Chapter4:ProcessesProcessConceptProcessSchedulingOperationonProcessesCooperatingProcessesInterprocessCommunicationProcessConceptAnoperatingsystemexecutesavarietyofprograms:Batchsystem–jobsTime-sharedsystems–userprogramsortasksTextbookusesthetermsjobandprocessalmostinterchangeably.Process–

aprograminexecutionAprocessincludes:programcounterstackdatasectionProcessConcept國內(nèi)學(xué)術(shù)界較為一致的定義：

進(jìn)程是一個(gè)具有一定獨(dú)立功能的程序，關(guān)于某個(gè)數(shù)據(jù)集合的一次可以并發(fā)執(zhí)行的運(yùn)行活動。FromWikipedia

Incomputing,aprocessisaninstanceofacomputerprogramthatisbeingsequentiallyexecutedbyacomputersystemthathastheabilitytorunseveralcomputerprogramsconcurrently.Processvs.ProgramDynamic/staticprocessisaprograminexecution,soitisdynamic,programisapassivecollectionofinstuctions,soitisastaticentity.Lifycycleprocesshasalifecycle.Itisanactiveentityinthesensethatitcanbecreated,executed,andterminatedduringitslifetime.Programcanexistforever.Structureprocess=program+data+PCBEachprocessisrepresentedintheOSbyaprocesscontrolblock(PCB),itcontainsallinformationassociatedwithaspecificprocess.Processvs.ProgramNoone-to-onemappingbetweenprocessesandprogramscanhavemulitpleprocessesofthesameprogramoneprocesscaninvokemultipleprogramsProcessisthebasicunitofbeingscheduledandresourceallocationfromtheviewpointofOS,whileprogramcannotberunwithoutprocesscreation.Processcanbeexecutedconcurrentlyorsimultaneously.ProcessStateAsaprocessexecutes,itchangesstatenew:Theprocessisbeingcreated.running:Instructionsarebeingexecuted.waiting:Theprocessiswaitingforsomeeventtooccur.ready:Theprocessiswaitingtobeassignedtoaprocessor.terminated:Theprocesshasfinishedexecution.ProcessControlBlock(PCB)Informationassociatedwitheachprocess：Processstate Programcounter CPUregisters CPUschedulinginformation Memory-managementinformationAccountinginformation I/OstatusinformationProcessControlBlock(PCB)CPUSwitch

FromProcesstoProcessProcessSchedulingProcessSchedulingQueuesJobqueue

–setofallprocessesinthesystem.Readyqueue

–setofallprocessesresidinginmainmemory,

readyandwaitingtoexecute.Devicequeues

–setofprocesseswaitingforanI/Odevice.Processmigrationbetweenthevariousqueues.ReadyQueueAnd

VariousI/ODeviceQueuesRepresentation

ofProcessSchedulingAdditionof

MediumTermSchedulingSchedulers(Cont.)Short-termschedulerisinvokedveryfrequently(milliseconds)T(mustbefast).調(diào)用頻率高Long-termschedulerisinvokedveryinfrequently(seconds,minutes)T(maybeslow).調(diào)用頻率不高Thelong-termschedulercontrolsthedegreeofmultiprogramming.

長程調(diào)度控制了多道程序的“道數(shù)”Processescanbedescribedaseither:I/O-boundprocess

–spendsmoretimedoingI/Othancomputations,manyshortCPUbursts.CPU-boundprocess

–spendsmoretimedoingcomputations;fewverylongCPUbursts.ContextSwitch上下文切換WhenCPUswitchestoanotherprocess,thesystemmustsavethestateoftheoldprocessandloadthesavedstateforthenewprocess.Context-switchtimeisoverhead;thesystemdoesnousefulworkwhileswitching.Timedependentonhardwaresupport.ProcessCreation(Cont.)AddressspaceChildduplicateofparent.Childhasaprogramloadedintoit.UNIXexamplesforksystemcallcreatesnewprocessexeclpsystemcallusedafteraforktoreplacetheprocess’memoryspacewithanewprogram.

