Chapter3ProcessesOperatingSystemConceptsChapter3:ProcessesProcessConceptProcessSchedulingOperationsonProcessesCooperatingProcessesInterprocessCommunicationCommunicationinClient-ServerSystemsOperatingSystemConceptsProcessConceptAnoperatingsystemexecutesavarietyofprograms:Batchsystem–jobsTime-sharedsystems–userprogramsortasksTextbookusesthetermsjobandprocessalmostinterchangeably.OperatingSystemConceptsProcesses

TheProcessModelMultiprogrammingoffourprogramsConceptualmodelof4independent,sequentialprocessesOnlyoneprogramactiveatanyinstantOperatingSystemConceptsProcessConcept(Cont.)Process–aprograminexecution;processexecutionmustprogressinsequentialfashion.Aprocessincludes:programcountercontentsoftheprocessor’sregistersstackdatasectionOperatingSystemConceptsProcessinMemoryOperatingSystemConceptsProcessStateAsaprocessexecutes,itchangesstatenew:Theprocessisbeingcreated.running:Instructionsarebeingexecuted.waiting:Theprocessiswaitingforsomeeventtooccur.ready:Theprocessiswaitingtobeassignedtoaprocessor.terminated:Theprocesshasfinishedexecution.OperatingSystemConceptsDiagramofProcessStateOperatingSystemConceptsProcessControlBlock(PCB)Informationassociatedwitheachprocess.ProcessstateProgramcounterCPUregistersCPUschedulinginformationMemory-managementinformationAccountinginformationI/OstatusinformationOperatingSystemConceptsProcessControlBlock(PCB)OperatingSystemConceptsCPUSwitchFromProcesstoProcessOperatingSystemConceptsChapter3:ProcessesProcessConceptProcessSchedulingOperationsonProcessesCooperatingProcessesInterprocessCommunicationCommunicationinClient-ServerSystemsOperatingSystemConceptsProcessSchedulingQueuesJobqueue–setofallprocessesinthesystem.Readyqueue–setofallprocessesresidinginmainmemory,readyandwaitingtoexecute.Devicequeues–setofprocesseswaitingforanI/Odevice.Processmigrationbetweenthevariousqueues.OperatingSystemConceptsReadyQueueAndVariousI/ODeviceQueuesOperatingSystemConceptsRepresentationofProcessSchedulingOperatingSystemConceptsSchedulersLong-termscheduler(orjobscheduler)–selectswhichprocessesshouldbeloadedintomemoryforexecution.Short-termscheduler(orCPUscheduler)–selectswhichprocessshouldbeexecutednextandallocatesCPU.OperatingSystemConceptsSchedulers(Cont.)Short-termschedulerisinvokedveryfrequently(milliseconds)(mustbefast).Long-termschedulerisinvokedveryinfrequently(seconds,minutes)(maybeslow).Thelong-termschedulercontrolsthedegreeofmultiprogramming.Processescanbedescribedaseither:I/O-boundprocess–spendsmoretimedoingI/Othancomputations,manyshortCPUbursts.CPU-boundprocess–spendsmoretimedoingcomputations;fewverylongCPUbursts.OperatingSystemConceptsAdditionofMedium-TermSchedulingOperatingSystemConceptsContextSwitchWhatisaprocesscontext?

ThecontextofaprocessincludesthevaluesofCPUregisters,theprocessstate,theprogramcounter,andothermemory/filemanagementinformation.OperatingSystemConceptsContextSwitch(Cont.)Whatisacontextswitch?

