operating systems_ cpu scheduling

2/19/2015 OperatingSystems:CPUScheduling

http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/5_CPU_Scheduling.html 1/20

CPUSchedulingReferences:

1. AbrahamSilberschatz,GregGagne,andPeterBaerGalvin,"OperatingSystemConcepts,EighthEdition",Chapter5

5.1BasicConcepts

AlmostallprogramshavesomealternatingcycleofCPUnumbercrunchingandwaitingforI/Oofsomekind.(EvenasimplefetchfrommemorytakesalongtimerelativetoCPUspeeds.)Inasimplesystemrunningasingleprocess,thetimespentwaitingforI/Oiswasted,andthoseCPUcyclesarelostforever.AschedulingsystemallowsoneprocesstousetheCPUwhileanotheriswaitingforI/O,therebymakingfulluseofotherwiselostCPUcycles.Thechallengeistomaketheoverallsystemas"efficient"and"fair"aspossible,subjecttovaryingandoftendynamicconditions,andwhere"efficient"and"fair"aresomewhatsubjectiveterms,oftensubjecttoshiftingprioritypolicies.

5.1.1CPUI/OBurstCycle

Almostallprocessesalternatebetweentwostatesinacontinuingcycle,asshowninFigure5.1below:ACPUburstofperformingcalculations,andAnI/Oburst,waitingfordatatransferinoroutofthesystem.

CPUburstsvaryfromprocesstoprocess,andfromprogramtoprogram,butanextensivestudyshows



frequencypatternssimilartothatshowninFigure5.2:

5.1.2CPUScheduler

WhenevertheCPUbecomesidle,itisthejoboftheCPUScheduler(a.k.a.theshorttermscheduler)toselectanotherprocessfromthereadyqueuetorunnext.ThestoragestructureforthereadyqueueandthealgorithmusedtoselectthenextprocessarenotnecessarilyaFIFOqueue.Thereareseveralalternativestochoosefrom,aswellasnumerousadjustableparametersforeachalgorithm,whichisthebasicsubjectofthisentirechapter.

5.1.3.PreemptiveScheduling

CPUschedulingdecisionstakeplaceunderoneoffourconditions:1. Whenaprocessswitchesfromtherunningstatetothewaitingstate,suchasforanI/Orequestor

invocationofthewait()systemcall.2. Whenaprocessswitchesfromtherunningstatetothereadystate,forexampleinresponsetoan

interrupt.3. Whenaprocessswitchesfromthewaitingstatetothereadystate,sayatcompletionofI/Oorareturn

fromwait().4. Whenaprocessterminates.

Forconditions1and4thereisnochoiceAnewprocessmustbeselected.Forconditions2and3thereisachoiceToeithercontinuerunningthecurrentprocess,orselectadifferentone.Ifschedulingtakesplaceonlyunderconditions1and4,thesystemissaidtobenonpreemptive,orcooperative.Undertheseconditions,onceaprocessstartsrunningitkeepsrunning,untiliteithervoluntarilyblocksoruntilitfinishes.Otherwisethesystemissaidtobepreemptive.WindowsusednonpreemptiveschedulinguptoWindows3.x,andstartedusingpreemptiveschedulingwithWin95.MacsusednonpreemptivepriortoOSX,andpreemptivesincethen.Notethatpreemptiveschedulingisonlypossibleonhardwarethatsupportsatimerinterrupt.Notethatpreemptiveschedulingcancauseproblemswhentwoprocessessharedata,becauseoneprocessmaygetinterruptedinthemiddleofupdatingshareddatastructures.Chapter6willexaminethisissueingreaterdetail.Preemptioncanalsobeaproblemifthekernelisbusyimplementingasystemcall(e.g.updatingcriticalkerneldatastructures)whenthepreemptionoccurs.MostmodernUNIXesdealwiththisproblembymaking



theprocesswaituntilthesystemcallhaseithercompletedorblockedbeforeallowingthepreemptionUnfortunatelythissolutionisproblematicforrealtimesystems,asrealtimeresponsecannolongerbeguaranteed.Somecriticalsectionsofcodeprotectthemselvesfromconcurrencyproblemsbydisablinginterruptsbeforeenteringthecriticalsectionandreenablinginterruptsonexitingthesection.Needlesstosay,thisshouldonlybedoneinraresituations,andonlyonveryshortpiecesofcodethatwillfinishquickly,(usuallyjustafewmachineinstructions.)

