The Abstraction The Process In this chapter we discuss one of the most fundamental abstr - PDF document

4TheAbstraction:TheProcessInthischapter,wediscussoneofthemostfundamentalabstractionsthattheOSprovidestousers:theprocess.Thedenitionofaprocess,infor-mally,isquitesimple:itisarunningprogram[V+65,BH70].Theprogramitselfisalifelessthing:itjustsitsthereonthedisk,abunchofinstructions(andmaybesomestaticdata),waitingtospringintoaction.Itistheoper-atingsystemthattakesthesebytesandgetsthemrunning,transformingtheprogramintosomethinguseful.Itturnsoutthatoneoftenwantstorunmorethanoneprogramatonce;forexample,consideryourdesktoporlaptopwhereyoumightliketorunawebbrowser,mailprogram,agame,amusicplayer,andsoforth.Infact,atypicalsystemmaybeseeminglyrunningtensorevenhundredsofprocessesatthesametime.Doingsomakesthesystemeasytouse,asoneneverneedbeconcernedwithwhetheraCPUisavailable;onesimplyrunsprograms.Henceourchallenge: THECRUXOFTHEPROBLEM:HOWTOPROVIDETHEILLUSIONOFMANYCPUS?AlthoughthereareonlyafewphysicalCPUsavailable,howcantheOSprovidetheillusionofanearly-endlesssupplyofsaidCPUs?TheOScreatesthisillusionbyvirtualizingtheCPU.Byrunningoneprocess,thenstoppingitandrunninganother,andsoforth,theOScanpromotetheillusionthatmanyvirtualCPUsexistwheninfactthereisonlyonephysicalCPU(orafew).Thisbasictechnique,knownastimesharingoftheCPU,allowsuserstorunasmanyconcurrentprocessesastheywouldlike;thepotentialcostisperformance,aseachwillrunmoreslowlyiftheCPU(s)mustbeshared.ToimplementvirtualizationoftheCPU,andtoimplementitwell,theOSwillneedbothsomelow-levelmachineryandsomehigh-levelin-telligence.Wecallthelow-levelmachinerymechanisms;mechanismsarelow-levelmethodsorprotocolsthatimplementaneededpieceoffunctionality.Forexample,we'lllearnlaterhowtoimplementacontext1 2THEABSTRACTION:THEPROCESS TIP:USETIMEHARING(ANDPACEHARING)TimesharingisabasictechniqueusedbyanOStosharearesource.Byallowingtheresourcetobeusedforalittlewhilebyoneentity,andthenalittlewhilebyanother,andsoforth,theresourceinquestion(e.g.,theCPU,oranetworklink)canbesharedbymany.Thecounterpartoftimesharingisspacesharing,wherearesourceisdivided(inspace)amongthosewhowishtouseit.Forexample,diskspaceisnaturallyaspace-sharedresource;onceablockisassignedtoale,itisnormallynotas-signedtoanotherleuntiltheuserdeletestheoriginalle.switch,whichgivestheOStheabilitytostoprunningoneprogramandstartrunninganotheronagivenCPU;thistime-sharingmechanismisemployedbyallmodernOSes.OntopofthesemechanismsresidessomeoftheintelligenceintheOS,intheformofpolicies.PoliciesarealgorithmsformakingsomekindofdecisionwithintheOS.Forexample,givenanumberofpossi-bleprogramstorunonaCPU,whichprogramshouldtheOSrun?AschedulingpolicyintheOSwillmakethisdecision,likelyusinghistori-calinformation(e.g.,whichprogramhasrunmoreoverthelastminute?),workloadknowledge(e.g.,whattypesofprogramsarerun),andperfor-mancemetrics(e.g.,isthesystemoptimizingforinteractiveperformance,orthroughput?)tomakeitsdecision.4.1TheAbstraction:AProcessTheabstractionprovidedbytheOSofarunningprogramissomethingwewillcallaprocess.Aswesaidabove,aprocessissimplyarunningprogram;atanyinstantintime,wecansummarizeaprocessbytakinganinventoryofthedifferentpiecesofthesystemitaccessesoraffectsduringthecourseofitsexecution.Tounderstandwhatconstitutesaprocess,wethushavetounderstanditsmachinestate:whataprogramcanreadorupdatewhenitisrunning.Atanygiventime,whatpartsofthemachineareimportanttotheexecu-tionofthisprogram?Oneobviouscomponentofmachinestatethatcomprisesaprocessisitsmemory.Instructionslieinmemory;thedatathattherunningpro-gramreadsandwritessitsinmemoryaswell.Thusthememorythattheprocesscanaddress(calleditsaddressspace)ispartoftheprocess.Alsopartoftheprocess'smachinestateareregisters;manyinstructionsexplicitlyreadorupdateregistersandthusclearlytheyareimportanttotheexecutionoftheprocess.Notethattherearesomeparticularlyspecialregistersthatformpartofthismachinestate.Forexample,theprogramcounter(PC)(sometimescalledtheinstructionpointerorIP)tellsuswhichinstructionofthepro-gramwillexecutenext;similarlyastackpointerandassociatedframeOPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS3 TIP:SEPARATEPOLICYANDMECHANISMInmanyoperatingsystems,acommondesignparadigmistoseparatehigh-levelpoliciesfromtheirlow-levelmechanisms[L+75].Youcanthinkofthemechanismasprovidingtheanswertoahowquestionaboutasystem;forexample,howdoesanoperatingsystemperformacontextswitch?Thepolicyprovidestheanswertoawhichquestion;forexample,whichprocessshouldtheoperatingsystemrunrightnow?Separatingthetwoallowsoneeasilytochangepolicieswithouthavingtorethinkthemechanismandisthusaformofmodularity,ageneralsoftwaredesignprinciple.pointerareusedtomanagethestackforfunctionparameters,localvari-ables,andreturnaddresses.Finally,programsoftenaccesspersistentstoragedevicestoo.SuchI/Oinformationmightincludealistofthelestheprocesscurrentlyhasopen.4.2ProcessAPIThoughwedeferdiscussionofarealprocessAPIuntilasubsequentchapter,herewerstgivesomeideaofwhatmustbeincludedinanyinterfaceofanoperatingsystem.TheseAPIs,insomeform,areavailableonanymodernoperatingsystem.Create:Anoperatingsystemmustincludesomemethodtocre-atenewprocesses.Whenyoutypeacommandintotheshell,ordouble-clickonanapplicationicon,theOSisinvokedtocreateanewprocesstoruntheprogramyouhaveindicated.Destroy:Asthereisaninterfaceforprocesscreation,systemsalsoprovideaninterfacetodestroyprocessesforcefully.Ofcourse,manyprocesseswillrunandjustexitbythemselveswhencomplete;whentheydon't,however,theusermaywishtokillthem,andthusanin-terfacetohaltarunawayprocessisquiteuseful.Wait:Sometimesitisusefultowaitforaprocesstostoprunning;thussomekindofwaitinginterfaceisoftenprovided.MiscellaneousControl:Otherthankillingorwaitingforaprocess,therearesometimesothercontrolsthatarepossible.Forexample,mostoperatingsystemsprovidesomekindofmethodtosuspendaprocess(stopitfromrunningforawhile)andthenresumeit(con-tinueitrunning).Status:Thereareusuallyinterfacestogetsomestatusinformationaboutaprocessaswell,suchashowlongithasrunfor,orwhatstateitisin.c\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES 4THEABSTRACTION:THEPROCESS Memory CPU Disk codestatic dataheapstackProcess codestatic dataProgram Loading:Takes on-disk programand reads it into theaddress space of process Figure4.1:Loading:FromProgramToProcess4.3ProcessCreation:ALittleMoreDetailOnemysterythatweshouldunmaskabitishowprogramsaretrans-formedintoprocesses.Specically,howdoestheOSgetaprogramupandrunning?Howdoesprocesscreationactuallywork?TherstthingthattheOSmustdotorunaprogramistoloaditscodeandanystaticdata(e.g.,initializedvariables)intomemory,intothead-dressspaceoftheprocess.Programsinitiallyresideondisk(or,insomemodernsystems,ash-basedSSDs)insomekindofexecutableformat;thus,theprocessofloadingaprogramandstaticdataintomemoryre-quirestheOStoreadthosebytesfromdiskandplacetheminmemorysomewhere(asshowninFigure4.1).Inearly(orsimple)operatingsystems,theloadingprocessisdoneea-gerly,i.e.,allatoncebeforerunningtheprogram;modernOSesperformtheprocesslazily,i.e.,byloadingpiecesofcodeordataonlyastheyareneededduringprogramexecution.Totrulyunderstandhowlazyloadingofpiecesofcodeanddataworks,you'llhavetounderstandmoreaboutOPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS5 