Unit-IV Distributed Computing Systems Distributed Computing models Issues in designing distributed

operating systems DCE

Computer Architecture Multiprocessors Types

• Tightly Coupled Multiprocessor System (Parallel )

Computer Architecture Multiprocessors Types

• Loosely Coupled Multiprocessor System(Distributed )

Parallel v/s Distributed Systems

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Parallel Systems Distributed Systems

Memory Tightly coupled shared memoryUMA, NUMA

Distributed memoryMessage passing, RPC, and/or used of distributed shared memory

Control Global clock controlSIMD, MIMD

No global clock controlSynchronization algorithms needed

Processor interconnection

Order of TbpsBus, mesh, tree, mesh of tree, and hypercube (-related) network

Order of GbpsEthernet(bus), token ring and SCI (ring), myrinet(switching network)

Main focus PerformanceScientific computing

Performance(cost and scalability)Reliability/availabilityInformation/resource sharing

What is a distributed system?

• Tanenbaum’s definition of a distributed system: “A distributed system is a collection of independent computers that appear to the users of the system as a single coherent system.”

An example of a Distributed System

• Nationalized Bank with multiple Branch Offices

Requirements of Distributed systems

• Security and reliability.• Consistency of replicated data.• Concurrent transactions (operations which

involve accounts in different banks; simultaneous access from several users, etc)

• Fault tolerance

What is a Distributed Computing System?

• "a collection of processors interconnected by a communication network in which each processor has its own local memory and other peripherals and the communication between any two processors of the system takes place by message passing over the communication network" (Sinha97)

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Evaluation of Distributed Computing System

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1945-1950s Loading monitor

1950s-1960s Batch system

1960s Multiprogramming

1960s-1970s Time sharing systems Multics, IBM360

1969-1973 WAN and LAN ARPAnet, Ethernet

1960s-early1980s

Minicomputers PDP, VAX

Early 1980s Workstations Alto

1980s – present

Workstation/Server models

Sprite, V-system

1990s Clusters Beowulf

Late 1990s Grid computing Globus, Legion

Distributed Computing System Models

• Minicomputer model• Workstation model• Workstation-server model• Processor-pool model• Cluster model• Grid computing

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Minicomputer Model

• Extension of Time sharing system– User must log on his/her home minicomputer.– Thereafter, he/she can log on a remote machine by telnet.

• Resource sharing– Database– High-performance devices

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Mini-computer

Mini-computer

Mini-computer

ARPAnet

Workstation Model

• Process migration– Users first log on his/her personal workstation.– If there are idle remote workstations, a heavy job may migrate to

one of them.• Problems:

– How to find an idle workstation?– How to migrate a job?– What if a user log on the remote machine?

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100MbpsLAN

Workstation

Workstation Workstation

WorkstationWorkstation

Workstation-Server Model

• Client workstations– Diskless– Graphic/interactive applications processed in local– All file, print, http and even cycle computation

requests are sent to servers.• Server minicomputers

– Each minicomputer is dedicated to one or more different types of services (print, database,…etc).

• Client-Server model of communication– RPC (Remote Procedure Call)– RMI (Remote Method Invocation)

• A Client process calls a server process’ function.• No process migration invoked• Example: NFS

• Disadvantage : capability of idle workstations is not utilized. 13

100GbpsLAN

Workstation

Workstation Workstation

Mini-Computerfile server

Mini-Computerhttp server

Mini-Computer

print server

Processor-Pool Model

• Clients:– They log in one of terminals (diskless

workstations or X terminals)– All services are dispatched to

servers.• Servers:

– Necessary number of processors are allocated to each user from the pool.

• Better utilization but less interactivity

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Server 1

100MbpsLAN

Server N

Cluster Model

• Client– Takes a client-server model

• Server– Consists of many

PC/workstations connected to a high-speed network.

– Puts more focus on performance: serves for requests in parallel.