5.1.4Dispatcher

ThedispatcheristhemodulethatgivescontroloftheCPUtotheprocessselectedbythescheduler.Thisfunctioninvolves:

Switchingcontext.Switchingtousermode.Jumpingtotheproperlocationinthenewlyloadedprogram.

Thedispatcherneedstobeasfastaspossible,asitisrunoneverycontextswitch.Thetimeconsumedbythedispatcherisknownasdispatchlatency.

5.2SchedulingCriteria

Thereareseveraldifferentcriteriatoconsiderwhentryingtoselectthe"best"schedulingalgorithmforaparticularsituationandenvironment,including:

CPUutilizationIdeallytheCPUwouldbebusy100%ofthetime,soastowaste0CPUcycles.OnarealsystemCPUusageshouldrangefrom40%(lightlyloaded)to90%(heavilyloaded.)ThroughputNumberofprocessescompletedperunittime.Mayrangefrom10/secondto1/hourdependingonthespecificprocesses.TurnaroundtimeTimerequiredforaparticularprocesstocomplete,fromsubmissiontimetocompletion.(Wallclocktime.)WaitingtimeHowmuchtimeprocessesspendinthereadyqueuewaitingtheirturntogetontheCPU.

(LoadaverageTheaveragenumberofprocessessittinginthereadyqueuewaitingtheirturntogetintotheCPU.Reportedin1minute,5minute,and15minuteaveragesby"uptime"and"who".)

ResponsetimeThetimetakeninaninteractiveprogramfromtheissuanceofacommandtothecommenceofaresponsetothatcommand.

Ingeneralonewantstooptimizetheaveragevalueofacriteria(MaximizeCPUutilizationandthroughput,andminimizealltheothers.)Howeversometimesonewantstodosomethingdifferent,suchastominimizethemaximumresponsetime.Sometimesitismostdesirabletominimizethevarianceofacriteriathantheactualvalue.I.e.usersaremoreacceptingofaconsistentpredictablesystemthananinconsistentone,evenifitisalittlebitslower.

5.3SchedulingAlgorithms

Thefollowingsubsectionswillexplainseveralcommonschedulingstrategies,lookingatonlyasingleCPUbursteachforasmallnumberofprocesses.ObviouslyrealsystemshavetodealwithalotmoresimultaneousprocessesexecutingtheirCPUI/Oburstcycles.

5.3.1FirstComeFirstServeScheduling,FCFS

FCFSisverysimpleJustaFIFOqueue,likecustomerswaitinginlineatthebankorthepostofficeoratacopyingmachine.Unfortunately,however,FCFScanyieldsomeverylongaveragewaittimes,particularlyifthefirstprocesstogettheretakesalongtime.Forexample,considerthefollowingthreeprocesses:

Process BurstTime

P1 24

P2 3

P3 3

InthefirstGanttchartbelow,processP1arrivesfirst.Theaveragewaitingtimeforthethreeprocessesis(0+24+27)/3=17.0ms.InthesecondGanttchartbelow,thesamethreeprocesseshaveanaveragewaittimeof(0+3+6)/3=3.0ms.Thetotalruntimeforthethreeburstsisthesame,butinthesecondcasetwoofthethreefinishmuchquicker,andtheotherprocessisonlydelayedbyashortamount.



FCFScanalsoblockthesysteminabusydynamicsysteminanotherway,knownastheconvoyeffect.WhenoneCPUintensiveprocessblockstheCPU,anumberofI/Ointensiveprocessescangetbackedupbehindit,leavingtheI/Odevicesidle.WhentheCPUhogfinallyrelinquishestheCPU,thentheI/OprocessespassthroughtheCPUquickly,leavingtheCPUidlewhileeveryonequeuesupforI/O,andthenthecyclerepeatsitselfwhentheCPUintensiveprocessgetsbacktothereadyqueue.

5.3.2ShortestJobFirstScheduling,SJF

TheideabehindtheSJFalgorithmistopickthequickestfastestlittlejobthatneedstobedone,getitoutofthewayfirst,andthenpickthenextsmallestfastestjobtodonext.(TechnicallythisalgorithmpicksaprocessbasedonthenextshortestCPUburst,nottheoverallprocesstime.)Forexample,theGanttchartbelowisbaseduponthefollowingCPUbursttimes,(andtheassumptionthatalljobsarriveatthesametime.)

