CGS 3763 Operating Systems Concepts Spring 2013

Dan C. Marinescu

Office: HEC 304

Office hours: M-Wd 11:30 - 12:30 AM

Last time: Answers to the midterm

Today: Answers from week 7 questions CPU Scheduling

Next time CPU scheduling

Reading assignments Chapter 5 and 6 of the textbook

Lecture 22 – Friday, February 29, 2013

2Lecture 22

Student Review Form 7 Summary BO KANG Feb 18th Monday: Key Points: Threads: single and multithreading processes. Questions: How threads, programs, and processes differ? The classification between user-level threads and kernel-level threads, and

the reasons that make each other unique. Among the different threading APIs, is there one that is more effective than

the other? What is meant by “race” conditions in relation to thread safeness? How do

you detect a race condition, and how are they handled? Can you explain why barrier synchronization is a challenge for multi-

threading? Are there any advantages to single threading over multithreading? Since threads share data, do the other threads have to wait to access the

information if one thread is using the data? How does a programmer know which thread model to use when writing

code?  

Lecture 22 3

Threads versus processes

In the traditional view a process has two roles: consumer of resources, e.g., address space, open files. code executing on a CPU, in one address space, there is a single line of

execution. The multi-threaded view of processes and threads the two roles are distinct:  

A process is a consumer of resources, it owns an address space and a set of open files.

A thread is an instance of code executing sequentially on a CPU. A thread needs an address space in which to execute, so we think of a thread

as belonging to a process. A single process may own many threads, running concurrently within its address

space.   Threads are “lightweight processes” less state information must be kept to

maintain a thread than a process.

 

Lecture 22 4

Threads versus processes Any  applications of threads can be implemented with separate

processes. Advantages of using threads instead of processes:

Threads share data in memory and can communicate without IPC (Inter-Process Communication) mechanisms, which require expensive system calls.

Thread creation and context switching are much less time-consuming than the same operations for processes.

Disadvantages Use of shared memory results in a need to synchronize access to

shared data. Libraries may need to be rewritten to make them “thread-safe”

Lecture 22 5

User and kernel threads User threads – run in user mode, are supported by a user-level thread

library. Kernel threads – run in user mode when executing user functions or

library calls; switch to kernel mode when executing system calls. Provide privileged services to applications e.g., system calls. The entities handled by the system scheduler. Used by the kernel to keep

track of all processes in the system and how much resources are allocated to each process.

When multiple user threads are mapped to a single kernel thread then this thread schedules individual user threads.

A kernel-only thread  - executes only in kernel mode environment.

Lecture 22 6

User and kernel threads (cont’d) Library code decides which user threads the mapping of user to

kernel threads. The use of system calls or other kernel services by an application:

If uses heavily system calls, the more user threads per kernel thread, the slower the applications will run, because the kernel thread becomes a bottleneck, all system calls pass through it.

If it uses rarely system calls, a large number of user threads can be assigned to a kernel thread without much performance penalty, other than the overhead of the context switch.

Increasing the number of kernel threads, adds overhead to the kernel in general, so while individual threads will be more responsive with respect to system calls, the system as a whole will become slower.

It is important to find a good balance between the number of kernel threads and the number of user threads per kernel thread.

Lecture 22 7

Race condition A race condition occurs in concurrent execution of multiple

threads/processes when the output is dependent on the timing or other uncontrollable events.

Example: two threads T1 and T2 which share a variable x.

in T1 there is a statement (SA): x = -2

in T2 there is a statement (SB): x=+17

The result depends the scheduler. If the order of execution is:

SA then SB then the final result is x=+17

SB then SA then the final result is x=-2

Lecture 22 8

Barrier synchronization

A barrier for a group of n threads /processes means any of them must stop and cannot proceed until all other threads/processes reach this barrier.

Barrier synchronization increases the execution time and must be avoided when possible.

Example: 10 threads, 9 of them finish after 10 seconds one needs 100 seconds. the first 9 processes have to wait 90 seconds.

Lecture 22 9

Feb 20th Wednesday: Key Points: Pthreads, Java threads, CPU Scheduling, CPU burst, I/O

burst Questions: Why would you want to use a thread instead of a pthread? If pthreads take

less time, why not just use pthreads instead of threads all the time? Are Pthreads just universally accepted threads used across different OS’s? Pthreads is a library that allows multiple threads to use the same code. Can

multiple threads access the same code simultaneously? Is Pthreads a set of instructions that can be used in any programming

language or what is specifically? What decides what type of scheduling is used, preemptive vs. non-

preemptive)? What are the advantages and disadvantages of preemptive vs. non-preemptive modes?

CPU scheduling, and the two subtypes of scheduling that it also uses, non-preemptive and preemptive. I don’t know what each of the subcategories specifically

Lecture 22 10

Feb 22th Friday: Key Points: CPU scheduling metrics, Scheduling Objectives, Scheduling

Policies – first-come, first-served (FCFS) Questions: What are batch and interactive systems used for? Please further clarify the

difference between interactive and batch processing policies. When to use these different forms of scheduling? Are rules for scheduling polices such as one being faster than another or

when to use one over anther? Can a higher priority thread prevent a lower priority from executing? Which example of process scheduling is most efficient? FCFS, SJF, RR or

priority? Where and when does Marshaling take place? Is it required for all

processes?

