OperatingSystems

OPERATING SYSTEMS:

Lesson 4:Process Scheduling

1

Jesús Carretero PérezDavid Expósito SinghJosé Daniel García SánchezFrancisco Javier García BlasFlorin Isaila

OperatingSystems

Content

• Processcreation.• Processtermination.• Processlifecycle.• Kindsofscheduling.• Schedulingalgorithms.

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OperatingSystems

Processcreation

• OSprovidesmechanismtoallowaprocesstocreateotherprocesses→Systemcall.

• Processcreationcanberepeatedrecursivelyleadingtoa“familystructure” →Processtree.

• Resourceallocationfornewprocess:– DirectlyobtainedfromtheOS.– Parentmustgiveoutitsresourcestothechildprocessorsharepart

ofthemwithit.• Toavoidasystemhangbyindefinitelyreplicatingtheprocessresources.

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OperatingSystems

Processhierarchy(pstree)

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OperatingSystems

Processcreation

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• Whenaprocessiscreated:

– Intermsofexecution:• Parentprocessrunsinparallelwithchildren.• Parentprocesswaitsuntilsomeorallofitschildrenhaveterminated.

– Intermsofmemoryspace:• Childprocessisacloneoftheparentprocess.• Childprocessisconvertedintoanotherprogramloadedinmemory.

OperatingSystems

UNIXprocesscreation

¨Unixfamilymakesdistinctionbetweenprocesscreationandnewprogramexecution.

¨ Systemcalltocreateanewprocessisfork()

¨ Thissystemcallcreatesacopyalmostidenticaloftheparentprocess.¨ Bothprocesses,parentandchild,

continueexecutioninparallel.¨ Parentgetsasaresultfromthefork()

callthechildPIDandchildgetsa0.¨ Someresourcesarenotinherited

(e.g.:pendingsignals).

Process HProcess P

Kernel

fork()pid H

0

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OperatingSystems

Linuxprocesscreation

Get free entry in Process Table

Copy PCB from parent

fork:

Duplicate Memory Map from parent (including stacks)

State ß ready

PCB in ready queue

Return PID to parent

Return 0 to child

“Copies parent process and gives new identity to

child”

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Also clean up signals, events,…

OperatingSystems

UNIXprocesscreation

• Childprocessmayinvoketheexec*() systemcall.– Changesitsmemoryimagewithadifferentprogram.

• Parentmaycontinuecreatingmorechildren,orwaitinguntilthecreatedchildfinishes.– wait() takestheprocessfromthe“Ready”queueuntilthechildhasterminated.

shProcess H

Kernel Diskvi

exec()

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OperatingSystems

ProcesscreationinLinux

exec:

“Change memory image from a process using as a

new container a new one”

Free process memory image

Read executable

Create new image M à PCB

Load .text and .data sections

Create initial U stackCreate S stack: dir.

program start

Init PCB: regs.; PC ß dir OS RETI

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OperatingSystems

CopyonWrite(COW)

¨ Inefficienciesofthefork()model:¨ Alotofdataiscopied,butdatacouldbe

sharedinstead.¨ Iffinallyanothermemoryimageis

loaded,isevenworseasallcopiesarediscarded

¨ManyUNIXsystemsuseCOW:¤ Copy-on-Write isatechniquetodelayor

avoidcopyingdatawhenperformingafork.

¤ DatapagesaresharedandmarkedasCOWsothatifamodification isapplied,thenacopyisdonetheprocess(parentandchild).

¤ Nowfork()onlycopiesthepagetablefromparent(butnotthepages)andcreatesanewPCBforchild.

Memory pages ofParent process

Page table in parent

process

PCB parent PCB child

Sharing go avoid duplication

Page table in child

process

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OperatingSystems

Content

• Processcreation.• Processtermination.• Processlifecycle.• Kindsofscheduling.• Schedulingalgorithms.

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OperatingSystems

Processtermination

• When aprocessfreesallitsallocatedresources.– Memory,openfiles,entriesintables,…

• andthekernelnotifiesthattoparentprocess.

• Aprocessmayterminatein2ways:• Voluntarily:exit()systemcall.• Involuntarily:

– Exceptions:dividebyzero,segmentviolation,…– Abortedbyuser(ctrl-c)orotherprocess(kill)

» i.e.:signalsthatcannotbehandledorignored.

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OperatingSystems

Processtermination

• Whenaprocessterminatestwooutcomesarepossible:– Itschildrenarenotaffected.– Allchildrenalsoterminate →cascadetermination(e.g.VMSoperatingsystem)

• In Unix,– Terminatedchildprocessesarenowchildrenoftheinitprocess.

– Terminatedprocesschangestozombie stateuntilparentprocessgetsitsterminationcode.

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OperatingSystems

WhenisPCBeliminated?

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• ProcessterminationandPCBeliminationaretwodifferenttasks:

– Whenparentgetsinformationfromchild,datastructurescanberemoved.

– wait()systemcall:• Blocksprocessuntilachildterminates.• ReturnsPID oftheterminatedchild.

