Ch (1)6cpu Scheduling

8/3/2019 Ch (1)6cpu Scheduling

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CPU Scheduling

Chapter 5


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CPU Scheduling

Basic Concepts

Scheduling Criteria

Scheduling Algorithms Multiple-Processor Scheduling

Thread Scheduling

UNIX example


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Basic Concepts

Maximum CPU utilization obtained

with multiprogramming

CPUI/O Burst Cycle Processexecution consists of a cycle of CPU

execution and I/O wait

CPU times are generally much

shorter than I/O times.


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CPU-I/O Burst Cycle

Process AProcess B


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Histogram of CPU-burst Times


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Schedulers

Process migrates among several queues

Device queue, job queue, ready queue

Scheduler selects a process to run from these queues

Long-term scheduler: load a job in memory

Runs infrequently

Short-term scheduler:

Select ready process to run on CPU Should be fast

Middle-term scheduler

Reduce multiprogramming or memory consumption


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CPU Scheduler

CPU scheduling decisions may take place when aprocess:

1. Switches from running to waiting state (by sleep).

2. Switches from running to ready state (by yield).3. Switches from waiting to ready (by an interrupt).

4. Terminates (by exit).

Scheduling under 1 and 4 is nonpreemptive. All other scheduling is preemptive.


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Dispatcher

Dispatcher module gives control of theCPU to the process selected by the short-term scheduler; this involves: switching context

switching to user mode

jumping to the proper location in the userprogram to restart that program

Dispatch latency time it takes for thedispatcher to stop one process and startanother running


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Scheduling Criteria

CPU utilization keep the CPU as busy as possible

Throughput # of processes that complete their execution

per time unit

Turnaround time (TAT) The interval of time ofsubmission of a process to the time of completion i.e

amount of time to execute a particular process

Waiting time amount of time a process has been waiting

in the ready queue

Response time amount of time it takes from when a

request was submitted until the first response is produced,

not output (for time-sharing environment)


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The perfect CPU scheduler

Minimize latency: response or job completion time

Maximize throughput: Maximize jobs / time.

Maximize utilization: keep I/O devices busy.

Recurring theme with OS scheduling

Fairness: everyone makes progress, no one starves


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Algorithms


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Scheduling Algorithms FCFS

First-come First-served (FCFS) (FIFO) Jobs are scheduled in order of arrival

Non-preemptive

Problem: Average waiting time depends on arrival order

Troublesome for time-sharing systems

Convoy effectshort process behind long process

Advantage: really simple!


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First Come First Served

Scheduling Example: Process Burst Time

P1 24

P2 3

P3 3 Suppose that the processes arrive in the order: P1 ,

P2 , P3

Suppose that the processes arrive in the order: P2 ,P3 , P1 .

P1P3P263 300

P1 P2 P324 27 300

Waiting time forP1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

Waiting time forP1 = 6; P2= 0;P3= 3

Average waiting time: (6 + 0 + 3)/3 = 3


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Shortest-Job-First (SJR)

Scheduling Associate with each process the length of itsnext CPU burst. Use these lengths to schedulethe process with the shortest time

Two schemes: nonpreemptive once CPU given to the process it

cannot be preempted until completes its CPU burst

preemptive if a new process arrives with CPU burstlength less than remaining time of current executingprocess, preempt. This scheme is know as the

Shortest-Remaining-Time-First (SRTF)

SJFisoptimal gives minimum average waitingtime for a given set of processes


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Shortest Job First Scheduling

Example: Process Arrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4 Non preemptive SJF

P1 P3 P2

7

P1(7)

160

P4

8 12

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

2 4 5

P2(4)

P3(1)

P4(4)

P1s wating time = 0

P2s wating time = 6

P3s wating time = 3

P4s wating time = 7


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Contd Example: Process Arrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

Preemptive SJF

P1 P3P2

42 110

P4

5 7

P2 P1

16

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

P1

(7)

P2(4)

P3(1)

P4(4)

P1s wating time = 9

P2s wating time = 1

P3s wating time = 0

P4s wating time = 2

P1(5)

P2(2)


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Contd

Optimal scheduling

However, there are no accurate

estimations to know the length of the next

CPU burst


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Optimal for minimizing queueing time, but impossible to

implement. Tries to predict the process to schedule based on

previous history.

Predicting the time the process will use on its next schedule:

t( n+1 ) = w * t( n ) + ( 1 - w ) * T( n )

Here: t(n+1) is time of next burst.

t(n) is time of current burst.T(n) is average of all previous bursts .

