Chapter4ThreadsOperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadLibrariesThreadingIssuesOperatingSystemExamplesOperatingSystemConceptsWhatisathread?Athread,alsoknownaslightweightprocess

(LWP),isabasicunitofCPUexecution.AthreadhasathreadID,aprogramcounter,aregisterset,andastack.Thus,itissimilartoaprocesshas.However,athreadshares

withotherthreadsinthesame

processitscodesection,datasection,andotherOSresources(e.g.,filesandsignals).Aprocess,orheavyweightprocess,hasasinglethreadofcontrol.ProcessRecalltheconceptofProcessAddressspacePCBAsinglecontrolthread…OperatingSystemConceptsOperatingSystemConceptsSingleandMultithreadedProcessesOperatingSystemConceptsItemssharedbyallthreadsinaprocessItemsprivatetoeachthreadOperatingSystemConceptsThreadUsage(1)AwordprocessorwiththreethreadsOperatingSystemConceptsThreadUsage(2)AmultithreadedWebserverOperatingSystemConceptsThreadUsage(3)Roughoutlineofcodeforpreviousslide(a)Dispatcherthread(b)WorkerthreadMulti-threadVSsingle-threadMultithreadserver:blockingRead()Singlethreadserver:non-blockingRead()interruptOperatingSystemConceptsOperatingSystemConceptsBenefitsResponsivenessAllowsaprogramcontinuerunningevenifpartofitisblockedResourceSharingDifferentthreadsofactivitywithinthesameaddressspaceEconomyProcesscreating/contextswitchismuchslowerUtilizationofMPArchitecturesIncreaseconcurrencyOperatingSystemConceptsUserThreadsThreadmanagementdonebyuser-levelthreadslibrary

Examples -POSIXPthreads(StandarddefiningAPIforthreadoperation,Demo) -MachC-threads -SolarisUI-threadsDemoPthreadsOperatingSystemConceptsOperatingSystemConceptsUserThreads(Cont.)Userthreadsaresupportedattheuserlevel.Thekernelisnotawareofuserthreads.Alibraryprovidesallsupportforthreadcreation,termination,joining,andscheduling.Thereisnokernelintervention,and,hence,userthreadsareusuallymoreefficient.Unfortunately,sincethekernelonlyrecognizesthecontainingprocess(ofthethreads),ifonethreadisblocked,everyotherthreadsofthesameprocessarealsoblocked

becausethecontainingprocessisblocked.OperatingSystemConceptsImplementingThreadsinUserSpaceAuser-levelthreadspackageImplementingThreadsinUserSpaceBenefitsEfficientthreadscheduling.(Why?)Weaknessblockingonethreadwillnotcauseotherthreadsofthesameprocesstoblock.(Why?)OperatingSystemConceptsOperatingSystemConceptsKernelThreadsSupportedbytheKernel

Examples -Windows95/98/NT/2000 -Solaris -Tru64UNIX -BeOS -LinuxOperatingSystemConceptsKernelThreads(Cont.)Kernelthreadsaredirectlysupportedbythekernel.Thekerneldoesthreadcreation,termination,joining,andschedulinginkernelspace.Kernelthreadsareusuallyslowerthantheuserthreads.However,blockingonethreadwillnotcauseotherthreadsofthesameprocesstoblock.Thekernelsimplyrunsotherthreads(?).Inamultiprocessorenvironment,thekernelcanschedulethreadsondifferentprocessorsOperatingSystemConceptsImplementingThreadsintheKernelAthreadspackagemanagedbythekernelOperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadingIssuesWindowsXPThreadsLinuxThreadsJavaThreadsWindowsThreadsAPIPthreadsOperatingSystemConceptsMultithreadingModelsMany-to-One

One-to-One

Many-to-ManyOperatingSystemConceptsMany-to-OneManyuser-levelthreadsmappedtosinglekernelthread.Theentireprocesswillblockifathreadmakesablockingsystemcall

Usedonsystemsthatdonotsupportkernelthreads.OperatingSystemConceptsMany-to-OneModelOperatingSystemConceptsMany-to-OneModel(Cont.)OperatingSystemConceptsOne-to-OneEachuser-levelthreadmapstokernelthread.Creatingauserthreadrequirescreatingakernelthread,thusburdentheperformanceoftheapplication