Code1.c

OperatingSystems

Content

• Processcreation.• Processtermination.• Processlifecycle.• Kindsofscheduling.• Schedulingalgorithms.

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OperatingSystems

Processbasiclifecycle

Running

BlockedReady

Wait for event

End of blocking by event

End of slice

Dispatching

As many as processors

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New Processes

Termination

OperatingSystems

• Information associated with each process• Process state• Program counter• CPU registers• CPU scheduling information• Memory-management information• Accounting information• I/O status information

• PCBs are stored in a global process table

Process Control Block (PCB)

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OperatingSystems

Ready Queue And Various I/O Device Queues

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OperatingSystems

Spawningtodisk(swap)

• Whenthere aremanyprocessinexecution,performancemaydegradeduetoexcessivememorypaging.– Solution:OperatingSystemmayneedtosendaprocesstotheswapareaindisk.

• Newprocessstates.– Blockedandsuspended.– Readyandsuspended.

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OperatingSystems

Processlifecycle

Running

BlockedReady

Wait for event

End of blocking by event

End ofslice

Dispaching

As many as processors

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New Processes

Termination

Ready andSuspended

Blocked and Suspended

Suspension SuspensionRecovery

End of blocking by event

OperatingSystems

Content

• Processcreation.• Processtermination.• Processlifecycle.• Kindsofscheduling.• Schedulingalgorithms.

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OperatingSystems

BasicConcepts

• MaximumCPUutilizationobtainedwithmultiprogramming– HavingwhenpossibleatanytimetheCPUusedbyaprocess

• CPU–I/OBurstCycle– Processexecutionconsistsofacycle ofCPUexecutionandI/Owait

• CPUburstdistribution

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OperatingSystems

AlternatingSequenceofCPUAndI/OBursts

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OperatingSystems

Schedulinglevels

¨Shorttermscheduling:¤Selectsnextprocesstoexecute.

¨Mediumtermscheduling:¨ Selectprocesstobeaddedorretired(suspendedtoswap)

frommainmemory.

¨Longtermscheduling:¤Performadmissioncontrolofprocesses.¤Veryusedinbatchsystems.

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OperatingSystems

Kindsofscheduling

• Non-preemptive.– ProcessinexecutionkeepsrunningonCPUwhileitisabletousetheCPU.

• Preemptive:– OperatingSystemmayacquiretheCPU,evicttheprocessandchangetheexecutiontoanotherprocess.

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OperatingSystems

Schedulingdecisionpoints

• PointsintimewhenOSmayperformprocessscheduling:• Whenaprocessblockswaitingforanevent.

• Performasystemcall.• I/Orequest• Wait()invocation

• Whenaninterrupthappens.• Clockinterrupt.• I/Ointerrupt.

• Whenaprocessswitchesfromwaitingstatetoreadystate• I/Ocompletion

• Whenaprocess ends.• Nonpreemptive scheduling:1and4.

• Windows95,MacOS before8.• Preemptivescheduling:1,2,3,and4.

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OperatingSystems

Dispatcher

• DispatchermodulegivescontroloftheCPUtotheprocessselectedbytheshort-termscheduler;thisinvolves:– switchingcontext– switchingtousermode– jumpingtotheproperlocationintheuserprogramtorestart

thatprogram

• Dispatchlatency– timeittakesforthedispatchertostoponeprocessandstartanotherrunning

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OperatingSystems

Processqueues

• Readyprocessesarekeptinaqueue.

• Alternatives:– Singlequeue.– Queuesbytypesofprocesses.– Priorityqueues.

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OperatingSystems

Content

• Processcreation.• Processtermination.• Processlifecycle.• Kindsofscheduling.• Schedulingalgorithms.

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OperatingSystems

Scheduling:metrics

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• CPUutilization:• PercentageoftimeCPUisused.• Goal:Maximize.

• Throughput:• Numberofjobsfinishedbyunitoftime.• Goal:Maximize.

• Turnaroundtime(Tq)• Overalltimeaprocessisinsystem.• Tq =Tf – Ti

• Tf:Finalizationtime.• Ti:Initiationtime.

• Goal:Minimize.

OperatingSystems

Scheduling:Metrics

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• Servicetime(Ts):• Timedevotedtoproductivetasks(cpu,I/O).• Ts=Tcpu+TI/O

• Waitingtime(Tw):• Timeaprocessspendsinwaitingqueues.• Tw=Tq – Ts

• NormalizedTurnaroundtime(Tn):• RatiobetweenTurnaroundtimeandservicetime.• Tn =Tq/Ts• Indicationofexperienceddelay.