themachineryofpagingandswapping,topicswe'llcoverinthefuturewhenwediscussthevirtualizationofmemory.Fornow,justrememberthatbeforerunninganything,theOSclearlymustdosomeworktogettheimportantprogrambitsfromdiskintomemory.Oncethecodeandstaticdataareloadedintomemory,thereareafewotherthingstheOSneedstodobeforerunningtheprocess.Somemem-orymustbeallocatedfortheprogram'srun-timestack(orjuststack).Asyoushouldlikelyalreadyknow,Cprogramsusethestackforlocalvariables,functionparameters,andreturnaddresses;theOSallocatesthismemoryandgivesittotheprocess.TheOSwillalsolikelyinitial-izethestackwitharguments;specically,itwillllintheparameterstothemain()function,i.e.,argcandtheargvarray.TheOSmayalsoallocatesomememoryfortheprogram'sheap.InCprograms,theheapisusedforexplicitlyrequesteddynamically-allocateddata;programsrequestsuchspacebycallingmalloc()andfreeitex-plicitlybycallingfree().Theheapisneededfordatastructuressuchaslinkedlists,hashtables,trees,andotherinterestingdatastructures.Theheapwillbesmallatrst;astheprogramruns,andrequestsmoremem-oryviathemalloc()libraryAPI,theOSmaygetinvolvedandallocatemorememorytotheprocesstohelpsatisfysuchcalls.TheOSwillalsodosomeotherinitializationtasks,particularlyasre-latedtoinput/output(I/O).Forexample,inUNIXsystems,eachprocessbydefaulthasthreeopenledescriptors,forstandardinput,output,anderror;thesedescriptorsletprogramseasilyreadinputfromtheterminalandprintoutputtothescreen.We'lllearnmoreaboutI/O,ledescrip-tors,andthelikeinthethirdpartofthebookonpersistence.Byloadingthecodeandstaticdataintomemory,bycreatingandini-tializingastack,andbydoingotherworkasrelatedtoI/Osetup,theOShasnow(nally)setthestageforprogramexecution.Itthushasonelasttask:tostarttheprogramrunningattheentrypoint,namelymain().Byjumpingtothemain()routine(throughaspecializedmechanismthatwewilldiscussnextchapter),theOStransferscontroloftheCPUtothenewly-createdprocess,andthustheprogrambeginsitsexecution.4.4ProcessStatesNowthatwehavesomeideaofwhataprocessis(thoughwewillcontinuetorenethisnotion),and(roughly)howitiscreated,letustalkaboutthedifferentstatesaprocesscanbeinatagiventime.Thenotionthataprocesscanbeinoneofthesestatesaroseinearlycomputersystems[DV66,V+65].Inasimpliedview,aprocesscanbeinoneofthreestates:Running:Intherunningstate,aprocessisrunningonaprocessor.Thismeansitisexecutinginstructions.Ready:Inthereadystate,aprocessisreadytorunbutforsomereasontheOShaschosennottorunitatthisgivenmoment.c\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES 6THEABSTRACTION:THEPROCESS RunningReady I/O: initiateI/O: done Figure4.2:Process:StateTransitionsBlocked:Intheblockedstate,aprocesshasperformedsomekindofoperationthatmakesitnotreadytorununtilsomeothereventtakesplace.Acommonexample:whenaprocessinitiatesanI/Orequesttoadisk,itbecomesblockedandthussomeotherprocesscanusetheprocessor.Ifweweretomapthesestatestoagraph,wewouldarriveatthedi-agraminFigure4.2.Asyoucanseeinthediagram,aprocesscanbemovedbetweenthereadyandrunningstatesatthediscretionoftheOS.Beingmovedfromreadytorunningmeanstheprocesshasbeensched-uled;beingmovedfromrunningtoreadymeanstheprocesshasbeendescheduled.Onceaprocesshasbecomeblocked(e.g.,byinitiatinganI/Ooperation),theOSwillkeepitassuchuntilsomeeventoccurs(e.g.,I/Ocompletion);atthatpoint,theprocessmovestothereadystateagain(andpotentiallyimmediatelytorunningagain,iftheOSsodecides).Let'slookatanexampleofhowtwoprocessesmighttransitionthroughsomeofthesestates.First,imaginetwoprocessesrunning,eachofwhichonlyusetheCPU(theydonoI/O).Inthiscase,atraceofthestateofeachprocessmightlooklikethis(Figure4.3).TimeProcess0Process1Notes 