Process BurstTime

P1 6

P2 8

P3 7

P4 3

Inthecaseabovetheaveragewaittimeis(0+3+9+16)/4=7.0ms,(asopposedto10.25msforFCFSforthesameprocesses.)SJFcanbeproventobethefastestschedulingalgorithm,butitsuffersfromoneimportantproblem:HowdoyouknowhowlongthenextCPUburstisgoingtobe?

Forlongtermbatchjobsthiscanbedonebaseduponthelimitsthatuserssetfortheirjobswhentheysubmitthem,whichencouragesthemtosetlowlimits,butriskstheirhavingtoresubmitthejobiftheysetthelimittoolow.HoweverthatdoesnotworkforshorttermCPUschedulingonaninteractivesystem.Anotheroptionwouldbetostatisticallymeasuretheruntimecharacteristicsofjobs,particularlyifthesametasksarerunrepeatedlyandpredictably.Butonceagainthatreallyisn'taviableoptionforshorttermCPUschedulingintherealworld.Amorepracticalapproachistopredictthelengthofthenextburst,basedonsomehistoricalmeasurementofrecentbursttimesforthisprocess.Onesimple,fast,andrelativelyaccuratemethodistheexponentialaverage,whichcanbedefinedasfollows.(Thebookusestauandtfortheirvariables,butthosearehardtodistinguishfromoneanotheranddon'tworkwellinHTML.)

estimate[i+1]=alpha*burst[i]+(1.0alpha)*estimate[i]

Inthisschemethepreviousestimatecontainsthehistoryofallprevioustimes,andalphaservesasaweightingfactorfortherelativeimportanceofrecentdataversuspasthistory.Ifalphais1.0,thenpasthistoryisignored,andweassumethenextburstwillbethesamelengthasthelastburst.Ifalphais0.0,thenallmeasuredbursttimesareignored,andwejustassumeaconstantbursttime.Mostcommonlyalphaissetat0.5,asillustratedinFigure5.3:



SJFcanbeeitherpreemptiveornonpreemptive.PreemptionoccurswhenanewprocessarrivesinthereadyqueuethathasapredictedbursttimeshorterthanthetimeremainingintheprocesswhoseburstiscurrentlyontheCPU.PreemptiveSJFissometimesreferredtoasshortestremainingtimefirstscheduling.Forexample,thefollowingGanttchartisbaseduponthefollowingdata:

Process ArrivalTime BurstTime

P1 0 8

P2 1 4

P3 2 9

p4 3 5

Theaveragewaittimeinthiscaseis((53)+(101)+(172))/4=26/4=6.5ms.(Asopposedto7.75msfornonpreemptiveSJFor8.75forFCFS.)

5.3.3PriorityScheduling

PriorityschedulingisamoregeneralcaseofSJF,inwhicheachjobisassignedapriorityandthejobwiththehighestprioritygetsscheduledfirst.(SJFusestheinverseofthenextexpectedbursttimeasitspriorityThesmallertheexpectedburst,thehigherthepriority.)Notethatinpractice,prioritiesareimplementedusingintegerswithinafixedrange,butthereisnoagreeduponconventionastowhether"high"prioritiesuselargenumbersorsmallnumbers.Thisbookuseslownumberforhighpriorities,with0beingthehighestpossiblepriority.Forexample,thefollowingGanttchartisbasedupontheseprocessbursttimesandpriorities,andyieldsan



averagewaitingtimeof8.2ms:

Process BurstTime Priority

P1 10 3

P2 1 1

P3 2 4

P4 1 5

P5 5 2

Prioritiescanbeassignedeitherinternallyorexternally.InternalprioritiesareassignedbytheOSusingcriteriasuchasaveragebursttime,ratioofCPUtoI/Oactivity,systemresourceuse,andotherfactorsavailabletothekernel.Externalprioritiesareassignedbyusers,basedontheimportanceofthejob,feespaid,politics,etc.Priorityschedulingcanbeeitherpreemptiveornonpreemptive.Priorityschedulingcansufferfromamajorproblemknownasindefiniteblocking,orstarvation,inwhichalowprioritytaskcanwaitforeverbecausetherearealwayssomeotherjobsaroundthathavehigherpriority.

Ifthisproblemisallowedtooccur,thenprocesseswilleitherruneventuallywhenthesystemloadlightens(atsay2:00a.m.),orwilleventuallygetlostwhenthesystemisshutdownorcrashes.(Therearerumorsofjobsthathavebeenstuckforyears.)Onecommonsolutiontothisproblemisaging,inwhichprioritiesofjobsincreasethelongertheywait.Underthisschemealowpriorityjobwilleventuallygetitspriorityraisedhighenoughthatitgetsrun.

