

supported by appropriate network interconnections.

iii. Fundamental models take an abstract perspective in order to examine individual

aspects of a distributed system. In this chapter we introduce fundamental models

that examine three important aspects of distributed systems: interaction models,

which consider the structure and sequencing of the communication between the

elements of the system; failure models, which consider the ways in which a system

may fail to operate correctly and; security models, which consider how the system

is protected against attempts to interfere with its correct operation or to steal its

data.

Figure 4.9: Comparison

4.6 Models of Middleware

i. Middleware as the name suggests, sits in between the Operating System and the

Application programs.

ii. The term middleware applies to a software layer that provides programming

abstraction, Masks the heterogeneity of the underlying networks, hardware,

operating systems and programming languages.

iii. Middleware is software glue. Middleware is the slash in Client/Server.


Anuradha Bhatia 83

iv. Middleware is an important class of technology that is helping to decrease the

cycle-time, level of effort, and complexity associated with developing high-quality,

flexible, and interoperable distributed systems.

v. Increasingly, these types of systems are developed using reusable software

(middleware) component services, rather than being implemented entirely from

scratch for each use. When implemented properly, middleware can help to: Shield

developers of distributed systems from low-level, tedious, and error-prone

platform details, such as socket-level network programming.

Figure 4.10: Distributed System Services

A. .Message Oriented Middleware: This is a large category and includes

communication via message exchange. It represents asynchronous

interactions between systems. It reduces complexity of developing

applications that span multiple operating systems and network protocols by

insulating the application developer from details of various operating system

and network interfaces. APIs that extend across diverse platforms and

networks are typically provided by the MOM. MOM is software that resides in

both portions of client/server architecture and typically supports

asynchronous calls between the client and server applications. Message

queues provide temporary storage when the destination program is busy or

not connected. MOM reduces the involvement of application developers with

the complexity of the master-slave nature of the client/server mechanism. E.g.

Sun’s JMS.


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B. Object Request Broker: In distributed computing, an object request broker

(ORB) is a piece of middleware software that allows programmers to make

program calls from one computer to another via a network. ORB's handle the

transformation of in-process data structures to and from the byte sequence,

which is transmitted over the network. This is called marshaling or

serialization. Some ORB's, such as CORBA-compliant systems, use an Interface

Description Language (IDL) to describe the data which is to be transmitted on

remote calls. E.g. CORBA

C. RPC Middleware: This type of middleware provides for calling procedures on

remote systems, so called as Remote Procedure Call. Unlike message oriented

middleware, RPC middleware represents synchronous interactions between

systems and is commonly used within an application. Thus, the programmer

would write essentially the same code whether the subroutine is local to the

executing program, or remote. When the software in question is written using

object-oriented principles, RPC may be referred to as remote invocation or

remote method invocation. Client makes calls to procedures running on

remote systems, which can be asynchronous or synchronous. E.g. DCE RPC.

D. Database Middleware: Database middleware allows direct access to data

structures and provides interaction directly with databases. There are

database gateways and a variety of connectivity options. Extract, Transform,

and Load (ETL) packages are included in this category. E.g. CRAVE is a web-

accessible JAVA application that accesses an underlying MySQL database of

ontologies via a JAVA persistent middleware layer Chameleon).

E. Transaction Middleware: This category as used in the Middleware Resource

Center includes traditional transaction processing monitors (TPM) and web

application servers. e.g. IBM’s CICS.

F. Portals: Enterprise portal servers are included as middleware largely because

they facilitate “front end” integration. They allow interaction between the

user’s desktop and back end systems and services. e.g Web Logic.


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Figure 4.11: Middleware

4.7 Services offered by middleware

i. Distributed system services: Critical communications, program-to-program, and

data management services. This type of service includes RPCs, MOMs and ORBs.

ii. Application enabling services: Access to distributed services and the underlying

network. This type of services includes transaction processing monitors and

database services such as Structured Query Language (SQL).

iii. Middleware management services: Which enable applications and system

functions to be continuously monitored to ensure optimum performance of the

distributed environment.


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Figure 4.12: Layers of middle ware

iv. Provide a more functional set of Application Programming Interfaces (API) than

the operating system and network services to allow an application to

Naming, Location, Service discovery, Replication

Protocol handling, Communication faults, QoS

Synchronization, Concurrency, Transactions, Storage

Access control, Authentication

4.8 Client Server model.

i. Basic client-server model, processes in a distributed system are divided into two

(possibly overlapping) groups. A server is a process implementing a specific

service, for example, a file system service or a database service. A client is a

process that requests a service from a server by sending it a request and

subsequently waiting for the server's reply. This client-server interaction, also

known as request-reply behavior is shown in Figure.


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Figure 4.13: Client Server Model

ii. Communication between a client and a server can be implemented by means of a

simple connectionless protocol when the underlying network is fairly reliable as in

many local-area networks.

iii. In these cases, when a client requests a service, it simply packages a message for

the server, identifying the service it wants, along with the necessary input data.

The message is then sent to the server.

iv. The latter, in turn, will always wait for an incoming request, subsequently process

it, and package the results in a reply message that is then sent to the client.


Anuradha Bhatia

5. Communication CONTENTS

5.1 Layered Protocols

5.2 Remote Procedure Call

5.3 Remote Object Invocation

5.4 Message Oriented Communication

5.5 Stream Oriented Communication

Parallel & Distributed Systems 5. Communication

Anuradha Bhatia 89

1. Interprocess communication is at the heart of all distributed systems.

2. It makes no sense to study distributed systems without carefully examining the

ways that processes on different machines can exchange information.

3. Communication in distributed systems is always based on low-level message

passing as offered by the underlying network.

4. Expressing communication through message passing is harder than using

primitives based on shared memory, as available for non-distributed platforms.

5. Modem distributed systems often consist of thousands or even millions of

processes scattered across a network with unreliable communication such as the

Internet.

6. Unless the primitive communication facilities of computer networks are replaced

by something else, development of large-scale distributed applications is

extremely difficult.

5.1 Layered Protocols

1. Due to the absence of shared memory, all communication in distributed systems

is based on sending and receiving (low level) messages.

2. When process A wants to communicate with process B, it first builds a message in

its own address space.

3. Then.it executes a system call that causes the operating system to send the

message over the network to B.

4. Although this basic idea sounds simple enough, in order to prevent chaos, A and

B have to agree on the meaning of the bits being sent.

5. If A sends a brilliant new novel written in French and encoded in IBM's EBCDIC

character code, and B expects the inventory of a supermarket written in English

and encoded in ASCII, communication will be less than optimal.


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Figure 5.1: Layers, Interface and Protocols in the OSI Model

Figure 5.2: A typical message as it appears on the network

6. The collection of protocols used in a particular system is called a protocol suite

or protocol stack.

7. It is important to distinguish a reference model from its actual protocols.


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Figure 5.3: Discussion between a receiver and a sender in the data link layer

8. The OSI protocols were never popular. In contrast, protocols developed for the

Internet, such as TCP and IP, are mostly used. In the following sections, we will

briefly examine each of the OSI layers in turn, starting at the bottom.

Figure 5.4: Client-Server TCP


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a) Normal operation of TCP

Each layer deals with one specific aspect of the communication. In this way, the

problem can be divided up into manageable pieces, each of which can be solved

independent of the others. Each layer provides an interface to the one above it.

The interface consists of a set of operations that together define the service the

layer is prepared to offer its users. When process A on machine 1 wants to

communicate with process B on machine 2, it builds a message and passes the

message to the application layer on its machine. This layer might be a library

procedure, for example, but it could also be implemented in some other way (e.g.,

inside the operating system, on an external network processor, etc.). The

application layer software then adds a header to the front of the message and

passes the resulting message across the layer 6/7 interface to the presentation

layer. The presentation layer in tum adds its own header and passes the result

down to the session layer, and so on. Some layers add not only a header to the

front, but also a trailer to the end. When it hits the bottom, the physical layer

actually transmits the message by putting it onto the physical transmission

medium.

b) Transactional TCP

A. Lower-Level Protocols: The physical layer is concerned with transmitting the

Os and Is. How many volts to use for 0 and 1, how many bits per second can

be sent, and whether transmission can take place in both directions

simultaneously are key issues in the physical layer. The physical layer protocol

deals with standardizing the electrical, mechanical, and signaling interfaces so

that when one machine sends a 0 bit it is actually received as a 0 bit and not a

1 bit. On a LAN, there is usually no need for the sender to locate the receiver.

It just puts the message out on the network and the receiver takes it off. A

wide-area network, however, consists of a large number of machines, each

with some number of lines to other machines, rather like a large-scale map

showing major cities and roads connecting them. For a message to get from


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the sender to the receiver it may have to make a number of hops, at each one

choosing an outgoing line to use. The question of how to choose the best path

is called routing, and is essentially the primary task of the network layer. The

problem is complicated by the fact that the shortest route is not always the

best route. What really matters is the amount of delay on a given route, which,

in tum, is related to the amount of traffic and the number of messages queued

up for transmission over the various lines. The delay can thus change over the

course of time. Some routing algorithms try to adapt to changing loads,

whereas others are content to make decisions based on long-term averages.

B. Transport Protocols: The transport layer forms the last part of what could be

called a basic network protocol stack, in the sense that it implements all those

services that are not provided at the interface of the network layer, but which

are reasonably needed to build network applications. The transport layer turns

the underlying network into something that an application developer can use.

Packets can be lost on the way from the sender to the receiver. Although some

applications can handle their own error recovery, others prefer a reliable

connection. The job of the transport layer is to provide this service. The idea is

that the application layer should be able to deliver a message to the transport

layer with the expectation that it will be delivered without loss. Upon receiving

a message from the application layer, the transport layer breaks it into pieces

small enough for transmission, assigns each one a sequence number, and then

sends them all. The discussion in the transport layer header concerns which

packets have been sent, which have been received, how many more the

receiver has room to accept, which should be retransmitted, and similar

topics. Reliable transport connections (which by definition are connection

oriented) can be built on top of connection-oriented or connectionless

network services. In the former case all the packets will arrive in the correct

sequence (if they arrive at all), but in the latter case it is possible for one packet

to take a different route and arrive earlier than the packet sent before it. It is


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up to the transport layer software to put everything back in order to maintain

the illusion that a transport connection is like a big tube-you put messages into

it and they come out undamaged and in the same order in which they went in.

Providing this end-to-end communication behavior is an important aspect of

the transport layer.

C. Higher- Level Protocols: The session layer is essentially an enhanced version

of the transport layer. It provides dialog control, to keep track of which party

is currently talking, and it provides synchronization facilities. The latter are

useful to allow users to insert checkpoints into long transfers, so that in the

event of a crash, it is necessary to go back only to the last checkpoint, rather

than all the way back to the beginning. In practice, few applications are

interested in the session layer and it is rarely supported. It is not even present

in the Internet protocol suite. The session layer is essentially an enhanced

version of the transport layer. It provides dialog control, to keep track of which

party is currently talking, and it provides synchronization facilities. The latter

are useful to allow users to insert checkpoints into long transfers, so that in

the event of a crash, it is necessary to go back only to the last checkpoint,

rather than all the way back to the beginning. In practice, few applications are

interested in the session layer and it is rarely supported. It is not even present

in the Internet protocol suite. However, in the context of developing

middleware solutions, the concept of a session and its related protocols has

turned out to be quite relevant, notably when defining higher-level

communication protocols.

D. Middleware Protocols: Middleware is an application that logically lives

(mostly) in the application layer, but which contains many general-purpose

protocols that warrant their own layers, independent of other, more specific

applications. A distinction can be made between high-level communication

protocols and protocols for establishing various middleware services. There

are numerous protocols to support a variety of middleware services. There are


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various ways to establish authentication, that is, provide proof of a claimed

identity. Authentication protocols are not closely tied to any specific

application, but instead, can be integrated into a middleware system as a

general service. Likewise, authorization protocols by which authenticated

users and processes are granted access only to those resources for which they

have authorization. tend to have a general, application-independent nature.

Figure 5.5: An adapted reference model for networked communication

c) Types of Communication

To understand the various alternatives in communication that middleware can

offer to applications, we view the middleware as an additional service in client

server computing, as shown in Figure 5.6. Consider, for example an electronic mail

system. In principle, the core of the mail delivery system can be seen as a

middleware communication service. Each host runs a user agent allowing users to

compose, send, and receive e-mail. A sending user agent passes such mail to the

mail delivery system, expecting it, in tum, to eventually deliver the mail to the

intended recipient. Likewise, the user agent at the receiver's side connects to the

mail delivery system to see whether any mail has come in. If so, the messages are

transferred to the user agent so that they can be displayed and read by the user.


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An electronic mail system is a typical example in which communication is

persistent. With persistent communication, a message that has been submitted

for transmission is stored by the communication middleware as long as it takes to

deliver it to the receiver. In this case, the middleware will store the message at

one or several of the storage facilities shown in Figure 5.6. As a consequence, it is

not necessary for the sending application to continue execution after submitting

the message. Likewise, the receiving application need not be executing when the

message is submitted. Besides being persistent or transient, communication can

also be asynchronous or synchronous. The characteristic feature of asynchronous

communication is that a sender continues immediately after it has submitted its

message for transmission. This means that the message is (temporarily) stored

immediately by the middleware upon submission. With synchronous

communication, the sender is blocked until its request is known to be accepted.

There are essentially three points where synchronization can take place. First, the

sender may be blocked until the middleware notifies that it will take over

transmission of the request. Second, the sender may synchronize until its request

has been delivered to the intended recipient. Third, synchronization may take

place by letting the sender wait until its request has been fully processed, that is,

up the time that the recipient returns a response.

Figure 5.6: Viewing middleware as an intermediate (distributed) service in application-level

communication.


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5.2 Remote Procedure Call

1. Distributed systems have been based on explicit message exchange between

processes.

2. The procedures send and receive do not conceal communication at all, which is

important to achieve access transparency in distributed systems.

3. When a process on machine A calls' a procedure on machine B, the calling process

on A is suspended, and execution of the called procedure takes place on B.

Information can be transported from the caller to the callee in the parameters and

can come back in the procedure result. No message passing at all is visible to the

programmer. This method is known as Remote Procedure Call, or often just RPC.

A. Basic RPC Operation

To understand the basic operation of RPC we need to first understand the

conventional procedure call i.e the single machine call and then split the call

to a client and server part that can be executed on different machines.

i. Conventional Procedure Call: To understand the conventional procedure

call, consider the example

count = trial (fd, buf, nbytes)

Where fd is an .integer indicating a file, buf is an array of characters into

which data are read, and nbytes is another integer telling how many bytes

to read.

Several things are worth noting. For one, in C, parameters can be call-by

value or call-by-reference. A value parameter, such as fd or n bytes, is

simply copied to the stack as shown in Figure 5.7(b). To the called

procedure, a value parameter is just an initialized local variable. The called

procedure may modify it, but such changes do not affect the original value

at the calling side. A reference parameter in C is a pointer to a variable (i.e.,

the address of the variable), rather than the value of the variable. In the

call to read. The second parameter is a reference parameter because

arrays are always passed by reference in C. What is actually pushed onto


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the stack is the address of the character array. If the called procedure uses

this parameter to store something into the character array, it does modify

the array in the calling procedure. The difference between call-by-value

and call-by-reference is quite important for RPC, as we shall see. One other

parameter passing mechanism also exists, although it is not used in C. It is

called call-by-copy/restore. It consists of having the variable copied to the

stack by the caller, as in call-by-value, and then copied back after the call,

overwriting the caller's original value. Under most conditions, this achieves

exactly the same effect as call-by-reference, but in some situations. Such

as the same parameter being present multiple times in the parameter list.