Lecture 22 11

Scheduling policies

First-Come First-Serve (FCFS) Shortest Job First (SJF) Round Robin (RR) Priority scheduling

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Desirable properties of scheduling policies Fairness - Prevent starvation – ensure that a process/thread is able to run at

some point in time. Efficiency – context switching takes some time and if occurs very

often the overhead of it lowers the CPU utilization.

Lecture 22 13

Lecture 22

First-Come First-Served (FCFS)

Thread Burst Time

P1 24

P2 3

P3 3

Processes arrive in the order: P1 P2 P3

Gantt Chart for the schedule:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Convoy effect short process behind long process

P1 P2 P3

24 27 300

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Lecture 22

The effect of the release time on FCFS scheduling

Release time time when a job is available.

Now threads arrive in the order: P2 P3 P1 Gantt chart:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better!!

P1P3P2

63 300

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Lecture 22

Shortest-Job-First (SJF)

Use the length of the next burst to schedule the thread/process with the shortest time.

SJF is optimal minimum average waiting time for a given set of threads/processes

Two schemes: Non-preemptive the thread/process cannot be preempted until

completes its burst Preemptive if a new thread/process arrives with burst length less than

remaining time of current executing process, preempt. known as Shortest-Remaining-Time-First (SRTF)

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Lecture 22

Example of non-preemptive SJF

Thread Release time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4 SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

P1 P3 P2

73 160

P4

8 12

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Lecture 22

Example of Shortest-Remaining-Time-First (SRTF) (Preemptive SJF)

Thread Release time Burst time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4 Shortest-Remaining-Time-First

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16

18

Determining length of next CPU Burst

Needed by the SJF algorithm. Can be predicted by using:

1. The past history, the length of previous CPU bursts.

2. Exponential averaging based on the past lengths of the CPU bursts

:Define 4.

10 , 3.

burst CPU next the for value predicted 2.

burst CPU of length actual 1.

1n

thn nt

.1 1 nnn t

Lecture 22 19

Prediction of the Length of the Next CPU Burst

Lecture 22 20

Exponential Averaging =0

n+1 = n

Recent history does not count =1

n+1 = tn

Only the actual last CPU burst counts If we expand the formula, we get:

n+1 = tn+(1 - ) tn -1 + …

+(1 - )j tn -j + …

+(1 - )n +1 0

Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

Lecture 22 21

Lecture 22

Round Robin (RR)

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the thread/process is preempted and added to the end of the ready queue.

If there are n threads/processes in the ready queue and the time quantum is q, then each thread/process gets 1/n of the processor time in chunks of at most q time units at once. No thread/process waits more than (n-1)q time units.

Performance q large FIFO q small q must be large with respect to context switch,

otherwise overhead is too high

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Lecture 22

RR with time slice q = 20

Thread Burst Time

P1 53

P2 17

P3 68

P4 24

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

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Lecture 22

Time slice (quantum) and context switch time

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Comparison of FCFS, SJF, and RR

We have three processes A, B, and C. Each process is characterized by

1. Release time (R) – the time the process arrives in the system

2. Start time (S) – the time the scheduler allows it to run

3. Wait time (W) – the time till it has to wait until it is started: W = S - R

4. Work (Wrk) – the amount of time it needs to use the CPU

5. Finish time (F) – the time the process finishes: F=S+Wrk

6. Time in system (T) – the time from when the process arrives until it finishes: T = F- R.

We compare The average time in system The average waiting time

for three scheduling policies: FCFS, SJF, and RR.

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Lecture 22

Proc Release time

Work Start time Finish time Wait timetill start

Time in system

A 0 3 0 3 0 3

B 1 5 3 3 + 5 = 8 3 – 1 = 2 8 – 1 = 7

C 3 2 8 8 + 2 = 10 8 – 3 = 5 10 – 3 = 7

A 0 3 0 3 0 3

B 1 5 5 5 + 5 = 10 4 10 – 1 = 9

C 3 2 3 3 + 2 = 5 0 5 – 3 = 2

A 0 3 0 6 0 6 – 0 = 6

B 1 5 1 10 1 – 1 = 0 10 – 1 = 9

C 3 2 5 8 5 – 3 = 2 8 – 3 = 5

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Lecture 22

Scheduling policy

Average waiting time till the job started

Average time in system

FCFS 7/3 17/3

SJF 4/3 14/3

RR 3/3 20/3

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Lecture 22

Priority scheduling

Each thread/process has a priority and the one with the highest priority (smallest integer highest priority) is scheduled next. Preemptive Non-preemptive

SJF is a priority scheduling where priority is the predicted next CPU burst time

Problem Starvation – low priority threads/processes may never execute

Solution to starvation Aging – as time progresses increase the priority of the thread/process

Priority my be computed dynamically

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Lecture 22

Priority inversion

A lower priority thread/process prevents a higher priority one from running. T3 has the highest priority, T1 has the lowest priority; T1 and T3 share a lock. T1 acquires the lock, then it is suspended when T3 starts. Eventually T3 requests the lock and it is suspended waiting for T1 to release the

lock. T2 has higher priority than T1 and runs; neither T3 nor T1 can run; T1 due to its low

priority, T3 because it needs the lock help by T1.

