CPU Scheduling Algorithm

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Schedulin

g

Algorithm

s1Group No. 2


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Overview

Basic Concepts

Scheduling Criteria

Scheduling Algorithms

Implementation in C++

Demonstration

Involvement of Operating System

2CPU Scheduling Algorithms


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CPU Scheduling Algorithms 3

Basic Concepts

Main objective of multiprogramming is toeep on running processes all the time forma!imum C"# utili$ation%

Scheduling is fundamental function of OS%

&he tas of selecting the processes inmemory that are ready to e!ecute' andallocating them to the C"# is performedby the C"# Scheduler%


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4

CPU Scheduler

C"# scheduling decisions may taeplace (hen a process)o *% S(itches from running to (aiting

stateo % S(itches from running to ready

stateo ,% S(itches from (aiting to readyo -% &erminates

Scheduling under * and - is nonpreemptive%

All other scheduling is preemptive%

CPU Scheduling Algorithms


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5

.onpreemptive

Once a process is allocated the C"#' itdoes not leave unless)o it has to (ait' e%g%' for I/O re0uesto it terminates

"reemptiveo OS can force 2preempt3 a process

from C"# at anytimeo 4%g%' to allocate C"# to another

higher5priority process

CONT


CPU Scheduler


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6

Scheduling Criteria

C"# utili$ation)eep the C"# as busyas possible

Ma!imi$e&hroughput) .o of processes that

complete their e!ecution per time unit

Ma!imi$e&urnaround time) amount of time to

e!ecute a particular process 2timefrom submission to termination3

Minimi$e



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CONT


6aiting time)amount of time a processhas been (aiting in the ready 0ueue2sum of time (aiting in ready 0ueue3o Minimi$e

7esponse time 8 amount of time ittaes from (hen a re0uest (assubmitted until the 9rst response is

produced' not output 2for time5sharing environment3o Minimi$e

Scheduling Criteria


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Scheduling Algorithms

:irst Come' :irst Served

Shortest ;ob :irst

"riority

7ound 7obin



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Class)cpuschedule

Attributes)o n 8 number of processeso Bu< = 8 Array to store Burst

&imeo A< = 8 Array to store Arrival

&imeo 6t< = 8 Array to store 6aiting

&imeo &(t 8 &otal 6aiting &imeo A(t 8 Average 6aiting &ime


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o >etdata23 8 &o get number ofprocesses and Burst &imes fromthe user

o

:cfs23 8 :irst Come' :irst ServedAlgorithm

o Sjf23 8 Shortest ;ob :irst2normal3 Algorithm

o

Sjf"23 8 Shortest ;ob :irst2"reemption3 Algorithm

o Sjf.p23 8 Shortest ;ob :irst 2nonpreemption3 Algorithm

o

"riority23 8 "riority Algorithm

CONT

Operations)


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"rocess Burst &ime

P1 -

P2 ,

P3 ,

Suppose that the processes arrive in theorder)

P1'P2'P3

&he >antt Chart for the schedule is)

6aiting time forP1 ? @P2 ? -P3 ?

Average (aiting time) 2@ + - + 3/, ? *CPU Scheduling Algorithms

irst Come! irstServed

P1

P2

P3

24 27 3""


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Suppose that the processes arrive in theorder )

P2'P3'P12P1:-'P2:,'P3:,3

&he >antt chart for the schedule is)

6aiting time forP1=;P2? @;P3= ,

Average (aiting time) 2 + @ + ,3/, ? ,

Much better than previous case

Convoy eect 5 short process behind long

process

CONT


irst Come irstServed

P1

P3

P2

63 3""


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Shortest "o# irst

.ormal S;:

"rocess Burst &ime

P1

P2 , P3 -

&he >antt Chart for S;: 2.ormal3 is)

Average (aiting time ? 2@ + , + 3/, ?,%,,

P2

P3 P1

730 14


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"rocess Arrival &ime Burst &ime

P1 @%@

P2 %@ -P3 -%@ *

P4 E%@ -

&he >antt Chart for S;: 2non5preemptive3is)

Avera e (aitin time ? @ + + , + -

.on5"reemptive S;:


Shortest "o# irstCONT

P1

P3

P2

72 160

P4

8 124 5


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CONT

"rocess Arrival &ime Burst &imeP1 @%@ P2 %@ -P3 -%@ *P4 E%@ -

&he >antt Chart for S;: 2preemptive3 is)

Average (aiting time ? 2F + * + @ +3/- ? ,

P1 P3P2

42 11"

P4

5 7

P2 P1

16


"reemptive S;:

Shortest "o# irst


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Associate (ith each process the length ofits ne!t C"# burst%

#se these lengths to schedule the process(ith the shortest time%

&(o schemes)o .on5"reemptive) once C"# given to the

process it cannot be preempted untilcompletes its C"# burst%

o "reemptive) if a ne( process arrives (ithC"# burst length less than remaining

time of current e!ecuting process'preempt% &his scheme is no( as theShortest57emaining5&ime5:irst 2S7&:3%

S;: is optimal) gives minimum average(aiting time for a given set of processes%

Shortest "o# irstCONT



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17CPU Scheduling Algorithms

Priorit$

"rocess Burst &ime "riority P1 *@ , P2 * * P3 -

P4 * E P5 E

>antt Chart

Average (aiting time ? 2 + @ + * + *G +

*3/E ? G%

P2 P1P5

61 16"

P3

1

P4

1!


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A priority number 2integer3 is associated(ith each process%Hager the C"# burst lo(er the priority%&he C"# is allocated to the process (ith

the highest priority 2smallest integer

highest priority3Starvation 2In9nity blocing3) lo(

priority processes may never e!ecute%Aging) as time progresses increase the

priority of the process%

Priorit$CONT



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1!CPU Scheduling Algorithms

%ound %o#in

"rocess Burst &imeP1 -

P2 ,

P3 ,

uantum time ? - milliseconds&he >antt chart is)

Average (aiting time ? J


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&ypically' higher average turnaroundthan S;:' but better response

4ach process gets a small unit of C"#time 2time 0uantum3' usually *@5*@@milliseconds% After this time haselapsed' the process is preempted andadded to the end of the ready 0ueue%

"erformance

o0 large :C:S

o0 small 0 must be large (ithrespect to conte!t s(itch' other(iseoverhead is too high

CPU Scheduling Algorithms 2"

%ound %o#inCONT


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Involvement o& OSSource Code '(c)

Conversion

*ecuta#le '(ee)

Compiler

,icroso&t-indows

,icro./ernel

'(i ! (o)

,emor$

CPU

'load eecuta#ledirectl$ to memor$)

*ecut

e

SchedulingAlgorithms


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Scheduling

Algorithm

sGroup No 2 22

0roup No(1

CPU Scheduling Algorithm

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