The semantics are different. The call-by-copy/restore mechanism is not

used in many languages. The decision of which parameter passing

mechanism to use is normally made by the language designers and is a

fixed property of the language. Sometimes it depends on the data type

being passed. In C, for example, integers and other scalar types are always

passed by value, whereas arrays are always passed by reference, as we

have seen. Some Ada compilers use copy/restore for in out parameters,

but others use call-by-reference. The language definition permits either

choice, which makes the semantics a bit fuzzy.

Figure 5.7

a) Parameter passing in a local procedure call: the stack before the call to read


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b) The stack while the called procedure is active

ii. Client and Server Stubs: The idea behind RPC is to make a remote

procedure call look as much as possible like a local one. In other words, we

want RPC to be transparent-the calling procedure should not be aware that

the called procedure is executing on a different machine or vice versa. It

packs the parameters into a message and requests that message to be sent

to the server as illustrated below. Following the call to send, the client stub

calls receive, blocking itself until the reply comes back. RPC achieves its

transparency in an analogous way. When read is actually a remote

procedure (e.g., one that will run on the file server's machine), a different

version of read, called a client stub, is put into the library. Like the original

one, it, too, is called using the calling sequence of Figure 5.7 (b). Also like

the original one, it too, does a call to the local operating system. Only

unlike the original one, it does not ask the operating system to give it data.

Instead, it packs the parameters into a message and requests that message

to be sent to the server as illustrated in Figure 5.8. Following the call to

send, the client stub calls receive, blocking itself until the reply comes back.

When the message arrives at the server, the server's operating system

passes it up to a server stub. A server stub is the server-side equivalent of

a client stub: it is a piece of code that transforms requests coming in over

the network into local procedure calls. Typically the server stub will have

called receive and be blocked waiting for incoming messages. The server

stub unpacks the parameters from the message and then calls the server

procedure in the usual way. From the server's point of view, it is as though

it is being called directly by the client-the parameters and return address

are all on the stack where they belong and nothing seems unusual. The

server performs its work and then returns the result to the caller in the

usual way. For example, in the case of read, the server will fill the buffer,

pointed to by the second parameter, with the data. This buffer will be


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internal to the server stub. When the server stub gets control back after

the call has completed, it packs the result (the buffer) in a message and

calls send to return it to the client. After that, the server stub usually does

a call to receive again, to wait for the next incoming request. When the

message gets back to the client machine, the client's operating system sees

that it is addressed to the client process (or actually the client stub, but the

operating system cannot see the difference). The message is copied to the

waiting buffer and the client process unblocked. The client stub inspects

the message, unpacks the result, copies it to its caller, and returns in the

usual way. When the caller gets control following the call to read, all it

knows is that its data are available. It has no idea that the work was done

remotely instead of by the local operating system.

Figure 5.8: Principle of RPC between a client and server program

iii. Steps of a Remote Procedure Call

Client procedure calls client stub in normal way.

Client stub builds message, calls local OS.

Client's OS sends message to remote OS

Remote OS gives message to server stub

Server stub unpacks parameters, calls server

Server does work, returns result to the stub


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Server stub packs it in message, calls local OS

Server's OS sends message to client's OS

Client's OS gives message to client stub

Stub unpacks result, returns to client

B. Parameter Passing

i. The function of the client stub is to take its parameters, pack them into a

message, and send them to the server stub.

ii. While this sounds straightforward, it is not quite as simple as it at first

appears. In this section we will look at some of the issues concerned with

parameter passing in RPC systems.

iii. Passing Value Parameters: Packing parameters into a message is called

parameter marshaling. As a very simple example, consider a remote

procedure, add(i, j), that takes two integer parameters i and j and returns

their arithmetic sum as a result. The call to add, is shown in the left-hand

portion (in the client process) in Figure 5.8. The client stub takes its two

parameters and puts them in a message as indicated, it also puts the name

or number of the procedure to be called in the message because the server

might support several different calls, and it has to be told which one is

required. When the message arrives at the server, the stub examines the

message to see which procedure is needed and then makes the

appropriate call. If the server also supports other remote procedures, the

server stub might have a switch statement in it to select the procedure to

be called, depending on the first field of the message. The actual call from

the stub to the server looks like the original client call, except that the

parameters are variables initialized from the incoming message. When the

server has finished, the server stub gains control again. It takes the result

sent back by the server and packs it into a message. This message is sent

back to back to the client stub which unpacks it to extract the result and

returns the value to the waiting client procedure.


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Figure 5.8: Steps involved in doing remote computation through RPC

iv. Each parameter requires one 32-bit word. Figure below shows what the

parameter portion of a message built by a client stub on an Intel Pentium

might look like, the first word contains the integer parameter, 5 in this

case, and the second contains the string "JILL."

Figure 5.9 a) Original message on the Pentium, b) The message after receipt on the SPARC, c)

The message after being inverted. The little numbers in boxes indicate the address of each byte


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C. Passing Reference Parameters

We now come to a difficult problem: How are pointers, or in general,

references passed? The answer is: only with the greatest of difficulty, if at all.

Remember that a pointer is meaningful only within the address space of the

process in which it is being used. Getting back to our read example discussed

earlier, if the second parameter (the address of the buffer) happens to be 1000

on the client, one cannot just pass the number 1000 to the server and expect

it to work. Address 1000 on the server might be in the middle of the program

text. One solution is just to forbid pointers and reference parameters in

general. However, these are so important that this solution is highly

undesirable. In fact, it is not necessary either. In the read example, the client

stub knows that the second parameter points to an array of characters.

Suppose, for the moment, that it also knows how big the array is. One strategy

then becomes apparent: copy the array into the message and send it to the

server. The server stub can then call the server with a pointer to this array,

even though this pointer has a different numerical value than the second

parameter of read has. Changes the server makes using the pointer (e.g.,

storing data into it) directly affect the message buffer inside the server stub.

When the server finishes, the original message can be sent back to the client

stub, which then copies it back to the client. In effect, call-by-reference has

been replaced by copy/restore. Although this is not always identical, it

frequently is good enough. One optimization makes this mechanism twice as

efficient. If the stubs know whether the buffer is an input parameter or an

output parameter to the server, one of the copies can be eliminated. If the

array is input to the server (e.g., in a call to write) it need not be copied back.

If it is output, it need not be sent over in the first place. As a final comment, it

is worth noting that although we can now handle pointers to simple arrays and

structures, we still cannot handle the most general case of a pointer to an

arbitrary data structure such as a complex graph. Some systems attempt to


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deal with this case by actually passing the pointer to the server stub and

generating special code in the server procedure for using pointers. For

example, a request may be sent back to the client to provide the referenced

data.

D. Parameter Specification and Stub Generation

From what we have explained so far, it is clear that hiding a remote procedure

call requires that the caller and the callee agree on the format of the messages

they exchange, and that they follow the same steps when it comes to, for

example, passing complex data structures. In other words, both sides in an RPC

should follow the same protocol or the RPC will not work correctly. As a simple

example, consider the procedure of Figure 5.10(a). It has three parameters, a

character, a floating-point number, and an array of five integers. Assuming a

word is four bytes, the RPC protocol might prescribe that we should transmit

a character in the rightmost byte of a word (leaving the next 3 bytes empty), a

float as a whole word, and an array as a group of words equal to the array

length, preceded by a word giving the length, as shown in Figure 5.10(b). Thus

given these rules, the client stub for foobar knows that it must use the format

of Figure 5.10 (b), and the server stub knows that incoming messages for

foobar will have the format of Figure 5.10 (b).

Figure 5.10: (a) A procedure. (b) The corresponding message.


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Asynchronous RPC

1. As in conventional procedure calls, when a client calls a remote procedure, the

client will block until a reply is returned.

2. This strict request-reply behavior is unnecessary when there is no result to return,

and only leads to blocking the client while it could have proceeded and have done

useful work just after requesting the remote procedure to be called.

3. Examples of where there is often no need to wait for a reply include: transferring

money from one account to another, adding entries into a database, starting

remote services, batch processing, and so on.

4. To support such situations, RPC systems may provide facilities for what are called

asynchronous RPCs, by which a client immediately continues after issuing the RPC

request.

5. With asynchronous RPCs, the server immediately sends a reply back to the client

the moment the RPC request is received, after which it calls the requested

procedure.

6. The reply acts as an acknowledgment to the client that the server is going to

process the RPC.

7. The client will continue without further blocking as soon as it has received the

server's acknowledgment. Figure 5.11(b) shows how client and server interact in

the case of asynchronous RPCs. For comparison, Figure 5.11(a) shows the normal

request-reply behavior.

Figure 5.11: (a) The interaction between client and server in a traditional RPc.


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(b) The interaction using asynchronous RPc.

8. Asynchronous RPCs can also be useful when a reply will be returned but the client

is not prepared to wait for it and do nothing in the meantime. For example, a client

may want to prefetch the network addresses of a set of hosts that it expects to

contact soon. While a naming service is collecting those addresses, the client may

want to do other things. In such cases, it makes sense to organize the

communication between the client and server through two asynchronous RPCs,

as shown in Figure 5.12. The client first calls the server to hand over a list of host

names that should be looked up, and continues when the server has

acknowledged the receipt of that list. The second call is done by the server, who

calls the client to hand over the addresses it found. Combining two asynchronous

RPCs is sometimes also referred to as a deferred synchronous RPC.

Figure 5.12: A client and server interacting through two asynchronous RPCs.


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5.3 Message Oriented Communication

1. Remote procedure calls and remote object invocations contribute to hiding

communication in distributed systems, that is, they enhance access transparency.

2. Unfortunately, neither mechanism is always appropriate.

3. When the receiving side is executing at the time a request is issued, alternative

communication services are needed.

4. Likewise, the inherent synchronous nature of RPCs, by which a client is blocked

until its request has been processed, sometimes needs to be replaced by

something else.

A. Message-Oriented Transient Communication: Many distributed systems and

applications are built directly on top of the simple message-oriented model

offered by the transport layer. To better understand and appreciate the

message-oriented systems as part of middleware solutions, we first discuss

messaging through transport-level sockets.

i. Berkeley Sockets: Special attention has been paid to standardizing the

interface of the transport layer to allow programmers to make use of its

entire suite of (messaging) protocols through a simple set of primitives.

Also, standard interfaces make it easier to port an application to a different

machine. The listen primitive is called only in the case of connection-

oriented communication. It is a non-blocking call that allows the local

operating system to reserve enough buffers for a specified maximum

number of connections that the caller is willing to accept. A call to accept

blocks the caller until a connection request arrives. When a request arrives,

the local operating system creates a new socket with the same properties

as the original one, and returns it to the caller. This approach will allow the

server to, for example, fork off a process that will subsequently handle the

actual communication through the new connection. The server, in the

meantime, can go back and wait for another connection request on the

original socket. a socket must first be created using the socket primitive,


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but explicitly binding the socket to a local address is not necessary, since

the operating system can dynamically allocate a port when the connection

is set up. The connect primitive requires that the caller specifies the

transport-level address to which a connection request is to be sent. The

client is blocked until a connection has been set up successfully, after

which both sides can start exchanging information through the send and

receive primitives. Finally, closing a connection is symmetric when using

sockets, and is established by having both the client and server call the

close primitive. The general pattern followed by a client and server for

connection-oriented communication using sockets is shown in Figure 5.15.

Primitive Meaning

Socket Create a new communication endpoint

Bind Attach a local address to a socket

Listen Announce willingness to accept connections

Accept Block caller until a connection request arrives

Connect Actively attempt to establish a connection

Send Send some data over the connection

Receive Receive some data over the connection

Close Release the connection

Figure 5.14: Socket primitives for TCP/IP


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Figure 5.15: Connection-oriented communication pattern using sockets.

ii. The Message-Passing Interface (MPI): With the advent of high-performance

multicomputer, developers have been looking for message-oriented

primitives that would allow them to easily write highly efficient applications.

This means that the primitives should be at a convenient level of abstraction

(to ease application development), and that their implementation incurs

only minimal overhead. Sockets were deemed insufficient for two

reasons. There is also a blocking send operation, called MPLsend, of

which the semantics are implementation dependent. The primitive

MPLsend may either block the caller until the specified message has been

copied to the MPI runtime system at the sender's side, or until the receiver

has initiated a receive operation. Synchronous communication by which

the sender blocks until its request is accepted for further processing is

available through the MPI send primitive. Finally, the strongest form of

synchronous communication is also supported: when a sender calls MPL

sendrecv, it sends a request to the receiver and blocks until the latter

returns a reply. Basically, this primitive corresponds to a normal RPC.


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Figure 5.16: Some of the most intuitive message-passing primitives of MPI.

B. Message-Oriented Transient Communication: important class of message-

oriented middle ware services, generally known as message-queuing systems,

or just Message-Oriented Middleware (MOM). Message-queuing systems

provide extensive support for persistent asynchronous communication. The

essence of these systems is that they offer intermediate-term storage capacity

for messages, without requiring either the sender or receiver to be active

during message transmission. An important difference with Berkeley sockets

and MPI is that message-queuing systems are typically targeted to support

message transfers that are allowed to take minutes instead of seconds or

milliseconds.

i. Message-Queuing Model: The basic idea behind a message-queuing

system is that applications communicate by inserting messages in specific

queues. These messages are forwarded over a series of communication

servers and are eventually delivered to the destination, even if it was down

when the message was sent. In practice, most communication servers are

directly connected to each other. In other words, a message is generally

transferred directly to a destination server. In principle, each application

has its own private queue to which other applications can send messages.


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A queue can be read only by its associated application, but it is also

possible for multiple applications to share a single queue. An important

aspect of message-queuing systems is that a sender is generally given only

the guarantees that its message will eventually be inserted in the

recipient's queue. No guarantees are given about when, or even if the

message will actually be read, which is completely determined by the

behavior of the recipient.

Figure 5.17: Four combinations for loosely-coupled communications using queues.

Primitive Meaning

Put Append a message to a specified queue

Get Block until the specified queue is nonempty, and remove the first message

Poll Check a specified queue for messages, and remove the first. Never block.

Notify Install a handler to be called when a message is put into the specified queue.

Figure 5.18: Basic interface to a queue in a message-queuing system.

ii. General Architecture of a Message-Queuing System: One of the first

restrictions that we make is that messages can be put only into queues that

are local to the sender, that is, queues on the same machine, or no worse than

on a machine nearby such as on the same LAN that can be efficiently reached


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through an RPC. Such a queue is called the source queue. Likewise, messages

can be read only from local queues. However, a message put into a queue will

contain the specification of a destination queue to which it should be

transferred. It is the responsibility of a message-queuing system to provide

queues to senders and receivers and take care that messages are transferred

from their source to their destination queue.

Figure 5.19: The relationship between queue-level addressing and network-level addressing

Relays can thus generally help build scalable message-queuing systems.

However, as queuing networks grow, it is clear that the manual configuration

of networks will rapidly become completely unmanageable. The only solution

is to adopt dynamic routing schemes as is done for computer networks. In that

respect, it is somewhat surprising that such solutions are not yet integrated

into some of the popular message-queuing systems.


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Figure 5.20: The general organization of a message-queuing system with routers

C. Message Brokers: An important application area of message-queuing systems

is integrating existing and new applications into a single, coherent distributed

information system. Integration requires that applications can understand the

messages they receive. In practice, this requires the sender to have its

outgoing messages in the same format as that of the receiver. The problem

with this approach is that each time an application is added to the system that

requires a separate message format, each potential receiver will have to be

adjusted in order to produce that format. A message broker can be as simple

as a reformatted for messages. For example, assume an incoming message

contains a table from a database, in which records are separated by a special

end-oj-record delimiter and fields within a record have a known, fixed length.

If the destination application expects a different delimiter between records,

and also expects that fields have variable lengths, a message broker can be

used to convert messages to the format expected by the destination.


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Figure 5.21: The general organization of a message broker in a message queuing system.