5.3.4RoundRobinScheduling

RoundrobinschedulingissimilartoFCFSscheduling,exceptthatCPUburstsareassignedwithlimitscalledtimequantum.WhenaprocessisgiventheCPU,atimerissetforwhatevervaluehasbeensetforatimequantum.

Iftheprocessfinishesitsburstbeforethetimequantumtimerexpires,thenitisswappedoutoftheCPUjustlikethenormalFCFSalgorithm.Ifthetimergoesofffirst,thentheprocessisswappedoutoftheCPUandmovedtothebackendofthereadyqueue.

Thereadyqueueismaintainedasacircularqueue,sowhenallprocesseshavehadaturn,thentheschedulergivesthefirstprocessanotherturn,andsoon.RRschedulingcangivetheeffectofallprocessorssharingtheCPUequally,althoughtheaveragewaittimecanbelongerthanwithotherschedulingalgorithms.Inthefollowingexampletheaveragewaittimeis5.66ms.

Process BurstTime

P1 24

P2 3

P3 3

TheperformanceofRRissensitivetothetimequantumselected.Ifthequantumislargeenough,thenRRreducestotheFCFSalgorithmIfitisverysmall,theneachprocessgets1/nthoftheprocessortimeandsharetheCPUequally.BUT,arealsysteminvokesoverheadforeverycontextswitch,andthesmallerthetimequantumthemorecontextswitchesthereare.(SeeFigure5.4below.)Mostmodernsystemsusetimequantumbetween10and



100milliseconds,andcontextswitchtimesontheorderof10microseconds,sotheoverheadissmallrelativetothetimequantum.

Turnaroundtimealsovarieswithquantumtime,inanonapparentmanner.Consider,forexampletheprocessesshowninFigure5.5:

Ingeneral,turnaroundtimeisminimizedifmostprocessesfinishtheirnextcpuburstwithinonetimequantum.Forexample,withthreeprocessesof10msburstseach,theaverageturnaroundtimefor1msquantumis29,andfor10msquantumitreducesto20.However,ifitismadetoolarge,thenRRjustdegeneratestoFCFS.Aruleofthumbisthat80%ofCPUburstsshouldbesmallerthanthetimequantum.

5.3.5MultilevelQueueScheduling



Whenprocessescanbereadilycategorized,thenmultipleseparatequeuescanbeestablished,eachimplementingwhateverschedulingalgorithmismostappropriateforthattypeofjob,and/orwithdifferentparametricadjustments.Schedulingmustalsobedonebetweenqueues,thatisschedulingonequeuetogettimerelativetootherqueues.Twocommonoptionsarestrictpriority(nojobinalowerpriorityqueuerunsuntilallhigherpriorityqueuesareempty)androundrobin(eachqueuegetsatimesliceinturn,possiblyofdifferentsizes.)NotethatunderthisalgorithmjobscannotswitchfromqueuetoqueueOncetheyareassignedaqueue,thatistheirqueueuntiltheyfinish.

5.3.6MultilevelFeedbackQueueScheduling

Multilevelfeedbackqueueschedulingissimilartotheordinarymultilevelqueueschedulingdescribedabove,exceptjobsmaybemovedfromonequeuetoanotherforavarietyofreasons:

IfthecharacteristicsofajobchangebetweenCPUintensiveandI/Ointensive,thenitmaybeappropriatetoswitchajobfromonequeuetoanother.Agingcanalsobeincorporated,sothatajobthathaswaitedforalongtimecangetbumpedupintoahigherpriorityqueueforawhile.

Multilevelfeedbackqueueschedulingisthemostflexible,becauseitcanbetunedforanysituation.Butitisalsothemostcomplextoimplementbecauseofalltheadjustableparameters.Someoftheparameterswhichdefineoneofthesesystemsinclude:

Thenumberofqueues.Theschedulingalgorithmforeachqueue.Themethodsusedtoupgradeordemoteprocessesfromonequeuetoanother.(Whichmaybedifferent.)Themethodusedtodeterminewhichqueueaprocessentersinitially.



5.4ThreadScheduling

Theprocessschedulerschedulesonlythekernelthreads.UserthreadsaremappedtokernelthreadsbythethreadlibraryTheOS(andinparticularthescheduler)isunawareofthem.