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100MbpsLAN

Workstation

Workstation Workstation

Masternode

Slave1

SlaveN

Slave2

1Gbps SAN

http server1

http server2

http server N

Grid Computing

• Goal– Collect computing power of supercomputers

and clusters sparsely located over the nation and make it available as if it were the electric grid

• Distributed Supercomputing– Very large problems needing lots of CPU,

memory, etc.• High-Throughput Computing

– Harnessing many idle resources• On-Demand Computing

– Remote resources integrated with local computation

• Data-intensive Computing– Using distributed data

• Collaborative Computing– Support communication among multiple parties

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High-speedInformation high way

Super-computer

Cluster

Super-computer Cluster

Mini-computer

Workstation

Workstation Workstation

Why are Distributed Computing Systems Gaining Popularity?

• Inherently distributed applications– Distributed DB, worldwide airline reservation, banking system

• Information sharing among distributed users– CSCW (Computer Supported Cooperative Works )or groupware

• Resource sharing– Sharing DB/expensive hardware and controlling remote lab. devices

• Better cost-performance ratio / Performance– Emergence of Gbit network and high-speed/cheap MPUs– Effective for coarse-grained or embarrassingly parallel applications

• Reliability– Non-stopping (availability) and voting features.

• Scalability, Extensibility and Incremental Growth– Loosely coupled connection and hot plug-in

• Flexibility– Reconfigure the system to meet users’ requirements

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Distributed Computing Environment (DCE)

What is DCE?DCE ArchitectureDCE Components

DCE • Vendor Independent Environment • Defined by OSF a consortium of computer manufacturers .• Nither OS or an application ,integrated set of services • software that supplies a framework and toolkit for developing

client/server applications• The framework includes a Remote Procedure Call (RPC) mechanism, a naming (directory) service, a time service, authentication service, authorization service Distributed File System.

Distributed Computing Environment

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Platforms

Threads

Distributed File Service

RPC

Security

Name

Distributed Time Service

DCE Applications

DCE

• Support the development of distributed application in a single system

• Platform independent ,runs on all OS and heterogeneous DS

• Middle ware software between OS and networking layer and DCE applications layer.

• Hides the difference between machines by automatically performing data type conversion

DCE Architecture

DCE(Components ) • Fundamental distributed services provide tools for software developers to

create the end-user services needed for distributed computing. They include

– Remote Procedure Call, which provides portability, network independence, and secure distributed applications.

– Directory services, which provides a single naming model to allow programmers and maintainers to identify and access distributed resources more easily.

– Time service, which provides a mechanism to monitor and track clocks in a distributed environment and accurate time stamps to reduce the load on system administrator.

– Security service, which provides the network with authentication, authorization, and user account management services to maintain the integrity, privacy, and authenticity of the distributed system.

– Thread service, which provides a simple, portable, programming model for building concurrent applications.

DCE • Data-sharing services provide end users with capabilities built

upon the fundamental distributed services. These services require no programming on the part of the end user and facilitate better use of information. They include

– Distributed file system, which interoperates with the network file system to provide a high-performance, scalable, and secure file access system.

– Diskless support, which allows low-cost workstations to use disks on servers, possibly reducing the need/cost for local disks, and provides performance enhancements to reduce network overhead.

DCE components

DCE Cell Largest management unit in DCE is cell.(group of users, machines

and other resources) The highest privileges within a cell are assigned to a role called

cell administrator Breakdown a large system into small ,manageable units called cell.

Major components of DCE within every cell are:

– The security server that is responsible for authentication

– The Cell Directory Server (CDS) that is the repository of resources and Access Control Lists

– The Distributed Time Server that provides an accurate clock for proper functioning of the entire cell.

DCECell • Setting up a DCE system ,decide cell boundaries • Four factors1. Purpose :users and m/c’s with same task must be

put in same cells.2. Administration: to register new users in the

system.3. Security : machines of those users have trust in

each other put in same cell4. Overhead : m/c’s of users frequently interacting

put in same cell