D. Message Transfer: To transfer a message from one queue manager to another

(possibly remote) queue manager, it is necessary that each message carries its

destination address, for which a transmission header is used. An address in

MQ consists of two parts. The first part consists of the name of the queue

manager to which the message is to be delivered. The second part is the name

of the destination queue resorting under that manager to which the message

is to be appended. It is possible that a message needs to be transferred across

multiple queue managers before reaching its destination. Whenever such an

intermediate queue manager receives the message, it simply extracts the

name of the destination queue manager from the message header, and does

a routing-table look -up to find the local send queue to which the message

should be appended.


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Figure 5.22: The general organization of an MQ queuing network using routing tables

and aliases.

Following this approach of routing and aliasing leads to a programming interface

that, fundamentally, is relatively simple, called the Message Queue Interface

(MQI). The most important primitives of MQI are summarized as

Primitive Description

MQopen Open a (possibly remote) queue

MQclose Close a queue

MQput Put a message into an opened queue

MQget Get a message from a (local) queue

Figure 5.23: Primitive and its description

E. Managing Overlay Networks: A major issue with MQ is that overlay networks

need to be manually administrated. This administration not only involves

creating channels between queue managers, but also filling in the routing

tables. Obviously, this can grow into a nightmare. Unfortunately, management

support for MQ systems is advanced only in the sense that an administrator

can set virtually every possible attribute, and tweak any thinkable


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configuration. At the heart of overlay management is the channel control

function component, which logically sits between message channel agents.

This component allows an operator to monitor exactly what is going on at two

end points of a channel. In addition, it is used to create channels and routing

tables, but also to manage the queue managers that host the message channel

agents. In a way, this approach to overlay management strongly resembles the

management of cluster servers where a single administration server is used.

5.4 Stream Oriented Communication

1. There are also forms of communication in which timing plays a crucial role.

2. Considering, an audio stream built up as a sequence of 16-bit samples, each

representing the amplitude of the sound wave as is done through Pulse Code

Modulation (PCM).

3. Assuming that the audio stream represents CD quality, meaning that the original

sound wave has been sampled at a frequency of 44,100 Hz. To reproduce the

original sound, it is essential that the samples in the audio stream are played out

in the order they appear in the stream, but also at intervals of exactly 1/44,100

sec.

4. Playing out at a different rate will produce an incorrect version of the original

sound.

A. Support for Continuous Media: Support for the exchange of time-dependent

information is often formulated as support for continuous media. A medium

refers to the means by which information is conveyed. These means include

storage and transmission media, presentation media such as a monitor, and

so on. An important type of medium is the way that information is

represented. In other words, how is information encoded in a computer

system? Different representations are used for different types of information.

For example, text is generally encoded as ASCII or Unicode. Images can be

represented in different formats such as GIF or lPEG. Audio streams can be

encoded in a computer system by, for example, taking 16-bit samples using


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PCM. In continuous (representation) media, the temporal relationships

between different data items are fundamental to correctly interpreting what

the data actually means. We already gave an example of reproducing a sound

wave by playing out an audio stream. As another example, consider motion.

Motion can be represented by a series of images in which successive images

must be displayed at a uniform spacing T in time, typically 30-40 msec per

image. Correct reproduction requires not only showing the stills in the correct

order, but also at a constant frequency of liT images per second. In contrast to

continuous media, discrete (representation) media, is characterized by the

fact that temporal relationships between data items are not fundamental to

correctly interpreting the data. Typical examples of discrete media include

representations of text and still images, but also object code or executable

files.

B. Data Stream: To capture the exchange of time-dependent information,

distributed systems generally provide support for data streams. A data stream

is nothing but a sequence of data units. Data streams can be applied to

discrete as well as continuous media. For example, UNIX pipes or TCPIIP

connections are typical examples of (byte-oriented) discrete data streams.

Playing an audio file typically requires setting up a continuous data stream

between the file and the audio device. To capture the exchange of time-

dependent information, distributed systems generally provide support for

data streams. A data stream is nothing but a sequence of data units. Data

streams can be applied to discrete as well as continuous media. For example,

UNIX pipes or TCPIIP connections are typical examples of (byte-oriented)

discrete data streams. Playing an audio file typically requires setting up a

continuous data stream between the file and the audio device. Streams can be

simple or complex. A simple stream consists of only a single sequence of data,

whereas a complex stream consists of several related simple streams, called

sub streams. The relation between the sub streams in a complex stream is


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often also time dependent. For example, stereo audio can be transmitted by

means of a complex stream consisting of two sub streams, each used for a

single audio channel. In synchronous transmission mode, there is a maximum

end-to-end delay defined for each unit in a data stream. Whether a data unit

is transferred much faster than the maximum tolerated delay is not important.

For example, a sensor may sample temperature at a certain rate and pass it

through a network to an operator. In that case, it may be important that the

end-to-end propagation time through the network is guaranteed to be lower

than the time interval between taking samples, but it cannot do any harm if

samples are propagated much faster than necessary. Finally, in isochronous

transmission mode, it is necessary that data units are transferred on time. This

means that data transfer is subject to a maximum and minimum end-to-end

delay, also referred to as bounded (delay) jitter. Isochronous transmission

mode is particularly interesting for distributed multimedia systems, as it plays

a crucial role in representing audio and video. In this chapter, we consider only

continuous data streams using isochronous transmission, which we will refer

to simply as streams. Streams can be simple or complex. A simple stream

consists of only a single sequence of data, whereas a complex stream consists

of several related simple streams, called sub streams. The relation between

the sub streams in a complex stream is often also time dependent. For

example, stereo audio can be transmitted by means of a complex stream

consisting of two sub streams, each used for a single audio channel. It is

important, however, that those two sub streams are continuously

synchronized. In other words, data units from each stream are to be

communicated pairwise to ensure the effect of stereo. Another example of a

complex stream is one for transmitting a movie. Such a stream could consist

of a single video stream, along with two streams for transmitting the sound of

the movie in stereo. A fourth stream might contain subtitles for the deaf, or a

translation into a different language than the audio. Again, synchronization of


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the sub streams is important. If synchronization fails, reproduction of the

movie fails.

Figure 5.24: Setting up a stream between two processes across a network

C. Streams and Quality of Service: Timing (and other nonfunctional)

requirements are generally expressed as Quality of Service (QoS)

requirements. These requirements describe what is needed from the

underlying distributed system and network to ensure that, for example, the

temporal relationships in a stream can be preserved. QoS for continuous data

streams mainly concerns timeliness, volume, and reliability. In this section we

take a closer look at QoS and its relation to setting up a stream. Much has been

said about how to specify required QoS. From an application's perspective, in

many cases it boils down to specifying a few important properties :

1. The required bit rate at which data should be transported.

2. The maximum delay until a session has been set up (i.e., when an

application can start sending data).

3. The maximum end-to-end delay (i.e., how long it will take until a data unit

makes it to a recipient).

4. The maximum delay variance, or jitter.

5. The maximum round-trip delay.


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D. Enforcing QoS: Given that the underlying system offers only a best-effort

delivery service, a distributed system can try to conceal as much as possible of

the lack of quality of service. Fortunately, there are several mechanisms that

it can deploy. First, the situation is not really as bad as sketched so far. For

example, the Internet provides a means for differentiating classes of data by

means of its differentiated services. A sending host can essentially mark

outgoing packets as belonging to one of several classes, including an expedited

forwarding class that essentially specifies that a packet should be forwarded

by the current router with absolute priority (Davie et al., 2002). In addition,

there is also an assured forwarding class, by which traffic is divided into four

subclasses, along with three ways to drop packets if the network gets

congested. Assured forwarding therefore effectively defines a range of

priorities that can be assigned to packets, and as such allows applications to

differentiate time-sensitive packets from noncritical ones. Besides these

network-level solutions, a distributed system can also help in getting data

across to receivers. Although there are generally not many tools available, one

that is particularly useful is to use buffers to reduce jitter. The principle is

simple, as shown in Figure 5.25. Assuming that packets are delayed with a

certain variance when transmitted over the network, the receiver simply

stores them in a buffer for a maximum amount of time. This will allow the

receiver to pass packets to the application at a regular rate, knowing that there

will always be enough packets entering the buffer to be played back at that

rate.


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Figure 5.25: Using a buffer to reduce jitter.

One problem that may occur is that a single packet contains multiple audio and

video frames. As a consequence, when a packet is lost, the receiver may actually

perceive a large gap when playing out frames. This effect can be somewhat

circumvented by interleaving frames, as shown in Figure 5.26. In this way, when a

packet is lost, the resulting gap in successive frames is distributed over time. Note,

however, that this approach does require a larger receive buffer in comparison to

non-interleaving, and thus imposes a higher start delay for the receiving

application. For example, when considering Figure 5.26(b), to play the first four

frames, the receiver will need to have four packets delivered, instead of only one

packet in comparison to non-interleaved transmission.

Figure 5.26: The effect of packet loss in (a) non-interleaved transmission and

(b) Interleaved transmission.


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5.5 Stream Synchronization

1. An important issue in multimedia systems is that different streams, possibly in the

form of a complex stream, are mutually synchronized.

2. Synchronization of streams deals with maintaining temporal relations between

streams.

3. Two types of synchronization occur. The simplest form of synchronization is that

between a discrete data stream and a continuous data stream. Consider, for

example, a slide show on the Web that has been enhanced with audio. Each slide

is transferred from the server to the client in the form of a discrete data stream.

4. At the same time, the client should play out a specific (part of an) audio stream

that matches the current slide that is also fetched from the server.

5. In this case, the audio stream is to be 'synchronized with the presentation of slides.

A more demanding type of synchronization is that between continuous data

streams.

6. A daily example is playing a movie in which the video stream needs to be

synchronized with the audio, commonly referred to as lip synchronization.

7. Another example of synchronization is playing a stereo audio stream consisting of

two sub streams, one for each channel.

8. Proper play out requires that the two sub streams are tightly synchronized: a

difference of more than 20 usee can distort the stereo effect.

5.6 Multicast communication

1. An important topic in communication in distributed systems is the support for

sending data to multiple receivers, also known as multicast communication.

2. For many years, this topic has belonged to the domain of network protocols,

where numerous proposals for network-level and transport-level solutions have

been implemented and evaluated.

3. A major issue in all solutions was setting up the communication paths for

information dissemination.


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4. In practice, this involved a huge management effort, in many cases requiring

human intervention.

5. In addition, as long as there is no convergence of proposals, ISPs have shown to

be reluctant to support multicasting. With the advent of peer-to-peer technology,

and notably structured overlay management, it became easier to set up

communication paths.

6. As peer-to-peer solutions are typically deployed at the application layer, various

application-level multicasting techniques have been introduced. In this section,

we will take a brief look at these techniques.

7. Multicast communication can also be accomplished in other ways than setting up

explicit communication paths.

8. As we also explore in this section. Gossip-based information dissemination

provides simple (yet often less efficient) ways for multicasting.


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6. Resource and Process Management

CONTENTS

6.1 Desirable Features of global Scheduling algorithm

6.2 Task assignment approach

6.3 Load balancing approach

6.4 Load sharing approach

6.5 Introduction to process management

6.6 Process migration

6.7 Threads

6.8 Virtualization

6.9 Clients, Servers, Code Migration

Parallel & Distributed Systems 6. Resource and Process Management

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A resource can be a logical, such as a shared file, or physical, such as a CPU (a node of the

distributed system). One of the functions of a distributed operating system is to assign

processes to the nodes (resources) of the distributed system such that the resource

usage, response time, network congestion, and scheduling overhead are optimized.

There are three techniques for scheduling processes of a distributed system:

1. Task Assignment Approach, in which each process submitted by a user for

processing is viewed as a collection of related tasks and these tasks are scheduled

to suitable nodes so as to improve performance.

2. Load-balancing approach, in which all the processes submitted by the users are

distributed among the nodes of the system so as to equalize the workload among

the nodes.

3. Load-sharing approach, which simply attempts to conserve the ability of the

system to perform work by assuring that no node is idle while processes wait for

being processed.

The task assignment approach has limited applicability to practical situations

because it works on the assumption that the characteristics (e.g. execution time, IPC costs

etc) of all the processes to be scheduled are known in advance.

6.1 Desirable features of a good global scheduling algorithm

i. No a priori knowledge about the processes: Scheduling algorithms that operate

based on the information about the characteristics and resource requirements of

the processes pose an extra burden on the users who must provide this

information while submitting their processes for execution.

ii. Dynamic in nature: Process assignment decisions should be dynamic, I.e., be

based on the current load of the system and not on some static policy. It is

recommended that the scheduling algorithm possess the flexibility to migrate a

process more than once because the initial decision of placing a process on a

particular node may have to be changed after some time to adapt to the new

system load.


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iii. Quick decision making capability: Heuristic methods requiring less computational

efforts (and hence less time) while providing near-optimal results are preferable

to exhaustive (optimal) solution methods.

iv. Balanced system performance and scheduling overhead: Algorithms that provide

near-optimal system performance with a minimum of global state information

(such as CPU load) gathering overhead are desirable. This is because the overhead

increases as the amount of global state information collected increases. This is

because the usefulness of that information is decreased due to both the aging of

the information being gathered and the low scheduling frequency as a result of

the cost of gathering and processing the extra information.

v. Stability: Fruitless migration of processes, known as processor thrashing, must be

prevented. E.g. if nodes n1 and n2 observe that node n3 is idle and then offload a

portion of their work to n3 without being aware of the offloading decision made

by the other node. Now if n3 becomes overloaded due to this it may again start

transferring its processes to other nodes. This is caused by scheduling decisions

being made at each node independently of decisions made by other nodes.

vi. Scalability: A scheduling algorithm should scale well as the number of nodes

increases. An algorithm that makes scheduling decisions by first inquiring the

workload from all the nodes and then selecting the most lightly loaded node has

poor scalability. This will work fine only when there are few nodes in the system.

This is because the inquirer receives a flood of replies almost simultaneously, and

the time required to process the reply messages for making a node selection is too

long as the number of nodes (N) increase. Also the network traffic quickly

consumes network bandwidth. A simple approach is to probe only m of N nodes

for selecting a node.

vii. Fault tolerance: A good scheduling algorithm should not be disabled by the crash

of one or more nodes of the system. Also, if the nodes are partitioned into two or

more groups due to link failures, the algorithm should be capable of functioning

properly for the nodes within a group. Algorithms that have decentralized decision


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making capability and consider only available nodes in their decision making have

better fault tolerance capability.

viii. Fairness of service: Global scheduling policies that blindly attempt to balance the

load on all the nodes of the system are not good from the point of view of fairness

of service. This is because in any load-balancing scheme, heavily loaded nodes will

obtain all the benefits while lightly loaded nodes will suffer poorer response time

than in a stand-alone configuration. A fair strategy that improves response time

of the former without unduly affecting the latter is desirable. Hence load-

balancing has to be replaced by the concept of load sharing, that is, a node will

share some of its resources as long as its users are not significantly affected.

6.2 Task Assignment Approach

1. A process has already been split up into pieces called tasks. This split occurs along

natural boundaries (such as a method), so that each task will have integrity in itself

and data transfers among the tasks are minimized.

2. The amount of computation required by each task and the speed of each CPU are

known.

3. The cost of processing each task on every node is known. This is derived from

assumption 2.

4. The IPC costs between every pair of tasks is known. The IPC cost is 0 for tasks

assigned to the same node. This is usually estimated by an analysis of the static

program. If two tasks communicate n times and the average time for each inter-

task communication is t, the IPC costs for the two tasks is n * t.