AftertheCPUschedulerselectsaprocess(fromthereadyqueue)andbeforeallocatesCPUtoit,theCPUschedulermustsavethecontext

ofthecurrentlyrunningprocess,putitintoaqueue,loadthecontext

oftheselectedprocess,andletitrun.OperatingSystemConceptsCPUSwitchFromProcesstoProcessOperatingSystemConceptsContextSwitch(Cont.)WhenCPUswitchestoanotherprocess,thesystemmustsavethestateoftheoldprocessandloadthesavedstateforthenewprocess.Context-switchtimeisoverhead;thesystemdoesnousefulworkwhileswitching.Timedependentonhardwaresupport.OperatingSystemConceptsChapter3:ProcessesProcessConceptProcessSchedulingOperationsonProcessesCooperatingProcessesInterprocessCommunicationCommunicationinClient-ServerSystemsOperatingSystemConceptsProcessCreationParentprocesscreatechildrenprocesses,which,inturncreateotherprocesses,formingatreeofprocesses.ResourcesharingParentandchildrenshareallresources.Childrensharesubsetofparent’sresources.Parentandchildsharenoresources.ExecutionParentandchildrenexecuteconcurrently.Parentwaitsuntilchildrenterminate.OperatingSystemConceptsProcessCreation(Cont.)AddressspaceChildduplicateofparent.Childhasaprogramloadedintoit.UNIXexamplesforksystemcallcreatesnewprocessexecsystemcallusedafteraforktoreplacetheprocess’memoryspacewithanewprogram.OperatingSystemConceptsProcessCreation(UNIX)OperatingSystemConcepts

pid=fork(); if(pid<0){/*erroroccured*/ fprintf(stderr,“Forkfailed”); exit(-1);} elseif(pid==0){/*childprocess*/ execlp(“/bin/ls”,”ls”,NULL); } else{/*parentprocess*/ wait(NULL); printf(“ChildComplete”); exit(0); }OperatingSystemConceptsProcessesTreeonSolarisOperatingSystemConceptsProcessTerminationProcessexecuteslaststatementandaskstheoperatingsystemtodeleteit(exit).Outputdatafromchildtoparent(viawait).Process’resourcesaredeallocatedbyOS.Parentmayterminateexecutionofchildrenprocesses(abort).Childhasexceededallocatedresources.Taskassignedtochildisnolongerrequired.Parentisexiting.Operatingsystemdoesnotallowchildtocontinueifitsparentterminates.Cascadingtermination.ZOperatingSystemConceptsChapter3:ProcessesProcessConceptProcessSchedulingOperationsonProcessesInterprocessCommunicationCommunicationinClient-ServerSystemsOperatingSystemConceptsCooperatingProcessesIndependentprocesscannotaffectorbeaffectedbytheexecutionofanotherprocess.CooperatingprocesscanaffectorbeaffectedbytheexecutionofanotherprocessAdvantagesofprocesscooperationInformationsharingComputationspeed-upModularityConvenienceOperatingSystemConceptsCommunicationModelsMsgPassingSharedMemoryCommunicationmaytakeplaceusingeithermessagepassingorsharedmemory.Socket通信OperatingSystemConceptsOperatingSystemConceptsOperatingSystemConceptsCommunicationModelsMessagepassingSmallerdataexchangeIntercomputercommunicationSystemcallwithkernelinterventionSharedmemoryMaximumspeed/memoryConvenienceofcommunicationProtectionandsynchronizationRoutinememoryaccesswithoutkernelinterventionOperatingSystemConceptsProducer-ConsumerProblemParadigmforcooperatingprocesses,producerprocessproducesinformationthatisconsumedbyaconsumerprocess.unbounded-bufferplacesnopracticallimitonthesizeofthebuffer.bounded-bufferassumesthatthereisafixedbuffersize.OperatingSystemConceptsBounded-Buffer–Shared-MemorySolutionShareddata#defineBUFFER_SIZE10Typedefstruct{ ...}item;itembuffer[BUFFER_SIZE];intin=0;intout=0;OperatingSystemConceptsBounded-Buffer–ProducerProcess itemnextProduced;

while(1){ while(((in+1)%BUFFER_SIZE)==out) ;/*donothing*/ buffer[in]=nextProduced; in=(in+1)%BUFFER_SIZE; }OperatingSystemConceptsBounded-Buffer–ConsumerProcess itemnextConsumed;

while(1){ while(in==out) ;/*donothing*/ nextConsumed=buffer[out]; out=(out+1)%BUFFER_SIZE; }