ATreeofProcesses

OnATypicalUNIXSystemProcessTerminationProcessexecuteslaststatementandaskstheoperatingsystemtodeleteit(exit).Process’resourcesaredeallocatedbyoperatingsystem.Outputdatafromchildtoparent(viawait).ParentmayterminateexecutionofchildrenprocessesChildhasexceeded超過allocatedresources.Taskassignedtochildisnolongerrequired.Parentisexiting.Operatingsystemdoesnotallowchildtocontinueifitsparentterminates.Cascadingtermination.級聯(lián)終止Producer-ConsumerProblemParadigmforcooperatingprocesses,producerprocessproducesinformationthatisconsumedbyaconsumerprocess.unbounded-bufferplacesnopracticallimitonthesizeofthebuffer.

無界緩沖沒有對緩沖區(qū)大小的限制bounded-bufferassumesthatthereisafixedbuffersize.

有界緩沖對緩沖區(qū)大小作了限定Bounded-Buffer

——Shared-MemorySolutionShareddatavar

n;type

item=…;var

buffer.array[0..n–1]of

item;in,out:0..n–1;Producerprocess

repeatproduceaniteminnextp…while

in+1mod

n=out

do

no-op;buffer[in]:=nextp;in:=in+1mod

n;until

false;Threads(Cont.)Inamultiplethreadedtask,whileoneserverthreadisblockedandwaiting,asecondthreadinthesametaskcanrun.Cooperationofmultiplethreadsinsamejobconfershigherthroughputandimprovedperformance.Applicationsthatrequiresharingacommonbuffer(i.e.,producer-consumer)benefitfromthreadutilization.Threadsprovideamechanismthatallowssequentialprocessestomakeblockingsystemcallswhilealsoachievingparallelism.Kernel-supportedthreads(MachandOS/2).User-levelthreads;supportedabovethekernel,viaasetoflibrarycallsattheuserlevel.Hybridapproachimplementsbothuser-levelandkernel-supportedthreads(Solaris2).混合處理實(shí)現(xiàn)用戶級和內(nèi)核支持線程InterprocessCommunication-IPCMechanismforprocessestocommunicateandtosynchronizetheiractions.Messagesystem

–processescommunicatewitheachotherwithoutresortingtosharedvariables.IPCfacilityprovidestwooperationssend(message)–messagesizefixedorvariablereceive(message)IfPandQwishtocommunicate,theyneedto:establishacommunication

linkbetweenthemexchangemessagesviasend/receiveImplementationofcommunicationlinkphysical(e.g.,sharedmemory,hardwarebus)logical(e.g.,logicalproperties)ImplementationQuestionsHowarelinksestablished?Canalinkbeassociatedwithmorethantwoprocesses?Howmanylinkscantherebebetweeneverypairofcommunicatingprocesses?Whatisthecapacityofalink?？Isthesizeofamessagethatthelinkcanaccommodatefixedorvariable?Isalinkunidirectionalorbi-directional?DirectCommunicationProcessesmustnameeachotherexplicitly顯式:send(P,message)–sendamessagetoprocessPreceive(Q,message)–receiveamessagefromprocessQPropertiesofcommunicationlinkLinksareestablishedautomatically.Alinkisassociatedwithexactlyonepairofcommunicatingprocesses.Betweeneachpairthereexistsexactlyonelink.Thelinkmaybeunidirectional,butisusuallybi-directional.IndirectCommunicationMessagesaredirectedandreceivedfrommailboxes.Eachmailboxhasauniqueid.Processescancommunicateonlyiftheyshareamailbox.

PropertiesofcommunicationlinkLinkestablishedonlyifprocessesshareacommonmailboxAlinkmaybeassociatedwithmanyprocesses.Eachpairofprocessesmayshareseveralcommunicationlinks.Linkmaybeunidirectional.alorbi-directional.OperationscreateanewmailboxsendandreceivemessagesthroughmailboxdestroyamailboxTypesofMailboxPrivateMailbox:owner-Receiveotheruser-SendPublicMailbox:ownedbyOSSh