5.4.1ContentionScope

ContentionscopereferstothescopeinwhichthreadscompetefortheuseofphysicalCPUs.Onsystemsimplementingmanytooneandmanytomanythreads,ProcessContentionScope,PCS,occurs,becausecompetitionoccursbetweenthreadsthatarepartofthesameprocess.(Thisisthemanagement/schedulingofmultipleuserthreadsonasinglekernelthread,andismanagedbythethreadlibrary.)SystemContentionScope,SCS,involvesthesystemschedulerschedulingkernelthreadstorunononeormoreCPUs.Systemsimplementingonetoonethreads(XP,Solaris9,Linux),useonlySCS.PCSschedulingistypicallydonewithpriority,wheretheprogrammercansetand/orchangethepriorityofthreadscreatedbyhisorherprograms.Eventimeslicingisnotguaranteedamongthreadsofequalpriority.

5.4.2PthreadScheduling

ThePthreadlibraryprovidesforspecifyingscopecontention:PTHREAD_SCOPE_PROCESSschedulesthreadsusingPCS,byschedulinguserthreadsontoavailableLWPsusingthemanytomanymodel.PTHREAD_SCOPE_SYSTEMschedulesthreadsusingSCS,bybindinguserthreadstoparticularLWPs,effectivelyimplementingaonetoonemodel.

getscopeandsetscopemethodsprovidefordeterminingandsettingthescopecontentionrespectively:



Figure5.8

5.5MultipleProcessorScheduling

Whenmultipleprocessorsareavailable,thentheschedulinggetsmorecomplicated,becausenowthereismorethanoneCPUwhichmustbekeptbusyandineffectiveuseatalltimes.Loadsharingrevolvesaroundbalancingtheloadbetweenmultipleprocessors.Multiprocessorsystemsmaybeheterogeneous,(differentkindsofCPUs),orhomogenous,(allthesamekindofCPU).Eveninthelattercasetheremaybespecialschedulingconstraints,suchasdeviceswhichareconnectedviaaprivatebustoonlyoneoftheCPUs.Thisbookwillrestrictitsdiscussiontohomogenoussystems.

5.5.1ApproachestoMultipleProcessorScheduling

Oneapproachtomultiprocessorschedulingisasymmetricmultiprocessing,inwhichoneprocessoristhemaster,controllingallactivitiesandrunningallkernelcode,whiletheotherrunsonlyusercode.Thisapproachisrelativelysimple,asthereisnoneedtosharecriticalsystemdata.Anotherapproachissymmetricmultiprocessing,SMP,whereeachprocessorschedulesitsownjobs,eitherfromacommonreadyqueueorfromseparatereadyqueuesforeachprocessor.VirtuallyallmodernOSessupportSMP,includingXP,Win2000,Solaris,Linux,andMacOSX.

5.5.2ProcessorAffinity

Processorscontaincachememory,whichspeedsuprepeatedaccessestothesamememorylocations.Ifaprocessweretoswitchfromoneprocessortoanothereachtimeitgotatimeslice,thedatainthecache(forthatprocess)wouldhavetobeinvalidatedandreloadedfrommainmemory,therebyobviatingthebenefitofthecache.ThereforeSMPsystemsattempttokeepprocessesonthesameprocessor,viaprocessoraffinity.Softaffinityoccurswhenthesystemattemptstokeepprocessesonthesameprocessorbutmakesnoguarantees.LinuxandsomeotherOSessupporthardaffinity,inwhichaprocessspecifiesthatitisnottobemovedbetweenprocessors.Mainmemoryarchitecturecanalsoaffectprocessaffinity,ifparticularCPUshavefasteraccesstomemoryonthesamechiporboardthantoothermemoryloadedelsewhere.(NonUniformMemoryAccess,NUMA.)Asshownbelow,ifaprocesshasanaffinityforaparticularCPU,thenitshouldpreferentiallybeassignedmemorystoragein"local"fastaccessareas.

5.5.3LoadBalancing

Obviouslyanimportantgoalinamultiprocessorsystemistobalancetheloadbetweenprocessors,sothatoneprocessorwon'tbesittingidlewhileanotherisoverloaded.Systemsusingacommonreadyqueuearenaturallyselfbalancing,anddonotneedanyspecialhandling.Mostsystems,however,maintainseparatereadyqueuesforeachprocessor.