Allow a low priority thread holding a lock to run with the higher priority of the thread which requests the lock

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Multilevel queues

Ready queue is partitioned into separate queues: 1. foreground (interactive) 2. background (batch)

Each queue has its own scheduling algorithm foreground – RR background – FCFS

The CPU must be shared among the queues Fixed priority scheduling; (i.e., serve all from foreground then

from background). Possibility of starvation. Time slice – each queue gets a certain amount of CPU time

which it can schedule amongst its processes; i.e., 80% to foreground; RR scheduling algorithm 20% to background; FCFS scheduling algorithm

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Lecture 22 31

Multilevel feedback queue

Multiple queues are defined, each one with its own scheduling strategy and time quantum.

As the process ages it moves amongst the queues. Parameters of the multilevel-feedback-queue scheduler:

number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter

when that process needs service

Lecture 22 32

Three queues: Q0 – RR with time quantum 8 milliseconds

Q1 – RR time quantum 16 milliseconds

Q2 – FCFS Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

Lecture 22 33

Thread scheduling

Distinction between user-level and kernel-level threads Many-to-one and many-to-many models, thread library schedules

user-level threads to run on LWP Known as process-contention scope (PCS) since scheduling

competition is within the process Kernel thread scheduled onto available CPU is system-contention

scope (SCS) – competition among all threads in system

Lecture 22 34

Multiple-Processor Scheduling

CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Asymmetric multiprocessing – only one processor accesses the

system data structures, alleviating the need for data sharing Symmetric multiprocessing (SMP) – each processor is self-

scheduling, all processes in common ready queue, or each has its own private queue of ready processes.

NUMA – Non-Uniform Memory Access. Processor affinity – process has affinity for processor on which it is

currently running soft affinity hard affinity

Lecture 22 35

NUMA and CPU Scheduling

Lecture 22 36

Multicore Processors

Faster and consume less power Multiple threads per core; takes advantage of memory stall to make

progress on another thread while memory retrieve happens

Lecture 22 37

Solaris scheduling

The Solaris 10 kernel threads model consists of the following objects: kernel threads This is what is scheduled/executed on a processor user threads The user-level thread state within a process. process The object that tracks the execution environment of a program. lightweight process (lwp) Execution context for a user thread. Associates

a user thread with a kernel thread. Fair Share Scheduler (FSS) allows more flexible process priority

management. Each project is allocated a certain number of CPU shares via

the project.cpu-shares resource control. Each project is allocated CPU time based on its cpu-shares value divided

by the sum of the cpu-shares values for all active projects. Anything with a zero cpu-shares value will not be granted CPU time until

all projects with non-zero cpu-shares are done with the CPU.

Lecture 22 38

Scheduling classes TS (timeshare): default class for processes and their associated kernel

threads. Priority range 0-59; dynamically adjusted (vary during the lifetime of a process to allocate processor resources evenly.

IA (interactive): enhanced version of the TS class that applies to the in-focus window in the GUI. Its intent is to give extra resources to processes associated with that specific window. Like TS, IA's range is 0-59.

FSS (fair-share scheduler): Share-based rather than priority- based. Threads scheduled based on their associated shares and the processor's utilization. FSS also has a range 0-59.

FX (fixed-priority): The priorities for threads associated with this class are fixed, do not vary dynamically over the lifetime of the thread. Range 0-59.

SYS (system): Used to schedule kernel threads. Threads in this class are "bound" threads, which means that they run until they block or complete. Priorities for SYS threads are in the 60-99 range.

RT (real-time): Threads in the RT class are fixed-priority, with a fixed time quantum. Their priorities range 100-159, so an RT thread will preempt a system thread.

Lecture 22 39

Solaris dispatch table

Lecture 22 40

Solaris scheduling

Lecture 22 41

Linux scheduling Two priority ranges:

time-sharing real-time

Nice is used  Unix and Unix-like operating systems e.g., as Linux Invokes a utility or shell script with a particular priority and gives

a process more or less CPU time than other processes. −20 is the highest priority and 20 is the lowest priority. Default niceness for processes is inherited from its parent process,

usually 0. Real-time range from 0 to 99 and nice value from 100 to 140.

Lecture 22 42

Evaluation of scheduling algorithms

Deterministic modeling –defines the performance of each scheduling algorithm for a particular type of workload

Simulation – using a set of trace data. Data obtained from past execution of a certain type of workload.

Benchmarks –sets of programs to test hardware performance, scheduling algorithms, other types of software.

Lecture 22 43