5. Precedence relationships among the tasks are known.

6. Reassignment of tasks is not possible.

Goal is to assign the tasks of a process to the nodes of a distributed system in such a

manner as to achieve goals such as the following goals:

Minimization of IPC costs

Quick turnaround time for the complete process

A high degree of parallelism


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Efficient utilization of system resources in general

i. These goals often conflict. E.g., while minimizing IPC costs tends to assign all tasks

of a process to a single node, efficient utilization of system resources tries to

distribute the tasks evenly among the nodes. So also, quick turnaround time and

a high degree of parallelism encourage parallel execution of the tasks, the

precedence relationship among the tasks limits their parallel execution.

ii. Also note that in case of m tasks and q nodes, there are mq possible assignments

of tasks to nodes. In practice, however, the actual number of possible assignments

of tasks to nodes may be less than mq due to the restriction that certain tasks

cannot be assigned to certain nodes due to their specific requirements (e.g. need

a certain amount of memory or a certain data file).

iii. There are two nodes, {n1, n2} and six tasks {t1, t2, t3, t4, t5, t6}. There are two

task assignment parameters – the task execution cost (xab the cost of executing

task a on node b) and the inter-task communication cost (cij the inter-task

communication cost between tasks i and j).

Inter-task communication cost Execution costs

t1 t2 t3 t4 t5 t6 Nodes

t1 0 6 4 0 0 12 n1 n2

t2 6 0 8 12 3 0 t1 5 10

t3 4 8 0 0 11 0 t2 2

t4 0 12 0 0 5 0 t3 4 4

t5 0 3 11 5 0 0 t4 6 3

t6 12 0 0 0 0 0 t5 5 2

t6 4


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Task t6 cannot be executed on node n1 and task t2 cannot be executed on node n2 since

the resources they need are not available on these nodes.

Task assignment example

1) Serial assignment, where tasks t1, t2, t3 are assigned to node n1 and tasks t4, t5, t6 are

assigned to node n2:

Execution cost, x = x11 + x21 + x31 + x42 + x52 + x62 = 5 + 2 + 4 + 3 + 2 + 4 = 20

Communication cost, c = c14 + c15 + c16 + c24 + c25 + c26 + c34 + c35 + c36 = 0 + 0 +

12 + 12 + 3 + 0 + 0 + 11 + 0 = 38. Hence total cost = 58.

2) Optimal assignment, where tasks t1, t2, t3, t4, t5 are assigned to node n1 and task t6 is

assigned to node n2.

Execution cost, x = x11 + x21 + x31 + x41 + x51 + x62 = 5 + 2 + 4 + 6 + 5 + 4 = 26

Communication cost, c = c16 + c26 + c36 + c46 + c56= 12 + 0 + 0 + 0 + 0 = 12

Total cost = 38

Optimal assignments are found by first creating a static assignment graph. In this graph,

the weights of the edges joining pairs of task nodes represent inter-task communication

costs. The weight on the edge joining a task node to node n1 represents the execution

cost of that task on node n2 and vice-versa. Then we determine a minimum cutset in this

graph.

A cutset is defined to be a set of edges such that when these edges are removed, the

nodes of the graph are partitioned into two disjoint subsets such that nodes in one subset

are reachable from n1 and the nodes in the other are reachable from n2. Each task node

is reachable from either n1 or n2. The weight of a cutset is the sum of the weights of the

edges in the cutset. This sums up the execution and communication costs for that

assignment. An optimal assignment is found by finding a minimum cutset.

Basic idea: Finding an optimal assignment to achieve goals such as the following:

o Minimization of IPC costs

o Quick turnaround time of process

o High degree of parallelism


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o Efficient utilization of resources

6.3 Load balancing approach

A Taxonomy of Load-Balancing Algorithms

1. Load-balancing approach

2. Load-balancing algorithms

3. Dynamic

4. Static

5. Deterministic

6. Probabilistic

7. Centralized

8. Distributed

9. Cooperative

10. Non-cooperative

1. Load-balancing approach- Type of load-balancing algorithms

i. Static versus Dynamic

Static algorithms use only information about the average behavior of the system

Static algorithms ignore the current state or load of the nodes in the system

Dynamic algorithms collect state information and react to system state if it

changed

Static algorithms are much more simpler

Dynamic algorithms are able to give significantly better performance

2. Load-balancing approach Type of static load-balancing algorithms

i. Deterministic versus Probabilistic

Deterministic algorithms use the information about the properties of the nodes

and the characteristic of processes to be scheduled

Probabilistic algorithms use information of static attributes of the system (e.g.

number of nodes, processing capability, topology) to formulate simple process

placement rules

Deterministic approach is difficult to optimize


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Probabilistic approach has poor performance

3. Load-balancing approach Type of dynamic load-balancing algorithms

i.Centralized versus Distributed

Centralized approach collects information to server node and makes assignment

decision

Distributed approach contains entities to make decisions on a predefined set of

nodes

Centralized algorithms can make efficient decisions, have lower fault-tolerance

Distributed algorithms avoid the bottleneck of collecting state information and

react faster

4. Load-balancing approach Type of distributed load-balancing algorithms

i. Cooperative versus Noncooperative

In Noncooperative algorithms entities act as autonomous ones and make

scheduling decisions independently from other entities

In Cooperative algorithms distributed entities cooperate with each other

Cooperative algorithms are more complex and involve larger overhead

Stability of Cooperative algorithms are better

5. Issues in designing Load-balancing algorithms

Load estimation policy

determines how to estimate the workload of a node

Process transfer policy

determines whether to execute a process locally or remote

State information exchange policy

determines how to exchange load information among nodes

Location policy

determines to which node the transferable process should be sent

Priority assignment policy

determines the priority of execution of local and remote processes

Migration limiting policy


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determines the total number of times a process can migrate

6. Load estimation policy I.for Load-balancing algorithms

To balance the workload on all the nodes of the system, it is necessary to decide

how to measure the workload of a particular node

Some measurable parameters (with time and node dependent factor) can be the

following:

Total number of processes on the node

Resource demands of these processes

Instruction mixes of these processes

Architecture and speed of the node’s processor

Several load-balancing algorithms use the total number of processes to achieve

big efficiency

7. Load estimation policy II.for Load-balancing algorithms

In some cases the true load could vary widely depending on the remaining service

time, which can be measured in several way:

Memoryless method assumes that all processes have the same expected

remaining service time, independent of the time used so far

Past repeats assumes that the remaining service time is equal to the time used

so far

Distribution method states that if the distribution service times is known, the

associated process’s remaining service time is the expected remaining time

conditioned by the time already used

8. Load estimation policy III.for Load-balancing algorithms

None of the previous methods can be used in modern systems because of

periodically running processes and daemons

An acceptable method for use as the load estimation policy in these systems

would be to measure the CPU utilization of the nodes

Central Processing Unit utilization is defined as the number of CPU cycles actually

executed per unit of real time


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It can be measured by setting up a timer to periodically check the CPU state

(idle/busy)

9. Process transfer policy I.for Load-balancing algorithms

Most of the algorithms use the threshold policy to decide on whether the node is

lightly-loaded or heavily-loaded

Threshold value is a limiting value of the workload of node which can be

determined by

Static policy: predefined threshold value for each node depending on

processing capability

Dynamic policy: threshold value is calculated from average workload and a

predefined constant

Below threshold value node accepts processes to execute, above threshold value

node tries to transfer processes to a lightly-loaded node

Single-threshold policy may lead to unstable algorithm because under loaded

node could turn to be overloaded right after a process migration

To reduce instability double-threshold policy has been proposed which is also

known as high-low policy

10. Process transfer policy III. for Load-balancing algorithms

Double threshold policy

When node is in overloaded region new local processes are sent to run

remotely, requests to accept remote processes are rejected

When node is in normal region new local processes run locally, requests to

accept remote processes are rejected

When node is in under loaded region new local processes run locally, requests

to accept remote processes are accepted


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11. Location policy I. for Load-balancing algorithms

Threshold method

Policy selects a random node, checks whether the node is able to receive the

process, and then transfers the process. If node rejects, another node is

selected randomly. This continues until probe limit is reached.

Shortest method

L distinct nodes are chosen at random, each is polled to determine its load.

The process is transferred to the node having the minimum value unless its

workload value prohibits to accept the process.

Simple improvement is to discontinue probing whenever a node with zero load

is encountered.

12. Location policy II. for Load-balancing algorithms

Bidding method

Nodes contain managers (to send processes) and contractors (to receive

processes)

Managers broadcast a request for bid, contractors respond with bids (prices

based on capacity of the contractor node) and manager selects the best offer

Winning contractor is notified and asked whether it accepts the process for

execution or not

Full autonomy for the nodes regarding scheduling

Big communication overhead

Difficult to decide a good pricing policy

13. Location policy III. for Load-balancing algorithms

Pairing

Contrary to the former methods the pairing policy is to reduce the variance of

load only between pairs

Each node asks some randomly chosen node to form a pair with it

If it receives a rejection it randomly selects another node and tries to pair again


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Two nodes that differ greatly in load are temporarily paired with each other

and migration starts

The pair is broken as soon as the migration is over

A node only tries to find a partner if it has at least two processes

14. State information exchange policy I. for Load-balancing algorithms

Dynamic policies require frequent exchange of state information, but these extra

messages arise two opposite impacts:

Increasing the number of messages gives more accurate scheduling decision

Increasing the number of messages raises the queuing time of messages

State information policies can be the following:

Periodic broadcast

Broadcast when state changes

On-demand exchange

Exchange by polling

15. State information exchange policy II. for Load-balancing algorithms

Periodic broadcast

Each node broadcasts its state information after the elapse of every T units of

time

Problem: heavy traffic, fruitless messages, poor scalability since information

exchange is too large for networks having many nodes


Avoids fruitless messages by broadcasting the state only when a process

arrives or departures

Further improvement is to broadcast only when state switches to another

region (double-threshold policy)

16. State information exchange policy III. for Load-balancing algorithms

On-demand exchange

In this method a node broadcast a State-Information-Request message when

its state switches from normal to either underloaded or overloaded region.


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On receiving this message other nodes reply with their own state information

to the requesting node

Further improvement can be that only those nodes reply which are useful to

the requesting node

Exchange by polling

To avoid poor scalability (coming from broadcast messages) the partner node

is searched by polling the other nodes on by one, until poll limit is reached

17. Priority assignment policy for Load-balancing algorithms

Selfish

Local processes are given higher priority than remote processes. Worst

response time performance of the three policies.

Altruistic

Remote processes are given higher priority than local processes. Best response

time performance of the three policies.

Intermediate

When the number of local processes is greater or equal to the number of

remote processes, local processes are given higher priority than remote

processes. Otherwise, remote processes are given higher priority than local

processes.

18. Migration limiting policy for Load-balancing algorithms

This policy determines the total number of times a process can migrate

Uncontrolled

A remote process arriving at a node is treated just as a process originating

at a node, so a process may be migrated any number of times

Controlled

Avoids the instability of the uncontrolled policy

Use a migration count parameter to fix a limit on the number of time a

process can migrate

Irrevocable migration policy: migration count is fixed to 1


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For long execution processes migration count must be greater than 1 to

adapt for dynamically changing states

6.4 Load sharing approach

Drawbacks of Load-balancing approach

Load balancing technique with attempting equalizing the workload on all the

nodes is not an appropriate object since big overhead is generated by

gathering exact state information

Load balancing is not achievable since number of processes in a node is always

fluctuating and temporal unbalance among the nodes exists every moment

Basic ideas for Load-sharing approach

It is necessary and sufficient to prevent nodes from being idle while some

other nodes have more than two processes

Load-sharing is much simpler than load-balancing since it only attempts to

ensure that no node is idle when heavily node exists

Priority assignment policy and migration limiting policy are the same as that

for the load-balancing algorithms

Load estimation policies for Load-sharing algorithms

Since load-sharing algorithms simply attempt to avoid idle nodes, it is sufficient to

know whether a node is busy or idle

Thus these algorithms normally employ the simplest load estimation policy of

counting the total number of processes

In modern systems where permanent existence of several processes on an idle

node is possible, algorithms measure CPU utilization to estimate the load of a

node

Process transfer policies for Load-sharing algorithms

Algorithms normally use all-or-nothing strategy

This strategy uses the threshold value of all the nodes fixed to 1

Nodes become receiver node when it has no process, and become sender node

when it has more than 1 process


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To avoid processing power on nodes having zero process load-sharing algorithms

use a threshold value of 2 instead of 1

When CPU utilization is used as the load estimation policy, the double-threshold

policy should be used as the process transfer policy

Location policies I. for Load-sharing algorithms

Location policy decides whether the sender node or the receiver node of the

process takes the initiative to search for suitable node in the system, and this

policy can be the following:

Sender-initiated location policy

Sender node decides where to send the process

Heavily loaded nodes search for lightly loaded nodes

Receiver-initiated location policy

Receiver node decides from where to get the process

Lightly loaded nodes search for heavily loaded nodes

Location policies II. for Load-sharing algorithms

Sender-initiated location policy

Node becomes overloaded, it either broadcasts or randomly probes the other

nodes one by one to find a node that is able to receive remote processes

When broadcasting, suitable node is known as soon as reply arrives

Receiver-initiated location policy

Nodes becomes underloaded, it either broadcast or randomly probes the

other nodes one by one to indicate its willingness to receive remote processes

Receiver-initiated policy require preemptive process migration facility since

scheduling decisions are usually made at process departure epochs

Location policies III. For Load-sharing algorithms

Experiences with location policies

Both policies gives substantial performance advantages over the situation in

which no load-sharing is attempted

Sender-initiated policy is preferable at light to moderate system loads


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Receiver-initiated policy is preferable at high system loads

Sender-initiated policy provide better performance for the case when process

transfer cost significantly more at receiver-initiated than at sender-initiated

policy due to the preemptive transfer of processes

State information exchange policies for Load-sharing algorithms

In load-sharing algorithms it is not necessary for the nodes to periodically

exchange state information, but needs to know the state of other nodes when it

is either under loaded or overloaded


In sender-initiated/receiver-initiated location policy a node broadcasts State

Information Request when it becomes overloaded/under loaded

It is called broadcast-when-idle policy when receiver-initiated policy is used

with fixed threshold value of 1

Poll when state changes

In large networks polling mechanism is used

Polling mechanism randomly asks different nodes for state information until

find an appropriate one or probe limit is reached

It is called poll-when-idle policy when receiver-initiated policy is used with

fixed threshold value value of 1

Resource manager of a distributed system schedules the processes to optimize

combination of resources usage, response time, network congestion, scheduling

overhead

Three different approaches has been discussed

Task assignment approach deals with the assignment of task in order to

minimize inter process communication costs and improve turnaround time for

the complete process, by taking some constraints into account

In load-balancing approach the process assignment decisions attempt to

equalize the avarage workload on all the nodes of the system


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In load-sharing approach the process assignment decisions attempt to keep all

the nodes busy if there are sufficient processes in the system for all the nodes

6.5 Introduction to process management process migration.

1. The concept of virtualization has gained popularity.

2. Virtualization allows an application, and possibly also its complete environment

including the operating system, to run concurrently with other applications, but

highly independent of the underlying hardware and platforms, leading to a high

degree of portability.

3. Moreover, virtualization helps in isolating failures caused by errors or security

problems.

4. Process allocation deals with the process of deciding which process should be

assigned to which processor.

5. Process migration deals with the movement of a process from its current location

to the processor to which it has been assigned.

Figure 6.1: Process Migration

Threads deal with fine-grained parallelism for better utilization of the processing

capability of the system.

6. Although processes form a building block in distributed systems, practice indicates

that the granularity of processes as provided by the operating systems on which

distributed systems are built is not sufficient. Instead, it turns out that having a


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finer granularity in the form of multiple threads of control per process makes it

much easier to build distributed applications and to attain better performance.

6.6 Threads

1. To execute a program, an operating system creates a number of virtual

processors, each one for running a different program.

2. To keep track of these virtual processors, the operating system has a process table,

containing entries to store CPU register values, memory maps, open files,

accounting information. Privileges, etc.

3. A process is often defined as a program in execution, that is, a program that is

currently being executed on one of the operating system's virtual processors.

4. An important issue is that the operating system takes great care to ensure that

independent processes cannot maliciously or inadvertently affect the correctness

of each other's behavior.