OperatingSystemConceptsBounded-BufferSolutioniscorrect,butcanonlyuseBUFFER_SIZE-1elementsOperatingSystemConceptsInterprocessCommunication(IPC)Mechanismforprocessestocommunicateandtosynchronizetheiractions.Messagesystem–processescommunicatewitheachotherwithoutresortingtosharedvariables.OperatingSystemConceptsInterprocessCommunication(Cont.)IPCfacilityprovidestwooperations:send(message)–messagesizefixedorvariablereceive(message)IfPandQwishtocommunicate,theyneedto:establishacommunication

linkbetweenthemexchangemessagesviasend/receiveImplementationofcommunicationlinkphysical(e.g.,sharedmemory,hardwarebus)logical(e.g.,logicalproperties)OperatingSystemConceptsImplementationQuestionsHowarelinksestablished?Canalinkbeassociatedwithmorethantwoprocesses?Howmanylinkscantherebebetweeneverypairofcommunicatingprocesses?Whatisthecapacityofalink?Isthesizeofamessagethatthelinkcanaccommodatefixedorvariable?Isalinkunidirectionalorbi-directional?OperatingSystemConceptsDirectCommunicationProcessesmustnameeachotherexplicitly:send(P,message)–sendamessagetoprocessPreceive(Q,message)–receiveamessagefromprocessQPropertiesofcommunicationlinkLinksareestablishedautomatically.Alinkisassociatedwithexactlyonepairofcommunicatingprocesses.Betweeneachpairthereexistsexactlyonelink.Thelinkmaybeunidirectional,butisusuallybi-directional.OperatingSystemConceptsIndirectCommunicationMessagesaredirectedandreceivedfrommailboxes(alsoreferredtoasports).Eachmailboxhasauniqueid.cancommunicateonlyiftheyshareamailbox.PropertiesofcommunicationlinkLinkestablishedonlyifprocessesshareacommonmailboxAlinkmaybeassociatedwithmanyprocesses.Eachpairofprocessesmayshareseveralcommunicationlinks.Linkmaybeunidirectionalorbi-directional.OperatingSystemConceptsIndirectCommunicationOperationscreateanewmailboxsendandreceivemessagesthroughmailboxdestroyamailboxPrimitivesaredefinedas:

send(A,message)–sendamessagetomailboxA

receive(A,message)–receiveamessagefrommailboxAOperatingSystemConceptsIndirectCommunicationMailboxsharingP1,P2,andP3sharemailboxA.P1,sends;P2

andP3receive.Whogetsthemessage?SolutionsAllowalinktobeassociatedwithatmosttwoprocesses.Allowonlyoneprocessatatimetoexecuteareceiveoperation.Allowthesystemtoselectarbitrarilythereceiver.Senderisnotifiedwhothereceiverwas.OperatingSystemConceptsSynchronizationMessagepassingmaybeeitherblockingornon-blocking.BlockingisconsideredsynchronousNon-blockingisconsideredasynchronoussendandreceiveprimitivesmaybeeitherblockingornon-blocking.OperatingSystemConceptsBufferingQueueofmessagesattachedtothelink;implementedinoneofthreeways.1. Zerocapacity–0messages

Sendermustwaitforreceiver(rendezvous).2. Boundedcapacity–finitelengthofnmessages

Sendermustwaitiflinkfull.3. Unboundedcapacity–infinitelength

Senderneverblocks.OperatingSystemConceptsChapter3:ProcessesProcessConceptProcessSchedulingOperationsonProcessesCooperatingProcessesInterprocessCommunicationCommunicationinClient-ServerSystemsOperatingSystemConceptsClient-ServerCommunicationSocketsRemoteProcedure