Examples -Windows95/98/NT/2000 -OS/2OperatingSystemConceptsOne-to-oneModelOperatingSystemConceptsOne-to-oneModel(Cont.)OperatingSystemConceptsMany-to-ManyModelAllowsmanyuserlevelthreadstobemappedtomanykernelthreads.Allowstheoperatingsystemtocreateasufficientnumberofkernelthreads.Solaris2WindowsNT/2000withtheThreadFiberpackageOperatingSystemConceptsMany-to-ManyModelOperatingSystemConceptsMany-to-ManyModel(Cont.)OperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadingIssuesWindowsXPThreadsLinuxThreadsJavaThreadsWindowsThreadsAPIPthreadsOperatingSystemConceptsThreadingIssuesSemanticsoffork()andexec()systemcalls.Threadcancellation.SignalhandlingThreadpoolsThreadspecificdataSchedulerActivationsOperatingSystemConceptsSemanticsoffork()andexec()Doesfork()duplicateonlythecallingthreadorallthreads?TwoalternativesOperatingSystemConceptsThreadCancellationTerminatingathreadbeforeithasfinishedTwogeneralapproaches:AsynchronouscancellationterminatesthetargetthreadimmediatelyDeferredcancellationallowsthetargetthreadtoperiodicallycheckifitshouldbecancelledThepointathreadcanterminateitselfisa

cancellationpoint.OperatingSystemConceptsThreadCancellation(Cont.)Withasynchronouscancellation,ifthetargetthreadownssomesystem-wideresources,thesystemmaynotbeabletoreclaimallrecoursesWithdeferredcancellation,thetargetthreaddeterminesthetimetoterminateitself.Reclaimingresourcesisnotaproblem.Mostsystemsimplementasynchronouscancellationforprocesses(e.g.,usethekillsystemcall)andthreads.Pthreadsupportsdeferredcancellation.OperatingSystemConceptsSignalHandlingSignalsareusedinUNIXsystemstonotifyaprocessthataparticulareventhasoccurredAllsignalsfollowthesamepattern:SignalisgeneratedbyparticulareventSignalisdeliveredtoaprocessSignalishandledAsignalhandlerisusedtoprocesssignalsOperatingSystemConceptsSignalHandling(Cont.)Options:DeliverthesignaltothethreadtowhichthesignalappliesDeliverthesignaltoeverythreadintheprocess(example?)DeliverthesignaltocertainthreadsintheprocessAssignaspecificthreadtoreceiveallsignalsfortheprocessDemoSinglehandlerOperatingSystemConceptsOperatingSystemConceptsThreadPoolsCreateanumberofthreadsinapoolwheretheyawaitworkAdvantages:UsuallyslightlyfastertoservicearequestwithanexistingthreadthancreateanewthreadAllowsthenumberofthreadsintheapplication(s)tobeboundtothesizeofthepoolSchedulerActivationsBothM:MandTwo-levelmodelsrequirecommunicationtomaintaintheappropriatenumberofkernelthreadsallocatedtotheapplicationScheduleractivationsprovideupcalls-acommunicationmechanismfromthekerneltothethreadlibraryThiscommunicationallowsanapplicationtomaintainthecorrectnumberkernelthreadsOperatingSystemConceptsOperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadingIssuesWindowsXPThreadsLinuxThreadsJavaThreadsWindowsThreadsAPIPthreadsOperatingSystemConceptsWindowsXPThreadsImplementstheone-to-onemapping.Eachthreadcontains -athreadid -registerset -separateuserandkernelstacks -privatedatastorageareaOperatingSystemConceptsLinuxThreadsLinuxreferstothemastasksratherthanthreads.Threadcreationisdonethroughclone()systemcall.Clone()allowsachildtasktosharetheaddressspaceoftheparenttask(process)OperatingSystemConceptsJavaThreadsJavathreadsmaybecreatedby:

ExtendingThreadclassImplementingtheRunnableinterface

JavathreadsaremanagedbytheJVM.OperatingSystemConceptsJavaThreadStatesOperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadingIssuesWindowsXPThreadsLinuxThreadsJavaThreadsWindowsThreadAPIsPthreadsOperatingSystemConceptsWindowsThreadAPIsCreateThreadGetCurrentThreadId-returnsglobalIDGetCurrentThread-returnshandleSuspendThread/ResumeThreadExitThreadTerminateThreadGetExitCodeThreadGetThreadTimesOperatingSystemConceptsWindowsAPIThreadCreationHANDLECreateThread(