5. The fact that multiple processes may be concurrently sharing the same CPU and

other hardware resources is made transparent.

6. The operating system requires hardware support to enforce this separation.

7. Like a process, a thread executes its own piece of code, independently from other

threads.

8. In contrast to processes, no attempt is made to achieve a high degree of

concurrency transparency if this would result in performance degradation.

9. Therefore, a thread system generally maintains only the minimum information to

allow a CPU to be shared by several threads. In particular, a thread context often

consists of nothing more than the CPU context, along with some other information

for thread management.

10. For example, a thread system may keep track of the fact that a thread is currently

blocked on a mutex variable, so as not to select it for execution.

11. Information that is not strictly necessary to manage multiple threads is generally

ignored. For this reason, protecting data against inappropriate access by threads

within a single process is left entirely to application developers.


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Figure 6.2: Context switching as the result of IPC.

6.7 Thread Implementation

1. Threads are often provided in the form of a thread package. Such a package

contains operations to create and destroy threads as well as operations on

synchronization variables such as mutexes and condition variables.

2. There are basically two approaches to implement a thread package. The first

approach is to construct a thread library that is executed entirely in user mode.

3. The second approach is to have the kernel be aware of threads and schedule them.

4. A user-level thread library has a number of advantages. First, it is cheap to create

and destroy threads.

5. Because all thread administration is kept in the user's address space, the price of

creating a thread is primarily determined by the cost for allocating memory to set

up a thread stack.

6. Analogously, destroying a thread mainly involves freeing memory for the stack,

which is no longer used. Both operations are cheap. A second advantage of user-

level threads is that switching thread context can often be done in just a few

instructions.

7. Basically, only the values of the CPU registers need to be stored and subsequently

reloaded with the previously stored values of the thread to which it is being

switched.

8. There is no need to change memory maps, flush the TLB, do CPU accounting, and

so on. Switching thread context is done when two threads need to synchronize,

for example, when entering a section of shared data.


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9. A major drawback of user-level threads is that invocation of a blocking system call

will immediately block the entire process to which the thread belongs, and thus

also all the other threads in that process.

10. Threads are particularly useful to structure large applications into parts that could

be logically executed at the same time.

11. Blocking on I/O should not prevent other parts to be executed in the meantime.

For such applications, user level threads are of no help.

Figure 6.3: Combining kernel-level lightweight processes and user-level threads.

6.8 Virtualization

1. Threads and processes can be seen as a way to do more things at the same time.

In effect, they allow us build (pieces of) programs that appear to be executed

simultaneously.

2. On a single-processor computer, this simultaneous execution is, of course, an

illusion.

3. As there is only a single CPU, only an instruction from a single thread or process

will be executed at a time.

4. By rapidly switching between threads and processes, the illusion of parallelism is

created.

5. This separation between having a single CPU and being able to pretend there are

more can be extended to other resources as well, leading to what is known as

resource virtualization.


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6. This virtualization has been applied for many decades, but has received renewed

interest as (distributed) computer systems have become more commonplace and

complex, leading to the situation that application software is mostly always

outliving its underlying systems software and hardware.

7. First, while hardware and low-level systems software change reasonably fast,

software at higher levels of abstraction (e.g., middleware and applications), are

much more stable. In other words, we are facing the situation that legacy software

cannot be maintained in the same pace as the platforms it relies on.

8. Virtualization can help here by porting the legacy interfaces to the new platforms

and thus immediately opening up the latter for large classes of existing programs.

9. Equally important is the fact that networking has become completely pervasive. It

is hard to imagine that a modern computer is not connected to a network.

10. In practice, this connectivity requires that system administrators maintain a large

and heterogeneous collection of server computers, each one running very

different applications, which can be accessed by clients.

11. At the same time the various resources should be easily accessible to these

applications.

12. Virtualization can help a lot: the diversity of platforms and machines can be

reduced by essentially letting each application run on its own virtual machine,

possibly including the related libraries and operating system, which, in turn, run

on a common platform.

Figure 6.4: (a) General organization between a program, interface, and system.

(b) General organization of virtualizing system A on top of system B.


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6.9 Clients

1. A major task of client machines is to provide the means for users to interact with

remote servers.

2. There are roughly two ways in which this interaction can be supported.

3. First, for each remote service the client machine will have a separate counterpart

that can contact the service over the network.

4. A typical example is an agenda running on a user's PDA that needs to synchronize

with a remote, possibly.

Figure 6.5: (a) A networked application with its own protocol. (b) A general solution to allow

access to remote applications.

5. A second solution is to provide direct access to remote services by only offering a

convenient user interface. Effectively, this means that the client machine is used

only as a terminal with no need for local storage, leading to an application neutral

solution as shown in figure above

6. In the case of networked user interfaces, everything is processed and stored at

the server.

7. This thin-client approach is receiving more attention as Internet connectivity

increases, and hand-held devices are becoming more sophisticated.

8. A second solution is to provide direct access to remote services by only offering a

convenient user interface.


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9. Effectively, this means that the client machine is used only as a terminal with no

need for local storage, leading to an application neutral solution as shown in

Figure 6.5(b).

10. In the case of networked user interfaces, everything is processed and stored at

the server.

11. This thin-client approach is receiving more attention as Internet connectivity

increases, and hand-held devices are becoming more sophisticated. As we argued

in the previous chapter, thin-client solutions are also popular as they ease the task

of system management.

Figure 6.6: The basic organization of the X Window System.

6.10 Servers

1. A server is a process implementing a specific service on behalf of a collection for

clients.

2. In essence, each server is organized in the same way: it waits for an incoming

request from a client and subsequently ensures that the request is taken care of,

after which it waits for the next incoming request.

3. There are several ways to organize servers. In the case of an iterative server the

server itself handles the request and, if necessary, returns a response to the

requesting client.


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4. A concurrent server does not handle the request itself, but passes it to a separate

thread or another process, after which it immediately waits for the next incoming

request.

5. A multithreaded server is an example of a concurrent server. An alternative

implementation of a concurrent server is to fork a new process for each new

incoming request.

6. This approach is followed in many UNIX systems.

7. The thread or process that handles the request is responsible for returning a

response to the requesting client.

Figure 6.7: (a) Client-to-server binding using a daemon. (b) Client-to-server binding using a

super server.

6.11 Server Clusters

1. A server cluster is nothing else but a collection of machines connected through a

network, where each machine runs one or more servers.

2. The server clusters that we consider here, are the ones in which the machines are

connected through a local-area network, often offering high bandwidth and low

latency.

3. In most cases, a server cluster is logically organized into three tiers, as shown in Figure

6.8.


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4. The first tier consists of a (logical) switch through which client requests are routed.

Such a switch can vary widely.

5. For example, transport-layer switches accept incoming TCP connection requests and

pass requests on to one of servers in the cluster.

6. A completely different example is a Web server that accepts incoming HTTP requests,

but that partly passes requests to application servers for further processing only to

later collect results and return an HTTP response.

7. As in any multitier client-server architecture, many server clusters also contain servers

dedicated to application processing.

8. In cluster computing, these are typically servers running on high-performance

hardware dedicated to delivering compute power.

9. Enterprise server clusters, it may be the case that applications need only run on

relatively low-end machines, as the required compute power is not the bottleneck,

but access to storage is.

10. This brings us the third tier, which consists of data-processing servers, notably file and

database servers.

11. Again, depending on the usage of the server cluster, these servers may be running an

specialized machines, configured for high-speed disk access and having large server-

side data caches.

Figure 6.8: The general organization of a three-tiered server cluster.


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6.12 Code Migration

1. There are situations in which passing programs, sometimes even while they are being

executed, simplifies the design of a distributed system.

2. To start with by considering different approaches to code migration, followed by a

discussion on how to deal with the local resources that a migrating program uses.

3. Traditionally, code migration in distributed systems took place in the form of process

migration in which an entire process was moved from one machine to another.

4. Moving a running process to a different machine is a costly and intricate task, and

there had better be a good reason for doing so.

5. That reason has always been performance.

6. The basic idea is that overall system performance can be improved if processes are

moved from heavily-loaded to lightly-loaded machines.

7. Load is often expressed in terms of the CPU queue length or CPU utilization, but other

performance indicators are used as well.

8. Support for code migration can also help improve performance by exploiting

parallelism, but without the usual intricacies related to parallel programming.

9. A typical example is searching for information in the Web.

10. It is relatively simple to implement a search query in the form of a small mobile

program, called a mobile agent that moves from site to site.

11. By making several copies of such a program, and sending each off to different sites,

we may be able to achieve a linear speedup compared to using just a single program

instance.


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Figure 6.9: The principle of dynamically configuring a client to communicate to a server. The

client first fetches the necessary software, and then invokes the server

Models for Code Migration

i. Although code migration suggests that we move only code between machines, the term

actually covers a much richer area.

ii. Traditionally, communication in distributed systems is concerned with exchanging data

between processes.

iii. Code migration in the broadest sense deals with moving programs between machines,

with the intention to have those programs be executed at the target.

iv. In some cases, as in process migration, the execution status of a program, pending signals,

and other parts of the environment must be moved as well.

v. The code segment is the part that contains the set of instructions that make up the

program that is being executed.

vi. The resource segment contains references to external resources needed. by the process,

such as files, printers, devices, other processes, and so on.

vii. Finally, an execution segment is used to store the current execution state of a process,

consisting of private data, the stack, and, of course, the program counter.


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Figure 6.10: Alternatives for code migration.

Migration and Local Resources

i. The migration of the code and execution segment has been taken into account. The

resource segment requires some special attention.

ii. What often makes code migration so difficult is that the resource segment cannot always

be simply transferred along with the other segments without being changed.

iii. For example, suppose a process holds a reference to a specific TCP port through which it

was communicating with other (remote) processes. Such a reference is held in its resource

segment.

iv. When the process moves to another location, it will have to give up the port and request

a new one at the destination.

v. In other cases, transferring a reference need not be a problem.

vi. For example, a reference to a file by means of an absolute URL will remain valid

irrespective of the machine where the process that holds the URL resides.

vii. A weaker form of process-to-resource binding is when only the value of a resource is

needed.

viii. In that case, the execution of the process would not be affected if another resource

would provide that same value.


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ix. A typical example of binding by value is when a program relies on standard libraries, such

as those for programming in C or Java.

x. Such libraries should always be locally available, but their exact location in the local file

system may differ between sites.

xi. Not the specific files, but their content is important for the proper execution of the

process.

Migration in Heterogeneous Systems

i. Migration in such systems requires that each platform is supported, that is, that the code

segment can be executed on each platform. Also, we need to ensure that the execution

segment can be properly represented at each platform.

ii. The problems coming from heterogeneity are in many respects the same as those of

portability.

iii. Not surprisingly, solutions are also very similar. For example, at the end of the 1970s, a

simple solution to alleviate many of the problems of porting Pascal to different machines

was to generate machine-independent intermediate code for an abstract virtual machine

(Barron, 1981).

iv. That machine, of course, would need to be implemented on many platforms, but it would

then allow Pascal programs to be run anywhere.

v. Although this simple idea was widely used for some years, it never really caught on as the

general solution to portability problems for other languages, notably C.

Three ways to handle migration

1. Pushing memory pages to the new machine and resending the ones that are later

modified during the migration process.

2. Stopping the current virtual machine; migrate memory, and start the new virtual

machine.

3. Letting the new virtual machine pull in new pages as needed, that is,let processes start

on the new virtual machine immediately and copy memory pages on demand.


Anuradha Bhatia

7. Synchronization CONTENTS

7.1 Clock Synchronization, Logical Clocks, Election Algorithms, Mutual

Exclusion, Distributed Mutual Exclusion-Classification of mutual

Exclusion Algorithm, Requirements of Mutual Exclusion Algorithms,

Performance measure, Non Token based Algorithms: Lamport Algorithm,

Ricart–Agrawala’s Algorithm, Maekawa’s Algorithm.

7.2 Token Based Algorithms: Suzuki-Kasami’s Broardcast Algorithms,

Singhal’s Heurastic Algorithm, Raymond’s Tree based Algorithm,

Comparative Performance Analysis.

Parallel & Distributed Systems 7. Synchronization

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1. Communication is important, as closely related process cooperate and

synchronize with one another.

2. Cooperation is partly supported by means of naming, which allows processes to

at least share resources, or entities in general.

3. For example, it is important that multiple processes do not simultaneously access

a shared resource, such as printer, but instead cooperate in granting each other

temporary exclusive access.

4. Multiple processes may sometimes need to agree on the ordering of events, such

as whether message ml from process P was sent before or after message m2 from

process Q.

5. As it turns out, synchronization in distributed systems is often much more difficult

compared to synchronization in uniprocessor or multiprocessor systems.

7.1 Clock Synchronization:

In a centralized system, time is unambiguous. When a process wants to know the time, it

makes a system call and the kernel tells it. If process A asks for the time and then a little

later process B asks for the time, the value that B gets will be higher than (or possibly

equal to) the value A got. It will certainly not be lower. In a distributed system, achieving

agreement on time is not trivial.

Figure 7.1: When each machine has its own clock, an event that occurred after another event

may nevertheless be assigned an earlier time.


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i. Physical Clocks: all computers have a circuit for keeping track of time. Despite the

widespread use of the word "clock" to refer to these devices, they are not actually

clocks in the usual sense. Timer is perhaps a better word. A computer timer is

usually a precisely machined quartz crystal. When kept under tension, quartz

crystals oscillate at a well-defined frequency that depends on the kind of crystal,

how it is cut, and the amount of tension. Associated with each crystal are two

registers, a counter and a holding register. Each oscillation of the crystal

decrements the counter by one. When the counter gets to zero, an interrupt is

generated and the counter is reloaded from the holding register. In this way, it is

possible to program a timer to generate an interrupt 60 times a second, or at any

other desired frequency. Each interrupt is called one clock tick.

ii. Global Positioning System: As a step toward actual clock synchronization

problems, we first consider a related problem, namely determining one's

geographical position anywhere on Earth. This positioning problem is by itself

solved through a highly specific. Dedicated distributed system, namely GPS, which

is an acronym for global positioning system. GPS is a satellite-based distributed

system that was launched in 1978. Although it has been used mainly for military

applications, in recent years it has found its way to many civilian applications,

notably for traffic navigation. This principle of intersecting circles can be expanded

to three dimensions, meaning that we need three satellites to determine the

longitude, latitude, and altitude of a receiver on Earth. This positioning is all fairly

straightforward, but matters become complicated when we can no longer assume

that all clocks are perfectly synchronized.


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Figure 7.2: Computing a position in a two-dimensional space

iii. The Berkeley Algorithm: the time server (actually, a time daemon) is active,

polling every machine from time to time to ask what time it is there. Based on the

answers, it computes an average time and tells all the other machines to advance

their clocks to the new time or slow their clocks down until some specified

reduction has been achieved. This method is suitable for a system in which no

machine has a WWV receiver. The time daemon's time must be set manually by

the operator periodically.

Figure 7.3: (a) The time daemon asks all the other machines for their clock values. (b) The

machines answer. (c) The time daemon tells everyone how to adjust their clock.

7.2 Logical Clocks: Clock synchronization is naturally related to real time. However, we

have also seen that it may be sufficient that every node agrees on a current time, without

that time necessarily being the same as the real time. We can go one step further. For

running make, for example, it is adequate that two nodes agree that input.o is outdated


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by a new version of input.c. In this case, keeping track of each other's events (such as a

producing a new version of input.c) is what matters. For these algorithms, it is

conventional to speak of the clocks as logical clocks.

i. To synchronize logical clocks, Lamport defined a relation called happens-before.

The expression a ~ b is read "a happens before b" and means that all processes

agree that first event a occurs, then afterward, event b occurs. The happens-

before relation can be observed directly in two situations:

If a and b are events in the same process, and a occurs before b, then a ~

b is true.

If a is the event of a message being sent by one process, and b is the event

of the message being received by another process, then a ~ b is also true.