Balancingcanbeachievedthrougheitherpushmigrationorpullmigration:Pushmigrationinvolvesaseparateprocessthatrunsperiodically,(e.g.every200milliseconds),andmovesprocessesfromheavilyloadedprocessorsontolessloadedones.Pullmigrationinvolvesidleprocessorstakingprocessesfromthereadyqueuesofotherprocessors.Pushandpullmigrationarenotmutuallyexclusive.

Notethatmovingprocessesfromprocessortoprocessortoachieveloadbalancingworksagainsttheprincipleofprocessoraffinity,andifnotcarefullymanaged,thesavingsgainedbybalancingthesystemcanbelostinrebuildingcaches.Oneoptionistoonlyallowmigrationwhenimbalancesurpassesagiventhreshold.

5.5.4MulticoreProcessors

TraditionalSMPrequiredmultipleCPUchipstorunmultiplekernelthreadsconcurrently.RecenttrendsaretoputmultipleCPUs(cores)ontoasinglechip,whichappeartothesystemasmultipleprocessors.Computecyclescanbeblockedbythetimeneededtoaccessmemory,whenevertheneededdataisnotalreadypresentinthecache.(Cachemisses.)InFigure5.10,asmuchashalfoftheCPUcyclesarelosttomemorystall.

Byassigningmultiplekernelthreadstoasingleprocessor,memorystallcanbeavoided(orreduced)byrunningonethreadontheprocessorwhiletheotherthreadwaitsformemory.

Adualthreadeddualcoresystemhasfourlogicalprocessorsavailabletotheoperatingsystem.TheUltraSPARCT1CPUhas8coresperchipand4hardwarethreadspercore,foratotalof32logicalprocessorsperchip.Therearetwowaystomultithreadaprocessor:

1. Coarsegrainedmultithreadingswitchesbetweenthreadsonlywhenonethreadblocks,sayonamemoryread.Contextswitchingissimilartoprocessswitching,withconsiderableoverhead.

2. Finegrainedmultithreadingoccursonsmallerregularintervals,sayontheboundaryofinstructioncycles.Howeverthearchitectureisdesignedtosupportthreadswitching,sotheoverheadisrelativelyminor.

Notethatforamultithreadedmulticoresystem,therearetwolevelsofscheduling,atthekernellevel:TheOSscheduleswhichkernelthread(s)toassigntowhichlogicalprocessors,andwhentomakecontextswitchesusingalgorithmsasdescribedabove.Onalowerlevel,thehardwarescheduleslogicalprocessorsoneachphysicalcoreusingsomeotheralgorithm.



TheUltraSPARCT1usesasimpleroundrobinmethodtoschedulethe4logicalprocessors(kernelthreads)oneachphysicalcore.TheIntelItaniumisadualcorechipwhichusesa7levelpriorityscheme(urgency)todeterminewhichthreadtoschedulewhenoneof5differenteventsoccurs.

5.5.5VirtualizationandScheduling

Virtualizationaddsanotherlayerofcomplexityandscheduling.Typicallythereisonehostoperatingsystemoperatingon"real"processor(s)andanumberofguestoperatingsystemsoperatingonvirtualprocessors.TheHostOScreatessomenumberofvirtualprocessorsandpresentsthemtotheguestOSesasiftheywererealprocessors.TheguestOSesdon'trealizetheirprocessorsarevirtual,andmakeschedulingdecisionsontheassumptionofrealprocessors.Asaresult,interactiveandespeciallyrealtimeperformancecanbeseverelycompromisedonguestsystems.Thetimeofdayclockwillalsofrequentlybeoff.

5.6OperatingSystemExamples

5.6.1Example:SolarisScheduling

Prioritybasedkernelthreadscheduling.Fourclasses(realtime,system,interactive,andtimesharing),andmultiplequeues/algorithmswithineachclass.Defaultistimesharing.

Processprioritiesandtimeslicesareadjusteddynamicallyinamultilevelfeedbackpriorityqueuesystem.TimeslicesareinverselyproportionaltopriorityHigherpriorityjobsgetsmallertimeslices.InteractivejobshavehigherprioritythanCPUBoundones.Seethetablebelowforsomeofthe60prioritylevelsandhowtheyshift."Timequantumexpired"and"returnfromsleep"indicatethenewprioritywhenthoseeventsoccur.(Largernumbersareahigher,i.e.betterpriority.)



Figure5.12

Solaris9introducedtwonewschedulingclasses:Fixedpriorityandfairshare.Fixedpriorityissimilartotimesharing,butnotadjusteddynamically.FairshareusessharesofCPUtimeratherthanprioritiestoschedulejobs.AcertainshareoftheavailableCPUtimeisallocatedtoaproject,whichisasetofprocesses.