OperatingSystems

FCFS

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• FirsttoComeFirsttoServe.– Non-preemptive.– Penalizesshortprocesses:Convoyeffect shortprocessbehindlongprocess

Process

Arrival Service

A 0 3B 2 6C 4 4D 6 5E 8 2

A A AB B B B B B

C C C CD D D D D

E E

OperatingSystems

FCFS:NormalizedTurnaroundTime

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Process

Arrival Service Init End Turnaround

Wait NormalizedTurnaround

A 0 3 0 3 3 0 3/3=1B 2 6 3 9 7 1 7/6=1.16C 4 4 9 13 9 5 9/4=1.25D 6 5 13 18 12 7 12/5=2.4E 8 2 18 20 12 10 12/2=6

• AverageTurnaroundtime:4.6• AveragenormalizedTurnaroundtime:2.5

OperatingSystems

SJF

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• ShortestJobFirst.• Non-preemptivealgorithm.• Selectsshortestjob.• Itcanonlybeappliedifdurationofeachjobisknown

beforehand.• SJFisoptimal– givesminimumaveragewaitingtimefora

givensetofprocesses– ThedifficultyisknowingthelengthofthenextCPUrequest

• Starvationpossibility:– Ifshortjobsarecontinuouslyarriving,longerjobsneverarein

positiontobeexecuted.

OperatingSystems

SJF

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A A A

B B B B B B

E E

C C C C

D D D D D

Process

Arrival Service Init End Turnaround

Wait Normalized Turnaround

A 0 3 0 3 3 0 3/3=1B 2 6 3 9 7 1 7/6=1.16C 4 4 11 15 11 7 11/4=2.75D 6 5 15 20 14 9 14/5=2.8E 8 2 9 11 3 1 3/2=1.5

3.6 1.84

OperatingSystems

CyclicorRound-Robin

• KeepsaFIFOqueuewithprocessesready torun.• Aprocessisallocatedinto aprocessorforatimeslice.

• 10-100milliseconds• Aprocessgoesbacktotheready queuewhen:

• Itstimesliceexpires.• Aneventthattookittotheblockedqueuehappens.

• Aprocessgoestotheblocked queuewhen:• Startswaitingforanevent.

• Itisapreemptivealgorithm.• Itisimportanttoremindthateverycontextswitchleadstoadelay:

• Timeslice>>Contextswitchtime• Performance

– q large⇒ FIFO– qsmall⇒ qmustbelargewithrespecttocontextswitch,otherwiseoverheadis

toohigh

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OperatingSystems

Round-Robin(q=1)

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A A A

B B B B B B

EE

C C C C

DD D D D

Process

Arrival Service Init End Turnaround

Wait NormalizedTurnaround

A 0 3 0 4 4 1 4/3=1.33B 2 6 2 18 16 10 16/6=2.66C 4 4 5 17 13 9 13/4=3.25D 6 5 7 20 14 9 14/5=2.8E 8 2 10 15 7 5 7/2=3.5

6.8 2.71

OperatingSystems

Round-Robin(q=2)

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A A A

B B B B B B

EE

C C C C

DD D D D

Process

Arrival Service Init End Turnaround

Wait NormalizedTurnaround

A 0 3 0 5 4 1 4/3=1.33B 2 6 2 17 16 10 16/6=2.66C 4 4 5 13 13 9 13/4=3.25D 6 5 9 20 14 9 14/5=2.8E 8 2 13 15 7 5 7/2=3.5

6 2.54

OperatingSystems

Round-Robin(q=4)

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A A AB B B B B B

EE

C C C CDD D D D

Process

Arrival Service Init End Turnaround

Wait NormalizedTurnaround

A 0 3 0 3 3 0 3/3=1B 2 6 3 17 15 9 15/6=2.5C 4 4 7 11 7 3 7/4=1.75D 6 5 11 20 14 9 14/5=2.8E 8 2 17 19 11 9 11/2=5.5

6 2.71

OperatingSystems

PriorityScheduling

• Eachprocesshasapriorityassignedtoit.• Selectfirstprocesseswithhigherpriority.

– Preemptive– nonpreemptive

• Alternatives:– Fixedprioritiesà starvationproblem– lowpriorityprocessesmayneverexecute.

– Solution:agingmechanisms– astimeprogressesincreasethepriorityoftheprocess

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OperatingSystems

SchedulinginWindows

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• Maincharacteristics:• Prioritybased• Usestimeslices.• Preemptivescheduling.• Schedulingwithprocessoraffinity.

• Schedulingatthreadlevel(notprocesslevel).

• Athreadmayloosetheprocessorifanotherwithhigherprioritybecomesready.

• Schedulingdecisions:• Newthreadsà Ready.• Blockedthreadsreceivingitsevent à Ready.• Threadleavesprocessoriftimesliceexpires,itterminatesorbecomesblocked.

OperatingSystems

Summary

• ProcesscreationimpliesmemoryimageandPCBcreation.• Aprocesstransitionsthroughdifferentstatesduringits

execution.• Operatingsystemisresponsibleforprocessscheduling.• Schedulingmaybepreemptiveornon-preemptive.• Differentprocessschedulingalgorithmsmayfavoracertain

typeofprocesses.• ModernOperatingsystemsusepreemptivescheduling.

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OperatingSystems

OPERATING SYSTEMS:

Lesson 4:Process Scheduling

43

Jesús Carretero PérezDavid Expósito SinghJosé Daniel García SánchezFrancisco Javier García BlasFlorin Isaila