A message cannot be received before it is sent, or even at the same time

it is sent, since it takes a finite, nonzero amount of time to arrive.

ii. Happens-before is a transitive relation, so if a ~ band b ~ c, then a ~ c. If two events,

x and y, happen in different processes that do not exchange messages (not even

indirectly via third parties), then x ~ y is not true, but neither is y ~ x. These events

are said to be concurrent, which simply means that nothing can be said (or need

be said) about when the events happened or which event happened first.

Figure 7.4: (a) Three processes, each with its own clock. The clocks run at different rates. (b)

Lamport's algorithm corrects the clocks.


Anuradha Bhatia 158

7.3 Election Algorithms: Election algorithms attempt to locate the process with

the highest process number and designate it as coordinator. The algorithms differ

in the way they do the location. It is assumed that every process knows the process

number of every other process. What the processes do not know is which ones

are currently up and which ones are currently down. The goal of an election

algorithm is to ensure that when an election starts, it concludes with all processes

agreeing on who the new coordinator is to be.

i. Traditional Election Algorithm: We start with taking a look at two traditional

election algorithms to give an impression what whole groups of researchers have

been doing in the past decades. In subsequent sections, we pay attention to new

applications of the election problem.

The Bully Algorithm: As a first example, consider the bully algorithm

devised by Garcia-Molina (1982). When any process notices that the

coordinator is no longer responding to requests, it initiates an election. A

process, P, holds an election as follows:

1. P sends an ELECTION message to all processes with higher numbers.

2. If no one responds, P wins the election and becomes coordinator.

3. If one of the higher-ups answers, it takes over. P's job is done.


Anuradha Bhatia 159

Figure 7.5: Bully algorithm

The Ring Algorithm: Another election algorithm is based on the use of a

ring. Unlike some ring algorithms, this one does not use a token. We

assume that the processes are physically or logically ordered, so that each

process knows who its successor is. When any process notices that the

coordinator is not functioning, it builds an ELECTION message containing

its own process number and sends the message to' its successor. If the

successor is down, the sender skips over the successor and goes to the next

member along the ring. or the one after that, until a running process is

located. At each step along the way, the sender adds its own process

number to the list in the message effectively making itself a candidate to

be elected as coordinator.


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7.4 Mutual Exclusion

1. The problem of mutual exclusion frequently arises in distributed systems

whenever concurrent access to shared resources by several sites is involved.

2. For correctness, it is necessary that the shared resource be accessed by a single

site (or process) at a time.

3. A typical example is directory management, where an update to a directory must

be done atomically because if updates and reads to a directory proceed

concurrently, reads may obtain inconsistent information.

4. If an entry contains several fields, a read operation may read some fields before

the update and some after the update.

7.5 Distributed Mutual Exclusion

1. The problem of mutual exclusion in a single-computer system, where shared

memory exists.

2. In single-computer systems, the status of a shared resource and the status of users

is readily available in the shared memory, and solutions to the mutual exclusion

problem can be easily implemented using shared variables (e.g., semaphores).

3. In distributed systems, both the shared resources and the users may be

distributed and shared memory does not exist.

4. Consequently, approaches based on shared variables are not applicable to

distributed systems and approaches based on message passing must be used.

5. The problem of mutual exclusion becomes much more complex in distributed

systems (as compared to single-computer systems) because of the lack of both

shared memory and a common physical clock and because of unpredictable

message delays.

6. Owing to these factors, it is virtually impossible for a site in a distributed system

to have current and complete knowledge of the state of the system.


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7.6 Classification of mutual Exclusion Algorithm

1. The problem of mutual exclusion has received considerable attention and several

algorithms to achieve mutual exclusion in distributed systems have been

proposed.

2. They tend to differ in their communication topology (e.g., tree, ring, and any

arbitrary graph) and in the amount of information maintained by each site about

other sites.

3. These algorithms can be grouped into two classes. The algorithms in the first class

are no token-based.

4. These algorithms require two or more successive rounds of message exchanges

among the sites.

5. These algorithms are assertion based because a site can enter its critical section

(CS) when an assertion defined on its local variables becomes true.

6. Mutual exclusion is enforced because the assertion becomes true only at one site

at any given time.

7. The algorithms in the second class are token-based, in these algorithms, a unique

token (also known as the PRIVILEGE message) is shared among the sites.

8. A site is allowed to enter its CS if it possesses the token and it continues to hold

the token until the execution of the CS is over.

9. These algorithms essentially differ in the way a site carries out the search for the

token.

7.7 Requirements of Mutual Exclusion Algorithms

The primary objective of a mutual exclusion algorithm is to maintain mutual exclusion;

that is, to guarantee that only one request accesses the es at a time. In addition, the

following characteristics are considered important in a mutual exclusion algorithm:

i. Freedom from Deadlocks. Two or more sites should not endlessly wait for

messages that will never arrive.


Anuradha Bhatia 162

ii. Freedom from Starvation. A site should not be forced to wait indefinitely to

execute es while other sites are repeatedly executing es. That is, every requesting

site should get an opportunity to execute es in a finite time.

iii. Fairness. Fairness dictates that requests must be executed in the order they are

made (or the order in which they arrive in the system). Since a physical global clock

does not exist, time is determined by logical clocks. Note that fairness implies

freedom from starvation, but not vice-versa.

iv. Fault Tolerance. A mutual exclusion algorithm is fault-tolerant if in the wake of a

failure, it can reorganize itself so that it continues to function without any

(prolonged) disruptions.

7.8 Performance measure

1. The performance of mutual exclusion algorithms is generally measured by the

following four metrics:

2. First, the number of messages necessary per es invocation.

3. Second, the synchronization delay, which is the time required after a site leaves

the es and before the next site enters the es .

4. Note that normally one or more sequential message exchanges are required after

a site exits the es and before the next site enters the es.

5. Third, the response time, which is the time interval a request waits for its es

execution to be over after its request messages have been sent out.

6. Thus, response time does not include the time a request waits at a site before its

request messages have been sent out.

7. Fourth, the system throughput, which is the rate at which the system executes

requests for the es.

8. If sd is the synchronization delay and E is the average critical section execution

time, then the throughput is given by the following equation:


Anuradha Bhatia 163

Figure 7.6: System throughput l/(sd+E)

LOW AND HIGH LOAD PERFORMANCE. Performance of a mutual exclusion algorithm

depends upon the loading conditions of the system and is often studied under two special

loading conditions, viz., low load and high load. Under low load conditions, there is

seldom more than one request for mutual exclusion simultaneously in the system. Under

high load conditions, there is always a pending request for mutual exclusion at a site.

Thus, after having executed a request, a site immediately initiates activities to let the next

site execute its CS. A site is seldom in an idle state under high load conditions. For many

mutual exclusion algorithms, the performance metrics can be easily determined under

low and high loads through simple reasoning.

BEST AND WORST CASE PERFORMANCE. Generally, mutual exclusion algorithms have

best and worst cases for the performance metrics. In the best case, prevailing conditions

are such that a performance metric attains the best possible value. For example, in most

algorithms the best value of the response time is a round-trip message delay plus CS

execution time, 2T + E (where T is the average message delay and E is the average critical

section execution time).

Figure 7.7: Performance


Anuradha Bhatia 164

7.9 Non Token based Algorithms:

1. In non-taken-based mutual exclusion algorithms, a site communicates with a set

of other sites to arbitrate who should execute the CS next. For a site Si.

2. Request set Ri contains ids of all those sites from which site Si must acquire

permission before entering the CS.

3. Non-token-based mutual exclusion algorithms which are good representatives of

this class.

4. Non-token-based mutual exclusion algorithms use timestamps to order requests

for the CS and to resolve conflicts between simultaneous requests for the CS.

5. In all these algorithms, logical clocks are maintained and updated according to

Lamport's scheme [9]. Each request for the CS gets a timestamp, and smaller

timestamp requests have priority over larger timestamp requests.

i. Lamport Algorithm

Lamport was the first to give a distributed mutual exclusion algorithm as an

illustration of his clock synchronization scheme [9]. In Lamport's algorithm,

. I : 1 <= I <= N :: Ri = {S1,S2,…..,S3}

Every site Si keeps a queue, request.queuei which contains mutual exclusion

requests ordered by their timestamps. This algorithm requires messages to be

delivered in the FIFO order between every pair of sites.

The Algorithm

Requesting the critical section:

1. When a site Si wants to enter the CS, it sends a REQUEST(tsi, i) message to

all the sites in its request set Ri and places the request on

reques.queuei.((tsi, i) is the timestamp of the request.)

2. When a site H j receives the REQUEST(t8i, i) message from site Si, it retums

a timestamped REPLY message to and places site Si 's request on

reques.queue}.

Executing the critical section. Site Hi enters the CS when the two following

conditions hold:


Anuradha Bhatia 165

[L1:] Hi has received a message with timestamp larger than (t8i, i) from all other

sites.

[L2:] Hi'S request is at the top of requesLqueuei.

Releasing the critical section.

1. Site Si, upon exiting the CS, removes its request from the top of its request

queue and sends a timestamped RELEASE message to all the sites in its

request set.

2. When a site S} receives a RELEASE message from site Hi, it removes Si'S

request from its request queue.

When a site removes a request from its request queue, its own request may come

at the top of the queue, enabling it to enter the CS. The algorithm executes CS

requests in the increasing order of timestamps.

ii. Ricart–Agrawala’s Algorithm

The Ricart-Agrawala algorithm [16] is an optimization of Lamport's algorithm that

dispenses with RELEASE messages by cleverly merging them with REPLY messages. In this

algorithm also, I : 1 <= I <= N :: Ri = {S1,S2,…..,SN}

The Algorithm

Requesting the critical section.

1. When a site Si wants to enter the CS, it sends a timestamped REQUEST message

to all the sites in its request set.

2. When site Sj receives a REQUEST message from site Si, it sends a REPLY message

to site Si if site Sj is neither requesting nor executing the CS or if site Sj is requesting

and Si'S request's timestamp is smaller than site S/s own request's timestamp. The

request is deferred otherwise.

Executing the critical section

3. Site Si enters the CS after it has received REPLY messages from all the sites in its

request set.

Releasing the critical section

4. When site Si exits the CS, it sends REPLY messages to all the deferred requests.


Anuradha Bhatia 166

iii. Maekawa’s Algorithm

1. Maekawa's algorithm [10] is a departure from the general trend in the following

two ways:

2. First, a site does not request permission from every other site, but only from a

subset of the sites.

3. This is a radically different approach as compared to the Lamport and the Ricart-

Agrawala algorithms, where all sites participate in the conflict resolution of all

other sites.

4. In Maekawa's algorithm, the request set of each site is chosen such that

5. Consequently, every pair of sites has a site that mediates conflicts between that

pair. Second, in Maekawa's algorithm a site can send out only one REPLY message

at a time.

6. A site can only send a REPLY message only after it has received a RELEASE message

for the previous REPLY message.

7. Therefore, a site Si locks all the sites in Ri in exclusive mode before executing its

CS.

8. THE CONSTRUCTION OF REQUEST SETS. The request sets for sites in Maekawa's

algorithm are constructed to satisfy the following conditions:

9. Since there is at least one common site between the request sets of any two sites

(condition Ml), every pair of sites has a common site that mediates conflicts

between the pair.


Anuradha Bhatia 167

10. A site can have only one outstanding REPLY message at any time; that is, it grants

permission to an incoming request if it has not granted permission to some other

site. Therefore, mutual exclusion is guaranteed.

11. This algorithm requires the delivery of messages to be in the order they are sent

between every pair of sites.

The Algorithm

Maekawa's algorithm works in the following manner:

Requesting the critical section.

i. A site Si requests access to the CS by sending REQUEST(i) messages to all

the sites in its request set Ri .

ii. When a site Sj receives the REQUEST(i) message, it sends a REPLY(j)

message to Si provided it hasn't sent a REPLY message to a site from the

time it received the last RELEASE message. Otherwise, it queues up the

REQUEST for later consideration.

Executing the critical section.

iii. Site Si accesses the CS only after receiving REPLY messages from all the

sites in Ri’

Releasing the critical section

iv. After the execution of the CS is over, site Si sends RELEASE(i) message to

all the sites in Ri.

v. When a site Sj receives a RELEASE(i) message from site Si, it sends a

REPLY message to the next site waiting in the queue and deletes that entry

from the queue. If the queue is empty, then the site updates its state to

reflect that the site has not sent out any REPLY message.


Anuradha Bhatia 168

7.10 Token Based Algorithms

i. In token-based algorithms, a unique token is shared among all sites.

ii. A site is allowed to enter its CS if it possesses the token.

iii. Depending upon the way a site carries out its search for the token, there are

numerous token-based algorithms.

iv. Token-based algorithms use sequence numbers instead of timestamps. Every

request for the token contains a sequence number and the sequence numbers

of sites advance independently.

v. A site increments its sequence number counter every time it makes a request

for the token.

vi. A primary function of the sequence numbers is to distinguish between old and

current requests.

vii. Second, a correctness proof of token-based algorithms to ensure that mutual

exclusion is enforced is trivial because an algorithm guarantees mutual

exclusion so long as a site holds the token during the execution of the CS..

7.11 Suzuki-Kasami’s Broardcast Algorithms

1. In the Suzuki-Kasami's algorithm [21], if a site attempting to enter the CS does not

have the token, it broadcasts a REQUEST message for the token to all the other

sites.

2. A site that possesses the token sends it to the requesting site upon receiving its

REQUEST message.

3. If a site receives a REQUEST message when it is executing the CS, it sends the token

only after it has exited the CS.

4. A site holding the token can enter its CS repeatedly until it sends the token to

some other site.

5. The main design issues in this algorithm are:

Distinguishing outdated REQUEST messages from current REQUEST

messages


Anuradha Bhatia 169

Determining which site has an outstanding request for the CS.

The Algorithm

Requesting the critical section

i. If the requesting site 5i does not have the token, then it increments its

sequence number, RNi[i], and sends a REQUEST(i, sn) message to all other

sites. (sn is the updated value of RNdi].)

ii. When a site 5 j receives this message, it sets RNj[i] to max(RNj[i], sn). If 5 j has

the idle token, then it sends the token to 5 i if RNj [i]=LN[i]+1.

Executing the critical section.

iii. Site 5i executes the CS when it has received the token.

Releasing the critical section. Having finished the execution of the CS, site 5 i takes the

following actions:

iv. It sets LN[i] element of the token array equal to RNdi].

v. For every site 5 j whose 10 is not in the token queue, it appends its 10 to the

token queue if RNi [j]=LN[j]+1.

vi. If token queue is nonempty after the above update, then it deletes the top site

ID from the queue and sends the token to the site indicated by the 10.

Thus, after having executed its CS, a site gives priority to other sites with outstanding

requests for the CS (over its pending requests for the CS). The Suzuki-Kasami algorithm is

not symmetric because a site retains the token even if it does not have a request for the

CS, which is contrary to the spirit of Ricart and Agrawala's definition of a symmetric

algorithm: "no site possesses the right to access its CS when it has not been requested.”

7.12 Singhal’s Heurastic Algorithm

1. In Singhal's token-based heuristic algorithm [20], each site maintains information

about the state of other sites in the system and uses it to select a set of sites that

are likely to have the token.

2. The site requests the token only from these sites, reducing the number of

messages required to execute the CS.


Anuradha Bhatia 170

3. It is called a heuristic algorithm because sites are heuristically selected for sending

token request messages.

4. When token request messages are sent only to a subset of sites, it is necessary

that a requesting site sends a request message to a site that either holds the token

or is going to obtain the token in the near future.

5. Otherwise, there is a potential for deadlocks or starvation. Thus, one design

requirement is that a site must select a subset of sites such that at least one of

those sites is guaranteed to get the token in near future.