LPSECURITY_ATTRIBUTESlpsa,

DWORDcbStack,

LPTHREAD_START_ROUTINElpStartAddr,

LPVOIDlpvThreadParm,

DWORDfdwCreate,

LPDWORDlpIDThread)lpstartAddrpointstofunctiondeclaredas DWORDWINAPIThreadFunc(LPVOID)lpvThreadParmis32-bitargumentLPIDThreadpointstoDWORDthatreceivesthreadID

non-NULLpointer!cbStack==0:thread‘s

stacksizedefaultsto

primarythread‘ssizeOperatingSystemConceptsWindowsAPIThreadTermination VOIDExitThread(DWORDdevExitCode)Whenthelastthreadinaprocessterminates,theprocessitselfterminates

BOOLGetExitCodeThread(

HANDLEhThread,LPDWORDlpdwExitCode)ReturnsexitcodeorSTILL_ACTIVE

OperatingSystemConceptsSuspendingandResumingThreadsEachthreadhassuspendcountCanonlyexecuteifsuspendcount==0Threadcanbecreatedinsuspendedstate DWORDResumeThread(HANDLEhThread) DWORDSuspendThread(HANDLEhThread)Bothfunctionsreturnsuspendcountor0xFFFFFFFFonfailureOperatingSystemConceptsExample:ThreadCreationWhat’sWrong?OperatingSystemConceptsExampleExplainedMainthreadisprocessWhenprocessgoes,allthreadsgoNeedsomemethodsofwaitingforathreadtofinishOperatingSystemConceptsWaitingforWindows*ThreadBOOLthrdDone=FALSE;thrdDone=TRUE;while(!thrdDone);Notagoodidea!OperatingSystemConceptsWaitingforaThreadOperatingSystemConceptsWaitingforManyThreadsOperatingSystemConceptsNotesonWaitFor*FunctionsHandleasparameterUsedfordifferenttypesofobjectsKernelobjectshavetwostatesSignaledNon-signaledBehaviorisdefinedbyobjectreferredtobyhandleThread:signaledmeansterminatedOperatingSystemConceptsExample:WaitingformultiplethreadsOperatingSystemConceptsExample:HelloThreadsModifythepreviousexamplecodetoprintoutappropriate“HelloThread”messageUniquethreadnumberusefor-loopvariableofCreateThreadloopSampleoutput:OperatingSystemConceptsWhat’sWrong?WhatisprintedformyNum?OperatingSystemConceptsHelloThreadsTimelineOperatingSystemConceptsRaceConditionsConcurrentaccessofsamevariablebymultiplethreadsRead/WriteconflictWrite/WriteconflictMostcommonerrorinconcurrentprogramsMaynotbeapparentatalltimesHowtoavoiddataraces?LocalstorageControlsharedaccesswithcriticalregionsOperatingSystemConceptsHelloThread:LocalStoragesolutionOperatingSystemConceptsChapter4:ThreadsOverviewMultithreadingModelsThreadingIssuesWindowsXPThreadsLinuxThreadsJavaThreadsWindowsThreadsAPIPthreadsOperatingSystemConceptsPthreadsaPOSIXstandard(IEEE1003.1c)APIforthreadcreationandsynchronization.APIspecifiesbehaviorofthethreadlibrary,implementationisuptodevelopmentofthelibrary.CommoninUNIXoperatingsystems.OperatingSystemConceptspthread_createpthread_t*tidhandleofcreatedthreadconstpthread_attr_t*attrattributesofthreadtobecreatedvoid*(*function)(void*)functiontobemappedtothreadvoid*argsingleargumenttofunctionintpthread_create(tid,attr,function,arg);OperatingSystemConceptspthread_createexplainedspawnathreadrunningthefunctionthreadhandlereturnedviapthread_tstructurespecifyNULLtousedefaultattributessingleargumentsenttofunctionIfnoargumenttofunction,specifyNULLcheckerrorcodes!EAGAIN–insufficientresourcestocreatethreadEINVAL–invalidattributeOperatingSystemConceptsThreadsstatespthreadthreadshavetwostatesjoinableanddetachedthreadsarejoinablebydefaultResourcesarekeptuntilpthread_joincanbereset