1RunningReady2RunningReady3RunningReady4RunningReadyProcess0nowdone5–Running6–Running7–Running8–RunningProcess1nowdoneFigure4.3:TracingProcessState:CPUOnlyOPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS7 TimeProcess0Process1Notes 1RunningReady2RunningReady3RunningReadyProcess0initiatesI/O4BlockedRunningProcess0isblocked,5BlockedRunningsoProcess1runs6BlockedRunning7ReadyRunningI/Odone8ReadyRunningProcess1nowdone9Running–10Running–Process0nowdoneFigure4.4:TracingProcessState:CPUandI/OInthisnextexample,therstprocessissuesanI/Oafterrunningforsometime.Atthatpoint,theprocessisblocked,givingtheotherprocessachancetorun.Figure4.4showsatraceofthisscenario.Morespecically,Process0initiatesanI/Oandbecomesblockedwait-ingforittocomplete;processesbecomeblocked,forexample,whenread-ingfromadiskorwaitingforapacketfromanetwork.TheOSrecog-nizesProcess0isnotusingtheCPUandstartsrunningProcess1.WhileProcess1isrunning,theI/Ocompletes,movingProcess0backtoready.Finally,Process1nishes,andProcess0runsandthenisdone.NotethattherearemanydecisionstheOSmustmake,eveninthissimpleexample.First,thesystemhadtodecidetorunProcess1whileProcess0issuedanI/O;doingsoimprovesresourceutilizationbykeep-ingtheCPUbusy.Second,thesystemdecidednottoswitchbacktoProcess0whenitsI/Ocompleted;itisnotclearifthisisagooddeci-sionornot.Whatdoyouthink?ThesetypesofdecisionsaremadebytheOSscheduler,atopicwewilldiscussafewchaptersinthefuture.4.5DataStructuresTheOSisaprogram,andlikeanyprogram,ithassomekeydatastruc-turesthattrackvariousrelevantpiecesofinformation.Totrackthestateofeachprocess,forexample,theOSlikelywillkeepsomekindofpro-cesslistforallprocessesthatarereadyandsomeadditionalinforma-tiontotrackwhichprocessiscurrentlyrunning.TheOSmustalsotrack,insomeway,blockedprocesses;whenanI/Oeventcompletes,theOshouldmakesuretowakethecorrectprocessandreadyittorunagain.Figure4.5showswhattypeofinformationanOSneedstotrackabouteachprocessinthexv6kernel[CK+08].Similarprocessstructuresexistin“real”operatingsystemssuchasLinux,MacOSX,orWindows;lookthemupandseehowmuchmorecomplextheyare.Fromthegure,youcanseeacoupleofimportantpiecesofinforma-tiontheOStracksaboutaprocess.Theregistercontextwillhold,forac\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES 8THEABSTRACTION:THEPROCESS //theregistersxv6willsaveandrestore//tostopandsubsequentlyrestartaprocessstructcontext{inteip;intesp;intebx;intecx;intedx;intesi;intedi;intebp;};//thedifferentstatesaprocesscanbeinenumproc_state{UNUSED,EMBRYO,SLEEPING,RUNNABLE,RUNNING,ZOMBIE};//theinformationxv6tracksabouteachprocess//includingitsregistercontextandstatestructproc{char*mem;//Startofprocessmemoryuintsz;//Sizeofprocessmemorychar*kstack;//Bottomofkernelstack//forthisprocessenumproc_statestate;//Processstateintpid;//ProcessIDstructproc*parent;//Parentprocessvoid*chan;//If!zero,sleepingonchanintkilled;//If!zero,hasbeenkilledstructfile*ofile[NOFILE];//Openfilesstructinode*cwd;//Currentdirectorystructcontextcontext;//Switchheretorunprocessstructtrapframe*tf;//Trapframeforthe//currentinterrupt};Figure4.5:Thexv6ProcStructurestoppedprocess,thecontentsofitsregisters.Whenaprocessisstopped,itsregisterswillbesavedtothismemorylocation;byrestoringthesereg-isters(i.e.,placingtheirvaluesbackintotheactualphysicalregisters),theOScanresumerunningtheprocess.We'lllearnmoreaboutthistechniqueknownasacontextswitchinfuturechapters.Youcanalsoseefromthegurethattherearesomeotherstatesapro-cesscanbein,beyondrunning,ready,andblocked.Sometimesasystemwillhaveaninitialstatethattheprocessisinwhenitisbeingcreated.Also,aprocesscouldbeplacedinanalstatewhereithasexitedbutOPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS9 ASIDE:DATATRUCTURE—THEPROCESSLISTOperatingsystemsarerepletewithvariousimportantdatastructuresthatwewilldiscussinthesenotes.Theprocesslist(alsocalledthetasklist)istherstsuchstructure.Itisoneofthesimplerones,butcertainlyanyOSthathastheabilitytorunmultipleprogramsatoncewillhavesomethingakintothisstructureinordertokeeptrackofalltherunningprogramsinthesystem.Sometimespeoplerefertotheindividualstruc-turethatstoresinformationaboutaprocessasaProcessControlBlock(PCB),afancywayoftalkingaboutaCstructurethatcontainsinforma-tionabouteachprocess(alsosometimescalledaprocessdescriptor).hasnotyetbeencleanedup(inUNIX-basedsystems,thisiscalledthezombiestate1).Thisnalstatecanbeusefulasitallowsotherprocesses(usuallytheparentthatcreatedtheprocess)toexaminethereturncodeoftheprocessandseeifthejust-nishedprocessexecutedsuccessfully(usually,programsreturnzeroinUNIX-basedsystemswhentheyhaveaccomplishedatasksuccessfully,andnon-zerootherwise).Whenn-ished,theparentwillmakeonenalcall(e.g.,wait())towaitforthecompletionofthechild,andtoalsoindicatetotheOSthatitcancleanupanyrelevantdatastructuresthatreferredtothenow-extinctprocess.4.6SummaryWehaveintroducedthemostbasicabstractionoftheOS:theprocess.Itisquitesimplyviewedasarunningprogram.Withthisconceptualviewinmind,wewillnowmoveontothenitty-gritty:thelow-levelmechanismsneededtoimplementprocesses,andthehigher-levelpoli-ciesrequiredtoscheduletheminanintelligentway.Bycombiningmech-anismsandpolicies,wewillbuildupourunderstandingofhowanoper-atingsystemvirtualizestheCPU. 1Yes,thezombiestate.Justlikerealzombies,thesezombiesarerelativelyeasytokill.However,differenttechniquesareusuallyrecommended.c\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES 10THEABSTRACTION:THEPROCESS ASIDE:KEYPROCESSTERMSTheprocessisthemajorOSabstractionofarunningprogram.Atanypointintime,theprocesscanbedescribedbyitsstate:thecon-tentsofmemoryinitsaddressspace,thecontentsofCPUregisters(includingtheprogramcounterandstackpointer,amongothers),andinformationaboutI/O(suchasopenleswhichcanbereadorwritten).TheprocessAPIconsistsofcallsprogramscanmakerelatedtopro-cesses.Typically,thisincludescreation,destruction,andotheruse-fulcalls.Processesexistinoneofmanydifferentprocessstates,includingrunning,readytorun,andblocked.Differentevents(e.g.,gettingscheduledordescheduled,orwaitingforanI/Otocomplete)tran-sitionaprocessfromoneofthesestatestotheother.Aprocesslistcontainsinformationaboutallprocessesinthesys-tem.Eachentryisfoundinwhatissometimescalledaprocesscontrolblock(PCB),whichisreallyjustastructurethatcontainsinformationaboutaspecicprocess.OPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS11 References[BH70]“TheNucleusofaMultiprogrammingSystem”byPerBrinchHansen.Communica-tionsoftheACM,Volume13:4,April1970.Thispaperintroducesoneoftherstmicrokernelsinoperatingsystemshistory,calledNucleus.Theideaofsmaller,moreminimalsystemsisathemethatrearsitsheadrepeatedlyinOShistory;itallbeganwithBrinchHansen'sworkdescribedherein.