Preemptive and Non Preemptive Scheduling

8/6/2019 Preemptive and Non Preemptive Scheduling

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Chapter 5 CPU Scheduling

Bernard Chen

Spring 2007


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Outline

Basic Concepts Scheduling Criteria

Scheduling Algorithms


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Basic Concepts The objective of multiprogramming is to

have some process running at all times

Scheduling of this kind is a fundamentaloperating-system function


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CPU-I/O Burst Cycle Process execution consists of a cycle of

CPU execution and I/O request

Process execution begins with a CPUburst and followed by an I/O burst


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CPU Scheduler Whenever the CPU becomes idle, theOS must select one of the processes inthe ready queue to be executed

The selection process is carried out bythe short-term scheduler


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Diagram of Process State


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Preemptive scheduling vs.

non-preemptive schedulingCPU scheduling decisions may take place

when a process:

1. Switches from running to waiting state

2. Terminates3. Switches from waiting to ready

4. Switches from running to ready state


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non-preemptive scheduling When scheduling takes place only under

circumstances 1 and 2, we say that the

scheduling scheme is non-preemptive;otherwise, its called preemptive

Under non-preemptive scheduling, once the

CPU has been allocated to a process, theprocess keep the CPU until it release the CPUeither by terminating or by switching towaiting state. (Windows 95 and earlier)


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non-preemptive scheduling Preemptive scheduling incurs a cost

associated with access to share data.

Consider the case of two processes that sharea data. It is preemptive so that while one

process is updating the data, the secondprocess then tries to read the data, which arein an inconsistent state. (Ch6)


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Dispatcher Dispatcher module gives control of the CPU to

the process selected by the short-termscheduler

It should work as fast as possible, since it isinvoked during every process switch

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


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Scheduling CriteriaCPU utilization keep the CPU as busy as possible (from 0% to

100%)

Throughput # of processes that complete their execution pertime unit

Turnaround time amount of time to execute a particularProcess

Waiting time amount of time a process has been waiting in theready queue

Response time amount of time it takes from when a request wassubmitted until the first response is produced


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Optimization Criteria Max CPU utilization

Max throughput Min turnaround time

Min waiting time

Min Response time


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Scheduling Algorithems First Come First Serve Scheduling

Shortest Job First Scheduling Priority Scheduling

Round-Robin Scheduling

Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling


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First Come First Serve

Scheduling (FCFS)Process Burst time

P1 24

P2 3

P2 3


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Scheduling The average of waiting time in this

policy is usually quite long

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

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


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Scheduling Suppose we change the order of

arriving job P2, P3, P1


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Scheduling Consider if we have a CPU-bound process and

many I/O-bound processes

There is a convoy effect as all the otherprocesses waiting for one of the big processto get off the CPU

FCFS scheduling algorithm is non-preemptive


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Short job first scheduling (SJF) This algorithm associates with each

process the length of the processes

next CPU burst

If there is a tie, FCFS is used

In other words, this algorithm can bealso regard as shortest-next-cpu-burstalgorithm


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Short job first scheduling SJF is optimal gives minimum average

waiting time for a given set ofprocesses


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ExampleProcesses Burst time

P1 6

P2 8

P3 7

P4 3

FCFS average waiting time: (0+6+14+21)/4=10.25

SJF average waiting time: (3+16+9+0)/4=7


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Short job first schedulingTwo schemes:Non-preemptive once CPU given to the

process it cannot be preempted untilcompletes its CPU burst

Preemptive if a new process arrives with CPUburst length less than remaining time of

current executing process, preempt. Thisscheme is know as the Shortest-Remaining-Time-First (SRTF)


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Short job first scheduling-

Non-preemptive


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Short job first scheduling-

Preemptive


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Priority SchedulingA priority number (integer) is associated with each

processThe CPU is allocated to the process with the highest

priority(smallest integer highest priority) Preemptive Non-preemptive

SJF is a special priority scheduling where priority is thepredicted next CPU burst time, so that it can decidethe priority


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Priority SchedulingProcesses Burst time Priority Arrival time

P1 10 3

P2 1 1

P3 2 4

P4 1 5

P5 5 2The average waiting

time=(6+0+16+18+1)/5=8.2


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Priority SchedulingProcesses Burst time Priority Arrival time

P1 10 3 0.0

P2 1 1 1.0P3 2 4 2.0

P4 1 5 3.0

P5 5 2 4.0

Gantt chart for both preemptive and non-preemptive, also waiting time


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Priority SchedulingProblem : Starvation low priority

processes may never execute

Solution : Aging as time progressesincrease the priority of the process


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Round-Robin Scheduling The Round-Robin is designed especially

for time sharing systems.

It is similar FCFS but add preemptionconcept

A small unit of time, called time

quantum, is defined


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Round-Robin Scheduling Each process gets a small unit of CPU

time (time quantum), usually 10-100

milliseconds. After this time haselapsed, the process is preempted andadded to the end of the ready queue.


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Round-Robin Scheduling If there are n processes in the ready

queue and the time quantum is q, then

each process gets 1/n of the CPU timein chunks of at most q time units atonce. No process waits more than

(n-1)q time units.


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Round-Robin SchedulingPerformance

q large => FIFO

qsmall => qmust be large withrespect to context switch, otherwiseoverhead is too high

Typically, higher average turnaroundthan SJF, but better response


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Preemptive and Non Preemptive Scheduling