6. DATA STRUCTURES. A site Si maintains two arrays, viz., SVi[l ..N] and SNill..N], to

store the information about sites in the system. These arrays store the state and

the highest known sequence number for each site, respectively. Similarly, the

token contains two such arrays as well (denoted by TSV[l..N] and TSN[l..N]).

Sequence numbers are used to detect outdated requests. A site can be in one of

the following states:

The arrays are initialized as follows:

For a site Si (i = 1 to n),

Ri := {S1, S2,..., Si − 1, Si}

Ii := {Si}

Ci := 0

Requestingi = Executingi := False

Note that arrays SV[l..N] of sites are initialized such that for any two sites Si and

Sj, either SVi [j] = R or SVj [i] = R. Since the heuristic selects every site that is

requesting the CS according to local information (Le., the SV array), for any two

sites that are requesting the CS concurrently, one will always send a token request

message to the other. This ensures that sites are not isolated from each other and


Anuradha Bhatia 171

a site's request message reaches a site that either holds the token or is going to

get the token in near future.

The Algorithm

Step 1: (Request Critical Section)

Requesting = true;

Ci = Ci + 1;

Send REQUEST(Ci, i) message to all sites in Ri;

Wait until Ri = ∅;

/* Wait until all sites in Ri have sent a reply to Si */

Requesting = false;

Step 2: (Execute Critical Section)

Executing = true;

Execute CS;

Executing = false;

Step 3: (Release Critical Section)

For every site Sk in Ii (except Si) do

Begin

Ii= Ii – {Sk};

Send REPLY(Ci, i) message to Sk;

Ri= Ri + {Sk}

End

7.13 Raymond’s Tree based Algorithm

1. This algorithm uses a spanning tree to reduce the number of messages exchanged

per critical section execution.

2. The network is viewed as a graph, a spanning tree of a network is a tree that

contains all the N nodes.

3. The algorithm assumes that the underlying network guarantees message delivery.

All nodes of the network are ’completely reliable.


Anuradha Bhatia 172

4. The algorithm operates on a minimal spanning tree of the network topology or a

logical structure imposed on the network.

5. The algorithm assumes the network nodes to be arranged in an unrooted tree

structure. Figure 4 shows a spanning tree of seven nodes A, B, C, D, E, F, and G.

Messages between nodes traverse along the undirected edges of the tree.

Figure 7.9: Tree algorithm

6. A node needs to hold information about and communicate only to its immediate-

neighboring nodes.

7. The concept of tokens used in token-based algorithms, this algorithm uses a

concept of privilege.

8. Only one node can be in possession of the privilege (called the privileged node) at

any time, except when the privilege is in transit from one node to another in the

form of a PRIVILEGE message.

9. When there are no nodes requesting for the privilege, it remains in possession of

the node that last used it.

10. The HOLDER Variables Each node maintains a HOLDER variable that provides

information about the placement of the privilege in relation to the node itself. A

node stores in its HOLDER variable the identity of a node that it thinks has the

privilege or leads to the node having the privilege. For two nodes X and Y, if

HOLDERX = Y, we could redraw the undirected edge between the nodes X and Y

as a directed edge from X to Y. For instance, if node G holds the privilege, the


Anuradha Bhatia 173

Figure is redrawn as given below. The shaded node in Figure 5 represents the

privileged node.

Figure 7.9: Holder Variable

11. Now suppose node B that does not hold the privilege wants to execute the critical

section. B sends a REQUEST message to HOLDERB, i.e., C, which in turn forwards

the REQUEST message to HOLDERC, i.e., G. The privileged node G, if it no longer

needs the privilege, sends the PRIVILEGE message to its neighbor C, which made

a request for the privilege, and resets HOLDERG to C. Node C, in turn, forwards

the PRIVILEGE to node B, since it had requested the privilege on behalf of B. Node

C also resets HOLDERC to B.

Figure 7.10: B Request

The Algorithm

The algorithm consists of the following routines:

i. ASSIGN PRIVILEGE


Anuradha Bhatia 174

This is a routine sends a PRIVILEGE message. A privileged node sends a

PRIVILEGE message if

it holds the privilege but is not using it,

its REQUEST Q is not empty, and

The element at the head of its REQUEST Q is not “self.”

ii. MAKE REQUEST

This is a routine sends a REQUEST message. An unprivileged node sends a

REQUEST message if

it does not hold the privilege,

its REQUEST Q is not empty, i.e., it requires the privilege for itself, or

on behalf of one of its immediate neighboring nodes, and

it has not sent a REQUEST message already.


Anuradha Bhatia 175

The table of events

Figure 7.11: Functionality

7.14 Comparative Performance Analysis.

1. In the worst-case, the algorithm requires (2 * longest path length of the tree)

messages per critical section entry.

2. This happens when the privilege is to be passed between nodes at either ends of

the longest path of the minimal spanning tree.

3. The worst possible network topology for this algorithm is where all nodes are

arranged in a straight line and the longest path length will be N – 1, and thus the

algorithm will exchange 2 * (N – 1) messages per CS execution.

4. If all nodes generate equal number of REQUEST messages for the privilege, the

average number of messages needed per critical section entry will be

approximately 2N/3 because the average distance between a requesting node and

a privileged node is (N + 1)/3.

5. The best topology for the algorithm is the radiating star topology. The worst case

cost of this algorithm for this topology is O(logK−1N).

6. Trees with higher fan-outs are preferred over radiating star topologies.

7. The longest path length of such trees is typically O(log N).


Anuradha Bhatia 176

8. Thus, on an average, this algorithm involves the exchange of O(log N) messages

per critical section execution. Under heavy load, the algorithm exhibits an

interesting property:

9. “As the number of nodes requesting for the privilege increases, the number of

messages exchanged per critical section entry decreases.” In heavy load, the

algorithm requires exchange of only four messages per CS execution.


Anuradha Bhatia

8. Consistency and Replication

CONTENTS

8.1 Introduction, Data-Centric and Client-Centric Consistency Models, Replica

Management.

8.2 Introduction, good features of DFS, File models, File Accessing models,

File-Caching Schemes, File Replication, Network File System(NFS),

Andrew File System(AFS), Hadoop Distributed File System and Map

Reduce.

Parallel & Distributed Systems 8. Consistency and Replication

Anuradha Bhatia 178

8.1 Introduction

1. An important issue in distributed systems is the replication of data. Data are

generally replicated to enhance reliability or improve performance.

2. One of the major problems is keeping replicas consistent. Informally, this means

that when one copy is updated we need to ensure that the other copies are

updated as well; otherwise the replicas will no longer be the same.

3. Consistency models for shared data are often hard to implement efficiently in

large-scale distributed systems.

4. One specific class is formed by client-centric consistency models, which

concentrate, on consistency from the perspective of a single (possibly mobile)

client.

5. Reasons for Replication

There are two primary reasons for replicating data: reliability and

performance.

First, data are replicated to increase the reliability of a system.

If a file system has been replicated it may be possible to continue working

after one replica crashes by simply switching to one of the other replicas.

Also, by maintaining multiple copies, it becomes possible to provide better

protection against corrupted data.

For example, imagine there are three copies of a file and every read and

write operation is performed on each copy.

We can safeguard ourselves against a single, failing write operation, by

considering the value that is returned by at least two copies as being the

correct one.

Scaling with respect to the size of a geographical area may also require

replication.

The basic idea is that by placing a copy of data in the proximity of the

process using them, the time to access the data decreases. As a

consequence, the performance as perceived by that process increases.


Anuradha Bhatia 179

This example also illustrates that the benefits of replication for

performance may be hard to evaluate.

Although a client process may perceive better performance, it may also be

the case that more network bandwidth is now consumed keeping all

replicas up to date.

6. Replication as Scaling Technique

Replication and caching for performance are widely applied as scaling

techniques. Scalability issues generally appear in the form of performance

problems.

Placing copies of data close to the processes using them can improve

performance through reduction of access time and thus solve scalability

problems.

A possible trade-off that needs to be made is that keeping copies up to

date may require more network bandwidth.

Consider a process P that accesses a local replica N times per second,

whereas the replica itself is updated M times per second.

Assume that an update completely refreshes the previous version of the

local replica.

If N «M, that is, the access-to-update ratio is very low, we have the

situation where many updated versions of the local replica will never be

accessed by P, rendering the network communication for those versions

useless.

It may have been better not to install a local replica close to P, or to apply

a different strategy for updating the replica.


Anuradha Bhatia 180

8.2 Data-Centric and Client-Centric Consistency Models

1. Data Centric Consistency Model

A data store may be physically distributed across multiple machines. In

particular, each process that can access data from the store is assumed

to have a local (or nearby) copy available of the entire store.

Write operations are propagated to the other copies, as shown in

Figure.

A data operation is classified as a write operation when it changes the

data, and is otherwise classified as a read operation.

Figure 8.1: Data Centric Model

A consistency model is essentially a contract between processes and

the data store. It says that if processes agree to obey certain. Rules, the

store promises to work correctly.

Normally, a process that performs a read operation on a data item,

expects the operation to return a value that shows the results of the last

write operation on that data.

In the absence of a global clock, it is difficult to define precisely which

write operation is the last one. As an alternative, we need to provide

other definitions, leading to a range of consistency models.

Each model effectively restricts the values that a read operation on a

data item can return.


Anuradha Bhatia 181

As is to be expected, the ones with major restrictions are easy to use,

for example when developing applications, whereas those with minor

restrictions are sometimes difficult.

The tradeoff is, of course, that the easy-to-use models do not perform

nearly as well as the difficult ones.

2. Client-Centric Consistency Models

The consistency models described in the previous section aim at

providing a system wide consistent view on a data store.

An important assumption is that concurrent processes may be

simultaneously updating the data store, and that it is necessary to

provide consistency in the face of such concurrency.

For example, in the case of object-based entry consistency, the data

store guarantees that when an object is called, the calling process is

provided with a copy of the object that reflects all changes to the object

that have been made so far, possibly by other processes.

During the call, it is also guaranteed that no other process can interfere

that is, mutual exclusive access is provided to the calling process.

Eventual Consistency There are many examples in which concurrency

appears only in a restricted form. For example, in many database

systems, most processes hardly ever perform update operations; they

mostly read data from the database. Only one, or very few processes

perform update operations. The question then is how fast updates

should be made available to only reading processes.


Anuradha Bhatia 182

Figure 8.2: Client Centric Model

8.3 Replica Management

1. A key issue for any distributed system that supports replication is to decide where,

when, and by whom replicas should be placed, and subsequently which

mechanisms to use for keeping the replicas consistent.

2. The placement problem itself should be split into two sub problems: that of

placing replica servers, and that of placing content.

3. The difference is a subtle but important one and the two issues are often not

clearly separated.

4. Replica-server placement is concerned with finding the best locations to place a

server that can host (part of) a data store.

5. Content placement deals with finding the best servers for placing content.

6. Note that this often means that we are looking for the optimal placement of only

a single data item.

7. Replica-Server Placement: The placement of replica servers is not an

intensively studied problem for the simple reason that it is often more of a

management and commercial issue than an optimization problem.

Nonetheless, analysis of client and network properties are useful to come to

informed decisions. There are various ways to compute the .best placement of

replica servers, but all boil down to an optimization problem in which the best


Anuradha Bhatia 183

K out of N locations need to be selected (K < N). These problems are known to

be computationally complex and can be solved only through heuristics.

8. Content Replication and Placement: move away from server placement and

concentrate on content placement. When it comes to content replication and

placement, three different types of replicas can be distinguished logically

organized as shown in Figure 8.3.

Figure 8.3: Content Replication and Placement

9. Permanent Replicas: Permanent replicas can be considered as the initial set

of replicas that constitute a distributed data store. In many cases, the number

of permanent replicas is small. Consider, for example, a Web site. Distribution

of a Web site generally comes in one of two forms. The first kind of distribution

is one in which the files that constitute a site are replicated across a limited

number of servers at a single location. Whenever a request comes in, it is

forwarded to one of the servers, for instance, using a round-robin strategy. The

second form of distributed Web sites is what is called mirroring. In this case, a

Web site is copied to a limited number of servers, called mirror sites. Which

are geographically spread across the Internet. In most cases, clients simply

choose one of the various mirror sites from a list offered to them. Mirrored

web sites have in common with cluster-based Web sites that there are only a

few number of replicas, which are more or less statically configured.

10. Server-Initiated Replicas: The problem of dynamically placing replicas is

also being addressed in Web hosting services. These services offer a (relatively

static) collection of servers spread across the Internet that can maintain and

provide access to Web file belonging to third parties.


Anuradha Bhatia 184

8.4 Introduction, good features of DFS

1. A Distributed File System (DFS) enables programs to store and access remote files

exactly as they do local ones, allowing users to access files from any computer on

a network. This is an area of active research interest today.

2. The resources on a particular machine are local to itself. Resources on other

machines are remote.

3. A file system provides a service for clients. The server interface is the normal set

of file operations: create, read, etc. on files.

4. Key Features

Data sharing of multiple users.

User mobility

Location transparency

Location independence

Backups and System Monitoring

8.5 File models

1. File Transfer Protocol(FTP)

Connect to a remote machine and interactively send or fetch an arbitrary

file.

User connecting to an FTP server specifies an account and password.

Superseded by HTTP for file transfer.

2. Sun's Network File System.

One of the most popular and widespread distributed file system in use today

since its introduction in 1985.

Motivated by wanting to extend a UNIX file system to a distributed

environment. But, further extended to other OS as well.

Design is transparent. Any computer can be a server, exporting some of its

files, and a client, accessing files on other machines.

High performance: try to make remote access as comparable to local access

through caching and readahead.


Anuradha Bhatia 185

8.6 Network File System(NFS)

1. NFS is stateless All client requests must be self-contained

2. The virtual file system interface VFS operations, VNODE operations

3. Fast crash recovery, Reason behind stateless design

4. UNIX semantics at Client side, Best way to achieve transparent access.

Figure 8.4: NFS

8.6 Andrew File System(AFS)

1. Distributed network file system which uses a set of trusted servers to present a

homogeneous, location transparent file name space to all the client workstations.

2. Distributed computing environment developed at Carnegie Mellon University

(CMU) for use as a campus computing and information system [Morris et al. 1986].

3. Intention is to support information sharing on a large scale by minimizing client-

server communication

4. Achieved by transferring whole files between server and client computers and

caching them at clients until the servers receives a more up-to-date version.

5. Features

Features of AFS


Anuradha Bhatia 186

Uniform namespace

Location-independent file sharing

Client-side caching

Secure authentication

Replication

Whole-file serving

Whole-file caching

6. Working

Implemented as 2 software components that exists as UNIX processes

called Vice and Venus

Vice: Name given to the server software that runs as a user level UNIX

process in each server computer.

Venus: User level process that runs in each client computer and

corresponds to the client module in our abstract model.

Files available to user are either local or shared

Local files are stored on a workstation's disk and are available only to local

user processes.

Shared files are stored on servers, copies of them are cached on the local

disk of work stations.


Anuradha Bhatia 187

Figure 8.5: AFS

8.7 Hadoop Distributed File System

1. Underlying all of these components is the Hadoop Distributed File System (HDFS™).

2. This is the foundation of the Hadoop cluster.

Figure 8.6: HDFS

3. The HDFS file system manages how the datasets are stored in the Hadoop cluster.

4. It is responsible for distributing the data across the data nodes, managing replication

for redundancy and administrative tasks like adding, removing and recovery of data

nodes.


Anuradha Bhatia 188

A. Hadoop Cluster Architecture:

Figure 8.7: Hadoop Cluster Architecture

Hadoop Cluster would consists of

110 different racks

Each rack would have around 40 slave machine

At the top of each rack there is a rack switch

Each slave machine(rack server in a rack) has cables coming out it from both

the ends

Cables are connected to rack switch at the top which means that top rack

switch will have around 80 ports

There are global 8 core switches

The rack switch has uplinks connected to core switches and hence connecting

all other racks with uniform bandwidth, forming the Cluster

In the cluster, you have few machines to act as Name node and as JobTracker.