[CK+08]“Thexv6OperatingSystem”byRussCox,FransKaashoek,RobertMorris,NickolaiZeldovich.From:https://github.com/mit-pdos/xv6-public.ThecoolestrealandlittleOSintheworld.Downloadandplaywithittolearnmoreaboutthedetailsofhowoperatingsystemsactuallywork.Wehavebeenusinganolderversion(2012-01-30-1-g1c41342)andhencesomeexamplesinthebookmaynotmatchthelatestinthesource.[DV66]“ProgrammingSemanticsforMultiprogrammedComputations”byJackB.Dennis,EarlC.VanHorn.CommunicationsoftheACM,Volume9,Number3,March1966.Thispaperdenedmanyoftheearlytermsandconceptsaroundbuildingmultiprogrammedsystems.[L+75]“Policy/mechanismseparationinHydra”byR.Levin,E.Cohen,W.Corwin,F.Pollack,W.Wulf.SOSP'75,Austin,Texas,November1975.Anearlypaperabouthowtostructureoperat-ingsystemsinaresearchOSknownasHydra.WhileHydraneverbecameamainstreamOS,someofitsideasinuencedOSdesigners.[V+65]“StructureoftheMulticsSupervisor”byV.A.Vyssotsky,F.J.Corbato,R.M.Graham.FallJointComputerConference,1965.AnearlypaperonMultics,whichdescribedmanyofthebasicideasandtermsthatwendinmodernsystems.Someofthevisionbehindcomputingasautilityarenallybeingrealizedinmoderncloudsystems.c\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES 12THEABSTRACTION:THEPROCESS Homework(Simulation)Thisprogram,process-run.py,allowsyoutoseehowprocessstateschangeasprogramsrunandeitherusetheCPU(e.g.,performanaddinstruction)ordoI/O(e.g.,sendarequesttoadiskandwaitforittocomplete).SeetheREADMEfordetails.Questions1.Runprocess-run.pywiththefollowingags:-l5:100,5:100.WhatshouldtheCPUutilizationbe(e.g.,thepercentoftimetheCPUisinuse?)Whydoyouknowthis?Usethe-cand-pagstoseeifyouwereright.2.Nowrunwiththeseags:./process-run.py-l4:100,1:0.Theseagsspecifyoneprocesswith4instructions(alltousetheCPU),andonethatsimplyissuesanI/Oandwaitsforittobedone.Howlongdoesittaketocompletebothprocesses?Use-cand-ptondoutifyouwereright.3.Switchtheorderoftheprocesses:-l1:0,4:100.Whathappensnow?Doesswitchingtheordermatter?Why?(Asalways,use-cand-ptoseeifyouwereright)4.We'llnowexploresomeoftheotherags.Oneimportantagis-S,whichdetermineshowthesystemreactswhenaprocessis-suesanI/O.WiththeagsettoSWITCH ON END,thesystemwillNOTswitchtoanotherprocesswhileoneisdoingI/O,in-steadwaitinguntiltheprocessiscompletelynished.Whathap-penswhenyourunthefollowingtwoprocesses(-l1:0,4:100-c-SSWITCH ON END),onedoingI/OandtheotherdoingCPUwork?5.Now,runthesameprocesses,butwiththeswitchingbehaviorsettoswitchtoanotherprocesswheneveroneisWAITINGforI/O(-l1:0,4:100-c-SSWITCH ON IO).Whathappensnow?Use-cand-ptoconrmthatyouareright.6.OneotherimportantbehavioriswhattodowhenanI/Ocom-pletes.With-IIO RUN LATER,whenanI/Ocompletes,thepro-cessthatissueditisnotnecessarilyrunrightaway;rather,whateverwasrunningatthetimekeepsrunning.Whathappenswhenyourunthiscombinationofprocesses?(Run./process-run.py-l3:0,5:100,5:100,5:100-SSWITCH ON IO-IIO RUN LATER-c-p)Aresystemresourcesbeingeffectivelyutilized?7.Nowrunthesameprocesses,butwith-IIO RUN IMMEDIATEset,whichimmediatelyrunstheprocessthatissuedtheI/O.Howdoesthisbehaviordiffer?Whymightrunningaprocessthatjustcom-pletedanI/Oagainbeagoodidea?OPERATINGYSTEMS[VERSION1.01]WWW.OSTEP.ORG THEABSTRACTION:THEPROCESS13 8.Nowrunwithsomerandomlygeneratedprocesses:-s1-l3:50,3:50or-s2-l3:50,3:50or-s3-l3:50,3:50.Seeifyoucanpredicthowthetracewillturnout.Whathappenswhenyouusetheag-IIO RUN IMMEDIATEvs.-IIO RUN LATER?Whathap-penswhenyouuse-SSWITCH ON IOvs.-SSWITCH ON END?c\r2008–19,ARPACI-DUSSEAUTHREEEASYPIECES