Systemclassisreservedforkerneluse.(UserprogramsrunninginkernelmodeareNOTconsideredinthesystemschedulingclass.)



5.6.2Example:WindowsXPScheduling

WindowsXPusesaprioritybasedpreemptiveschedulingalgorithm.Thedispatcherusesa32levelpriorityschemetodeterminetheorderofthreadexecution,dividedintotwoclassesvariableclassfrom1to15andrealtimeclassfrom16to31,(plusathreadatpriority0managingmemory.)Thereisalsoaspecialidlethreadthatisscheduledwhennootherthreadsareready.WinXPidentifies7priorityclasses(rowsonthetablebelow),and6relativeprioritieswithineachclass(columns.)Processesarealsoeachgivenabaseprioritywithintheirpriorityclass.Whenvariableclassprocessesconsumetheirentiretimequanta,thentheirprioritygetslowered,butnotbelowtheirbasepriority.Processesintheforeground(activewindow)havetheirschedulingquantamultipliedby3,togivebetterresponsetointeractiveprocessesintheforeground.



Figure5.14

5.6.3Example:LinuxScheduling

ModernLinuxschedulingprovidesimprovedsupportforSMPsystems,andaschedulingalgorithmthatrunsinO(1)timeasthenumberofprocessesincreases.TheLinuxschedulerisapreemptiveprioritybasedalgorithmwithtwopriorityrangesRealtimefrom0to99andanicerangefrom100to140.UnlikeSolarisorXP,Linuxassignslongertimequantumstohigherprioritytasks.

Figure5.15

Arunnabletaskisconsideredeligibleforexecutionaslongasithasnotconsumedallthetimeavailableinit'stimeslice.Thosetasksarestoredinanactivearray,indexedaccordingtopriority.Whenaprocessconsumesitstimeslice,itismovedtoanexpiredarray.Thetasksprioritymaybereassignedaspartofthetransferal.Whentheactivearraybecomesempty,thetwoarraysareswapped.Thesearraysarestoredinrunqueuestructures.Onmultiprocessormachines,eachprocessorhasitsownschedulerwithitsownrunqueue.



Figure5.16

5.7AlgorithmEvaluation

Thefirststepindeterminingwhichalgorithm(andwhatparametersettingswithinthatalgorithm)isoptimalforaparticularoperatingenvironmentistodeterminewhatcriteriaaretobeused,whatgoalsaretobetargeted,andwhatconstraintsifanymustbeapplied.Forexample,onemightwantto"maximizeCPUutilization,subjecttoamaximumresponsetimeof1second".Oncecriteriahavebeenestablished,thendifferentalgorithmscanbeanalyzedanda"bestchoice"determined.Thefollowingsectionsoutlinesomedifferentmethodsfordeterminingthe"bestchoice".

5.7.1DeterministicModeling

Ifaspecificworkloadisknown,thentheexactvaluesformajorcriteriacanbefairlyeasilycalculated,andthe"best"determined.Forexample,considerthefollowingworkload(withallprocessesarrivingattime0),andtheresultingschedulesdeterminedbythreedifferentalgorithms:

Process BurstTime

P1 10

P2 29

P3 3

P4 7

P5 12



TheaveragewaitingtimesforFCFS,SJF,andRRare28ms,13ms,and23msrespectively.Deterministicmodelingisfastandeasy,butitrequiresspecificknowninput,andtheresultsonlyapplyforthatparticularsetofinput.Howeverbyexaminingmultiplesimilarcases,certaintrendscanbeobserved.(Likethefactthatforprocessesarrivingatthesametime,SJFwillalwaysyieldtheshortestaveragewaittime.)

5.7.2QueuingModels

Specificprocessdataisoftennotavailable,particularlyforfuturetimes.Howeverastudyofhistoricalperformancecanoftenproducestatisticaldescriptionsofcertainimportantparameters,suchastherateatwhichnewprocessesarrive,theratioofCPUburststoI/Otimes,thedistributionofCPUbursttimesandI/Obursttimes,etc.Armedwiththoseprobabilitydistributionsandsomemathematicalformulas,itispossibletocalculatecertainperformancecharacteristicsofindividualwaitingqueues.Forexample,Little'sFormulasaysthatforanaveragequeuelengthofN,withanaveragewaitingtimeinthequeueofW,andanaveragearrivalofnewjobsinthequeueofLambda,thenthesethreetermscanberelatedby:

N=Lambda*W

Queuingmodelstreatthecomputerasanetworkofinterconnectedqueues,eachofwhichisdescribedbyitsprobabilitydistributionstatisticsandformulassuchasLittle'sformula.Unfortunatelyrealsystemsandmodernschedulingalgorithmsaresocomplexastomakethemathematicsintractableinmanycaseswithrealsystems.