They are referred as Masters. These masters have different configuration

favoring more DRAM and CPU and less local storage.

Hadoop cluster has 3 components:

1. Client

2. Master


Anuradha Bhatia 189

3. Slave

The role of each components are shown in the below image.

Figure 8.8: Hadoop Component

1. Client:

i. It is neither master nor slave, rather play a role of loading the data into cluster,

submit MapReduce jobs describing how the data should be processed and then

retrieve the data to see the response after job completion.

Figure 8.9: Hadoop Client


Anuradha Bhatia 190

2. Masters:

The Masters consists of 3 components NameNode, Secondary Node name and JobTracker.

Figure 8.10: Masters

i. NameNode:

NameNode does NOT store the files but only the file's metadata. In later section we will

see it is actually the DataNode which stores the files.

Figure 8.11: Namenode

NameNode oversees the health of DataNode and coordinates access to the data stored

in DataNode.

Name node keeps track of all the file system related information such as to

Which section of file is saved in which part of the cluster

Last access time for the files

User permissions like which user have access to the file


Anuradha Bhatia 191

ii. JobTracker:

JobTracker coordinates the parallel processing of data using MapReduce.

To know more about JobTracker, please read the article All You Want to Know about MapReduce

(The Heart of Hadoop)

iii. Secondary Name Node:

Figure 8.12: Secondary NameNode

The job of Secondary Node is to contact NameNode in a periodic manner after

certain time interval (by default 1 hour).

NameNode which keeps all filesystem metadata in RAM has no capability to

process that metadata on to disk.

If NameNode crashes, you lose everything in RAM itself and you don't have any

backup of filesystem.

What secondary node does is it contacts NameNode in an hour and pulls copy of

metadata information out of NameNode.

It shuffle and merge this information into clean file folder and sent to back again

to NameNode, while keeping a copy for itself.

Hence Secondary Node is not the backup rather it does job of housekeeping.

In case of NameNode failure, saved metadata can rebuild it easily.


Anuradha Bhatia 192

3. Slaves:

i. Slave nodes are the majority of machines in Hadoop Cluster and are responsible

to

Store the data

Process the computation

Figure 8.13: Slaves

ii. Each slave runs both a DataNode and Task Tracker daemon which communicates

to their masters.

iii. The Task Tracker daemon is a slave to the Job Tracker and the DataNode daemon

a slave to the NameNode

I. Hadoop- Typical Workflow in HDFS:

Take the example of input file as Sample.txt.

Figure 8.14: Workflow


Anuradha Bhatia 193

1. How TestingHadoop.txt gets loaded into the Hadoop Cluster?

Figure 8.15: Cluster

Client machine does this step and loads the Sample.txt into cluster.

It breaks the sample.txt into smaller chunks which are known as "Blocks" in Hadoop

context.

Client put these blocks on different machines (data nodes) throughout the cluster.

2. Next, how does the Client knows that to which data nodes load the blocks?

Now NameNode comes into picture.

The NameNode used its Rack Awareness intelligence to decide on which DataNode to

provide.

For each of the data block (in this case Block-A, Block-B and Block-C), Client contacts

NameNode and in response NameNode sends an ordered list of 3 DataNodes.


Anuradha Bhatia 194

Figure 8.16: Blocks

For example in response to Block-A request, Node Name may send DataNode-2,

DataNode-3 and DataNode-4.

Block-B DataNodes list DataNode-1, DataNode-3, DataNode-4 and for Block C data

node list DataNode-1, DataNode-2, DataNode-3. Hence

Block A gets stored in DataNode-2, DataNode-3, DataNode-4

Block B gets stored in DataNode-1, DataNode-3, DataNode-4

Block C gets stored in DataNode-1, DataNode-2, DataNode-3

Every block is replicated to more than 1 data nodes to ensure the data recovery on

the time of machine failures. That's why NameNode send 3 DataNodes list for each

individual block

3. Who does the block replication?

Client write the data block directly to one DataNode. DataNodes then replicate the block

to other Data nodes.

When one block gets written in all 3 DataNode then only cycle repeats for next block.


Anuradha Bhatia 195

Notes:

Figure 8.17: Rack

In Hadoop Gen 1 there is only one NameNode wherein Gen2 there is active

passive model in NameNode where one more node "Passive Node" comes in

picture.

The default setting for Hadoop is to have 3 copies of each block in the cluster.

This setting can be configured with "dfs.replication" parameter of hdfs-

site.xml file.

Keep note that Client directly writes the block to the DataNode without any

intervention of NameNode in this process.

8.8 MapReduce

I. Distributed File Systems

1. Most computing is done on a single processor, with its main memory, cache,

and local disk (a compute node).

2. Applications that called for parallel processing, such as large scientific

calculations, were done on special-purpose parallel computers with many

processors and specialized hardware.


Anuradha Bhatia 196

3. The prevalence of large-scale Web services has caused more and more

computing to be done on installations with thousands of compute nodes

operating more or less independently.

4. In these installations, the compute nodes are commodity hardware, which

greatly reduces the cost compared with special-purpose parallel machines.

A. Physical Organization of Compute Nodes

i. The new parallel-computing architecture, sometimes called cluster

computing, is organized as follows. Compute nodes are stored on

racks, perhaps 8–64 on a rack.

ii. The nodes on a single rack are connected by a network, typically

gigabit Ethernet.

iii. There can be many racks of compute nodes, and racks are

connected by another level of network or a switch.

iv. The bandwidth of inter-rack communication is somewhat greater

than the intrarack Ethernet, but given the number of pairs of nodes

that might need to communicate between racks, this bandwidth

may be essential. Figure 4.1 suggests the architecture of a

largescale computing system. However, there may be many more

racks and many more compute nodes per rack.

v. Some important calculations take minutes or even hours on

thousands of compute nodes. If we had to abort and restart the

computation every time one component failed, then the

computation might never complete successfully.

vi. The solution to this problem takes two forms:

Files must be stored redundantly. If we did not duplicate the

file at several compute nodes, then if one node failed, all its

files would be unavailable until the node is replaced. If we


Anuradha Bhatia 197

did not back up the files at all, and the disk crashes, the files

would be lost forever.

Computations must be divided into tasks, such that if any

one task fails to execute to completion, it can be restarted

without affecting other tasks.

Figure 8.18: Racks of compute nodes

B. Large-Scale File-System Organization

i. To exploit cluster computing, files must look and behave somewhat

differently from the conventional file systems found on single

computers.

ii. This new file system, often called a distributed file system or DFS

(although this term has had other meanings in the past), is typically

used as follows.

iii. There are several distributed file systems of the type we have

described that are used in practice. Among these:

The Google File System (GFS), the original of the class.

Hadoop Distributed File System (HDFS), an open-source DFS

used with Hadoop, an implementation of map-reduce and

distributed by the Apache Software Foundation.

CloudStore, an open-source DFS originally developed by

Kosmix.


Anuradha Bhatia 198

iv. Files can be enormous, possibly a terabyte in size. If you have only

small files, there is no point using a DFS for them.

v. Files are rarely updated. Rather, they are read as data for some

calculation, and possibly additional data is appended to files from

time to time. For example, an airline reservation system would not

be suitable for a DFS, even if the data were very large, because the

data is changed so frequently.

II. MapReduce

1. Traditional Enterprise Systems normally have a centralized server to store and

process data.

2. The following illustration depicts a schematic view of a traditional enterprise

system. Traditional model is certainly not suitable to process huge volumes of

scalable data and cannot be accommodated by standard database servers.

3. Moreover, the centralized system creates too much of a bottleneck while

processing multiple files simultaneously.

Figure 8.19: MapReduce

4. Google solved this bottleneck issue using an algorithm called MapReduce.

MapReduce divides a task into small parts and assigns them to many

computers.

5. Later, the results are collected at one place and integrated to form the result

dataset.


Anuradha Bhatia 199

Figure 8.20: System

6. A MapReduce computation executes as follows:

Some number of Map tasks each are given one or more chunks from a

distributed file system. These Map tasks turn the chunk into a

sequence of key-value pairs. The way key-value pairs are produced

from the input data is determined by the code written by the user for

the Map function.

The key-value pairs from each Map task are collected by a master

controller and sorted by key. The keys are divided among all the

Reduce tasks, so all key-value pairs with the same key wind up at the

same Reduce task.

The Reduce tasks work on one key at a time, and combine all the values

associated with that key in some way. The manner of combination of

values is determined by the code written by the user for the Reduce

function.


Anuradha Bhatia 200

Figure 8.21: Schematic MapReduce Computation

A. The Map Task

i. We view input files for a Map task as consisting of elements, which

can be any type: a tuple or a document, for example.

ii. A chunk is a collection of elements, and no element is stored across

two chunks.

iii. Technically, all inputs to Map tasks and outputs from Reduce tasks

are of the key-value-pair form, but normally the keys of input

elements are not relevant and we shall tend to ignore them.

iv. Insisting on this form for inputs and outputs is motivated by the

desire to allow composition of several MapReduce processes.

v. The Map function takes an input element as its argument and

produces zero or more key-value pairs.

vi. The types of keys and values are each arbitrary.

vii. Further, keys are not “keys” in the usual sense; they do not have

to be unique.

viii. Rather a Map task can produce several key-value pairs with the

same key, even from the same element.


Anuradha Bhatia 201

B. Grouping by Key

i. As soon as the Map tasks have all completed successfully, the key-

value pairs are grouped by key, and the values associated with each

key are formed into a list of values.

ii. The grouping is performed by the system, regardless of what the

Map and Reduce tasks do.

iii. The master controller process knows how many Reduce tasks there

will be, say r such tasks.

iv. The user typically tells the MapReduce system what r should be.

v. Then the master controller picks a hash function that applies to

keys and produces a bucket number from 0 to r − 1.

vi. Each key that is output by a Map task is hashed and its key-value

pair is put in one of r local files. Each file is destined for one of the

Reduce tasks.1.

vii. To perform the grouping by key and distribution to the Reduce

tasks, the master controller merges the files from each Map task

that are destined for a particular Reduce task and feeds the merged

file to that process as a sequence of key-list-of-value pairs.

viii. That is, for each key k, the input to the Reduce task that handles

key k is a pair of the form (k, [v1, v2, . . . , vn]), where (k, v1), (k, v2),

. . . , (k, vn) are all the key-value pairs with key k coming from all the

Map tasks.

C. The Reduce Task

i. The Reduce function’s argument is a pair consisting of a key and its

list of associated values.

ii. The output of the Reduce function is a sequence of zero or more

key-value pairs.


Anuradha Bhatia 202

iii. These key-value pairs can be of a type different from those sent

from Map tasks to Reduce tasks, but often they are the same type.

iv. We shall refer to the application of the Reduce function to a single

key and its associated list of values as a reducer. A Reduce task

receives one or more keys and their associated value lists.

v. That is, a Reduce task executes one or more reducers. The outputs

from all the Reduce tasks are merged into a single file.

vi. Reducers may be partitioned among a smaller number of Reduce

tasks is by hashing the keys and associating each

vii. Reduce task with one of the buckets of the hash function.

D. Combiners

i. A Reduce function is associative and commutative. That is, the

values to be combined can be combined in any order, with the

same result.

ii. The addition performed in Example 2.2 is an example of an

associative and commutative operation. It doesn’t matter how we

group a list of numbers v1, v2, . . . , vn; the sum will be the same.

iii. When the Reduce function is associative and commutative, we can

push some of what the reducers do to the Map tasks

iv. These key-value pairs would thus be replaced by one pair with key

w and value equal to the sum of all the 1’s in all those pairs.

v. That is, the pairs with key w generated by a single Map task would

be replaced by a pair (w, m), where m is the number of times that

w appears among the documents handled by this Map task.


Anuradha Bhatia 203

E. Details of MapReduce task

The MapReduce algorithm contains two important tasks, namely Map and

Reduce.

i. The Map task takes a set of data and converts it into another set of

data, where individual elements are broken down into tuples (key-

value pairs).

ii. The Reduce task takes the output from the Map as an input and

combines those data tuples (key-value pairs) into a smaller set of

tuples.

iii. The reduce task is always performed after the map job.

Figure 8.22: Map Jobs

Input Phase − Here we have a Record Reader that translates

each record in an input file and sends the parsed data to the

mapper in the form of key-value pairs.

Map − Map is a user-defined function, which takes a series of

key-value pairs and processes each one of them to generate

zero or more key-value pairs.

Intermediate Keys − they key-value pairs generated by the

mapper are known as intermediate keys.

Combiner − A combiner is a type of local Reducer that groups

similar data from the map phase into identifiable sets. It takes


Anuradha Bhatia 204

the intermediate keys from the mapper as input and applies a

user-defined code to aggregate the values in a small scope of

one mapper. It is not a part of the main MapReduce algorithm;

it is optional.

Shuffle and Sort − The Reducer task starts with the Shuffle and

Sort step. It downloads the grouped key-value pairs onto the

local machine, where the Reducer is running. The individual key-

value pairs are sorted by key into a larger data list. The data list

groups the equivalent keys together so that their values can be

iterated easily in the Reducer task.

Reducer − The Reducer takes the grouped key-value paired data

as input and runs a Reducer function on each one of them. Here,

the data can be aggregated, filtered, and combined in a number

of ways, and it requires a wide range of processing. Once the

execution is over, it gives zero or more key-value pairs to the

final step.

Output Phase − In the output phase, we have an output

formatter that translates the final key-value pairs from the

Reducer function and writes them onto a file using a record

writer.

iv. The MapReduce phase


Anuradha Bhatia 205

Figure 8.23: MapReduce Phase

F. MapReduce-Example

Twitter receives around 500 million tweets per day, which is nearly 3000

tweets per second. The following illustration shows how Tweeter manages

its tweets with the help of MapReduce.

Figure 8.25: Twitter Example

i. Tokenize − Tokenizes the tweets into maps of tokens and writes them

as key-value pairs.

ii. Filter − Filters unwanted words from the maps of tokens and writes the

filtered maps as key-value pairs.

iii. Count − Generates a token counter per word.

iv. Aggregate Counters − Prepares an aggregate of similar counter values

into small manageable units.


Anuradha Bhatia 206

G. MapReduce – Algorithm

The MapReduce algorithm contains two important tasks, namely Map and

Reduce.

i. The map task is done by means of Mapper Class

Mapper class takes the input, tokenizes it, maps and sorts

it. The output of Mapper class is used as input by Reducer

class, which in turn searches matching pairs and reduces

them.

ii. The reduce task is done by means of Reducer Class.

MapReduce implements various mathematical algorithms

to divide a task into small parts and assign them to multiple

systems. In technical terms, MapReduce algorithm helps in

sending the Map & Reduce tasks to appropriate servers in a

cluster.

Figure 8.26: Reducer Class

H. Coping With Node Failures

i. The worst thing that can happen is that the compute node at which

the Master is executing fails. In this case, the entire MapReduce job

must be restarted.

ii. But only this one node can bring the entire process down; other

failures will be managed by the Master, and the MapReduce job

will complete eventually.


Anuradha Bhatia 207

iii. Suppose the compute node at which a Map worker resides fails.

This failure will be detected by the Master, because it periodically

pings the Worker processes.

iv. All the Map tasks that were assigned to this Worker will have to be

redone, even if they had completed. The reason for redoing

completed Map asks is that their output destined for the Reduce

tasks resides at that compute node, and is now unavailable to the

Reduce tasks.

v. The Master sets the status of each of these Map tasks to idle and

will schedule them on a Worker when one becomes available. The

vi. Master must also inform each Reduce task that the location of its

input from that Map task has changed. Dealing with a failure at the

node of a Reduce worker is simpler.

vii. The Master simply sets the status of its currently executing Reduce

tasks to idle. These will be rescheduled on another reduce worker

later.

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