CPU scheduling1

8/8/2019 CPU scheduling1

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CPU SCHEDULING


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PROCESS CONCEPT

An operating system executes a variety of

programs:

y Batch system jobs

y Time-shared systems user programs or tasks

Process a program in execution; processexecution must progress in sequential fashion.

Operating System Concepts


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PROCESS STATE

As a process executes, it changes statey new: The process is being created.

y running: Instructions are being executed.

y waiting: The process is waiting for some event to

occur.

y r

eady: The process is waiting to be assigned to aprocess.

y terminated: The process has finished execution.



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PROCESS CONTROL BLOCK(PCB)Information associated with each process.

Process state

Program counter

CPU registers

CPU scheduling information

Memory-management information

Accounting information

I/O status information



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PROCESS SCHEDULING QUEUES Job queue set of all PCBs in the system.

Ready queue set of all processes residing in mainmemory, ready and waiting to execute.

Device queues set of processes waiting for an I/Odevice.

PCB migration between the various queues.



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READYQUEUEANDVARIOUS I/O DEVICE QUEUES



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REPRESENTATION OF PROCESS

SCHEDULING



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CPU SWITCH FROM PROCESS TO PROCESS



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Long term scheduling: which determines whichprograms are admitted to the system for execution andwhen, and which ones should be exited.(disk tomemory)

Medium term scheduling: which determines whenprocesses are to be suspended and resumed;

Short term scheduling (or dispatching): whichdetermines which of the ready processes can have CPU

resources, and for how long.(disk to CPU)


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DISPATCHER Dispatcher module gives control of the CPU to

the process selected by the CPU scheduler.

This involves:

y Switching context

y Switching to user modey Setting the PC

Dispatch latency is the time taken to stop one

process and start another



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CPU SCHEDULING Basic Concepts

Scheduling Criteria

Scheduling Algorithms

Thread Scheduling

Multiple-Processor Scheduling

Operating Systems Examples

Algorithm Evaluation


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OBJECTIVES

To introduce CPU scheduling, which is the basis

for multiprogrammed operating systems

To describe various CPU-scheduling algorithms

To discuss evaluation criteria for selecting aCPU-scheduling algorithm for a particular

system


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CPU scheduling determines which process to run

when there are multiple runnable processes

CPU scheduling is important because it can have

big effect on resource utilization and overallperformance of the system


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ALTERNATING SEQUENCE OF CPU AND I/O

BURSTS


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CPU SCHEDULER

Selects from among the processes in memory that

are ready to execute, and allocates the CPU to

one of them

CPU scheduling decisions may take place when aprocess:

1. Switches from running to waiting state

2. Switches from running to ready state

3. Switches from waiting to ready

4. Terminates

Scheduling under 1 and 4 is nonpreemptive

All other scheduling is preemptive


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SCHEDULING CRITERIA CPUutilization keep the CPU as busy as

possible

Throughput # of processes that complete

their execution per time unit

Turnaround time amount of time to executea particular process

Waiting time amount of time a process has

been waiting in the ready queue

Response time amount of time it takes fromwhen a request was submitted until the first

response is produced, not output (for time-

sharing environment)


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SCHEDULINGALGORITHM OPTIMIZATION

CRITERIA

Max CPU utilization

Max throughput

Min turnaround time

Min waiting time Min response time


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FIRST-COME, FIRST-SERVED (FCFS) SCHEDULING

Process Burst Time

P1 24

P2 3

P3 3

Suppose that the processes arrive in the order: P1, P2 , P3The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3= 27

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

P1 P2 P3

24 27 300


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FCFS SCHEDULING (CONT)

Suppose that the processes arrive in the order

P2 , P3 , P1

The Gantt chart for the schedule is:

Waiting time for P1=6;P2= 0; P3=3

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

Much better than previous case

Convoy effect short process behind long process

P1P3P2

63 300


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SHORTEST-JOB-FIRST (SJF) SCHEDULING

Associate with each process the length of its next

CPU burst. Use these lengths to schedule the

process with the shortest time

SJF is optimal gives minimum average waitingtime for a given set of processes

y The difficulty is knowing the length of the next CPU

request


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BURST

Can only estimate the length

Can be done by using the length of previous CPU

bursts, using exponential averaging

:Define4.

10,3.burstCPUnexttheforvaluepredicted2.

burstCPUoflengthactual1.

ee

!

!

EE

X 1n

th

n nt

.11 nnn

t XEEX !!


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EXAMPLES OF EXPONENTIALAVERAGING

E =0

y Xn+1 =Xny Recent history does not count

E =1

y Xn+1 =E tny Only the actual last CPU burst counts

If we expand the formula, we get:Xn+1 =E tn+(1 - E)E tn -1 +

+(1 - E )jE tn -j

+

+(1 - E )n +1 X0

Since both E and (1 - E) are less than or equal to1, each successive term has less weight than itspredecessor


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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 ready

queue. If there are n processes in the ready queue

and the time quantum is q, then each process

gets 1/n of the CPU time in chunks of at most

q time units at once. No process waits more

than (n-1)q time units.

Performance

y q large FIFO

y q small q must be large with respect to context

switch, otherwise overhead is too high


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EXAMPLE OF RR WITH TIME QUANTUM = 4

Process Burst Time

P1 24

P2 3

P3 3

The Gantt chart is:

Typically, higher average turnaround than SJF,

but better response

P1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30


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TIME UANTUM AND ONTEXT WITCH

TIME


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MULTILEVEL QUEUE

Ready queue is partitioned into separate queues:

foreground (interactive)

background (batch)

Each queue has its own scheduling algorithmy foreground RR

y background FCFS

Scheduling must be done between the queues

y Fixed priority scheduling; (i.e., serve all from

foreground then from background). Possibility of

starvation.

y Time slice each queue gets a certain amount of

CPU time which it can schedule amongst its

processes; i.e., 80% to foreground in RR

y 20% to background in FCFS


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Batch system : In batch systems,typically want

good throughput or trunaround time.

Intractive system: In this systems both of this are

important but response time is given primaryconsideration


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MULTILEVEL QUEUE SCHEDULING


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MULTILEVEL FEEDBACKQUEUE

A process can move between the various queues;

aging can be implemented this way

Multilevel-feedback-queue scheduler defined by

the following parameters:

y number of queues

y scheduling algorithms for each queue

y method used to determine when to upgrade a

process

y

method used to determine when to demote a processy method used to determine which queue a process

will enter when that process needs service

E M F


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EXAMPLE OF MULTILEVEL FEEDBACK

QUEUE

Three queues:

y Q0 RR with time quantum 8 milliseconds

y Q1 RR time quantum 16 milliseconds

y Q2 FCFS

Scheduling

y 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.y At Q1job is again served FCFS and receives 16

additional milliseconds. If it still does not complete,

it is preempted and moved to queue Q2.


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MULTILEVEL FEEDBACKQUEUES


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END

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