5.7.3Simulations

Anotherapproachistoruncomputersimulationsofthedifferentproposedalgorithms(andadjustmentparameters)underdifferentloadconditions,andtoanalyzetheresultstodeterminethe"best"choiceofoperationforaparticularloadpattern.Operatingconditionsforsimulationsareoftenrandomlygeneratedusingdistributionfunctionssimilartothosedescribedabove.Abetteralternativewhenpossibleistogeneratetracetapes,bymonitoringandloggingtheperformanceofarealsystemundertypicalexpectedworkloads.Thesearebetterbecausetheyprovideamoreaccuratepictureofsystemloads,andalsobecausetheyallowmultiplesimulationstoberunwiththeidenticalprocessload,andnotjuststatisticallyequivalentloads.Acompromiseistorandomlydeterminesystemloadsandthensavetheresultsintoafile,sothatallsimulationscanberunagainstidenticalrandomlydeterminedsystemloads.Althoughtracetapesprovidemoreaccurateinputinformation,theycanbedifficultandexpensivetocollectandstore,andtheiruseincreasesthecomplexityofthesimulationssignificantly.Thereisalsosomequestionastowhetherthefutureperformanceofthenewsystemwillreallymatchthepastperformanceoftheoldsystem.(Ifthesystemrunsfaster,usersmaytakefewercoffeebreaks,andsubmitmoreprocessesperhourthanundertheoldsystem.Converselyiftheturnaroundtimeforjobsislonger,intelligentusersmaythinkmorecarefullyaboutthejobstheysubmitratherthanrandomlysubmittingjobsandhopingthatoneofthemworksout.)



Figure5.17

5.7.4Implementation

Theonlyrealwaytodeterminehowaproposedschedulingalgorithmisgoingtooperateistoimplementitonarealsystem.Forexperimentalalgorithmsandthoseunderdevelopment,thiscancausedifficultiesandresistanceamonguserswhodon'tcareaboutdevelopingOSesandareonlytryingtogettheirdailyworkdone.Eveninthiscase,themeasuredresultsmaynotbedefinitive,foratleasttwomajorreasons:(1)Systemworkloadsarenotstatic,butchangeovertimeasnewprogramsareinstalled,newusersareaddedtothesystem,newhardwarebecomesavailable,newworkprojectsgetstarted,andevensocietalchanges.(ForexampletheexplosionoftheInternethasdrasticallychangedtheamountofnetworktrafficthatasystemseesandtheimportanceofhandlingitwithrapidresponsetimes.)(2)Asmentionedabove,changingtheschedulingsystemmayhaveanimpactontheworkloadandthewaysinwhichusersusethesystem.(Thebookgivesanexampleofaprogrammerwhomodifiedhiscodetowriteanarbitrarycharactertothescreenatregularintervals,justsohisjobwouldbeclassifiedasinteractiveandplacedintoahigherpriorityqueue.)Mostmodernsystemsprovidesomecapabilityforthesystemadministratortoadjustschedulingparameters,eitherontheflyorastheresultofarebootorakernelrebuild.

5.8Summary

MaterialOmittedfromtheEighthEdition:

Was5.4.4SymmetricMultithreading(Omittedfrom8thedition)

AnalternativestrategytoSMPisSMT,SymmetricMultiThreading,inwhichmultiplevirtual(logical)CPUsareusedinsteadof(orincombinationwith)multiplephysicalCPUs.SMTmustbesupportedinhardware,aseachlogicalCPUhasitsownregistersandhandlesitsowninterrupts.(IntelreferstoSMTashyperthreadingtechnology.)TosomeextenttheOSdoesnotneedtoknowiftheprocessorsitismanagingarerealorvirtual.Ontheotherhand,someschedulingdecisionscanbeoptimizediftheschedulerknowsthemappingofvirtualprocessorstorealCPUs.(ConsidertheschedulingoftwoCPUintensiveprocessesonthearchitectureshownbelow.)



omitted

operating systems_ cpu scheduling

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