W is a weighting factor emphasizing current or previous

bursts.


Contd


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A priority number (integer) is associated with each process

The CPU is allocated to the process with the highest priority

(smallest integer| highest priority in Unix but lowest in Java).

Preemptive

Non-preemptive

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

burst time.

Problem | Starvation low priority processes may never execute.

Solution |Aging as time progresses increase the priority of the

process.

Priority Scheduling


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Round Robin (RR)

Each process gets a small unit of CPU time(time quantum), usually 10-100 milliseconds.

After this time has elapsed, the process is

preempted and added to the end of the readyqueue.

If there are n processes in the ready queue andthe time quantum is q, then each process gets

1/n of the CPU time in chunks of at most q timeunits at once. No process waits more than (n-1)q time units.


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time quantum = 20

Process Burst Time Wait Time

P1 53 57 +24 = 81

P2 17 20

P3

68 37 + 40 + 17= 94

P4 24 57 + 40 = 97

Round Robin Scheduling

P1 P2 P3 P4 P1 P3 P4 P1 P3 P30 20 37 57 77 97 117 121 134 154 162

Average wait time = (81+20+94+97)/4 = 73

57

20

37

57

24

40

40

17

P1(53)

P2(17)

P3(68)

P4(24)

P1(33) P1(13)

P3(48) P3(28) P3(8)

P4(4)


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Typically, higher average turnaround than SJF, but betterresponse.

Performance

q large FCFS

q small

q must be large with respect to context switch,otherwise overhead is too high.

Round Robin Scheduling


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Turnaround Time Varies With

The Time Quantum

TAT can be improved if most processfinish their next CPU burst in a single

time quantum.


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Multilevel Queue Ready queue is partitioned into separate queues:

EX:

foreground (interactive)background (batch)

Each queue has its own scheduling algorithm

EX foreground RR

background FCFS

Scheduling must be done between 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;

EX

80% to foreground in RR

20% to background in FCFS


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Multilevel Queue Scheduling


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Multi-level Feedback Queues

Implement multiple ready queues Different queues may be scheduled using different algorithms

Just like multilevel queue scheduling, but assignments are not static

Jobs move from queue to queue based on feedback

Feedback = The behavior of the job, EX does it require the full quantum for computation, or

does it perform frequent I/O ?

Need to select parameters for:

Number of queues Scheduling algorithm within each queue

When to upgrade and downgrade a job


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Example of Multilevel Feedback

Queue 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 (RR).If it does not finish in 8 milliseconds, job is moved toqueue Q1.

At Q1job is again served FCFS and receives 16additional milliseconds (RR). If it still does notcomplete, it is preempted and moved to queue Q2.

AT Q2job is served FCFS


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Multilevel Feedback Queues


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Multiple-Processor Scheduling CPU scheduling more complex when multiple

CPUs are available

y Different rules forhomogeneous orheterogeneousprocessors.

Loadsharing in the distribution of work, such that

all processors have an equal amount to do. Asymmetric multiprocessing only one processor

accesses the system data structures, alleviatingthe need for data sharing

Symmetric multiprocessing (SMP) each

processor is self-schedulingy Each processor can schedule from a common ready

queue OR each one can use a separate ready queue.


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Thread Scheduling

On operating system that support threads thekernel-threads (notprocesses) that are beingscheduled by the operating system.

Local Scheduling (process-contention-scopePCS ) How the threads library decides whichthread to put onto an available LWP

PTHREAD_SCOPE_PROCESS

Global Scheduling (system-contention-scopeSCS ) How the kernel decides which kernelthread to run next

PTHREAD_SCOPE_PROCESS


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Linux Scheduling

Two algorithms: time-sharing and real-time

Time-sharing Prioritized credit-based process with most credits is

scheduled next

Credit subtracted when timer interrupt occurs

When credit = 0, another process chosen

When all processes have credit = 0, recrediting occurs

Based on factors including priority and history

Real-time

Defined by Posix.1b Real time Tasks assigned static priorities. All other taskshave dynamic (changeable) priorities.


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The Relationship Between Priorities and

Time-slice length


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Conclusion

Weve looked at a number of different

scheduling algorithms.

Which one works the best is application

dependent.

General purpose OS will use priority based,

round robin, preemptive

Real Time OS will use priority, nopreemption.


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References

Some slides from

Text book slides

Kelvin Sung - University of Washington,

Bothell

Jerry Breecher - WPI

Einar Vollset - Cornell University

Ch (1)6cpu Scheduling

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