计算机线程 外文翻译 外文文献 英文文献Word格式.docx

计算机线程 外文翻译 外文文献 英文文献Word格式.docx

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...………………………………………….…..…691

ThreadOverhead…..…………………………………………………………………….…...692

StoptheMadness…………..………………………………………………………….……...696

CPUTrends……………………………………………………………………….…..….……699

NUMAArchitectureMachines…..……………………………….………………….……..700

CLRThreadsandWindowsThreads………………………………..……………….…….703

UsingaDedicatedThreadtoPerformanAsynchronous

Compute-BoundOperation…………………………………………………………….……704

ReasonstoUseThreads……..……………………………………………...…………….….706

ThreadSchedulingandPriorities……………….……………………..……………….…...708

ForegroundThreadsversusBackgroundThreads………………………………………..713

WhatNow?

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Inthischapter,Iintroducethebasicconceptsconcerningthreads,andIofferawayfordeveloperstoconceptualizeaboutthemandtheiruse.I’llexplainwhyMicrosoftWindowsintroducedtheconceptofthreads,CPUtrends,therelationshipbetweencommonlanguageruntime（CLR）threadsandWindowsthreads,theoverheadassociatedwithusingthreads,howWindowsschedulesthreads,theMicrosoft.NETFrameworkclassesthatexposethreadproperties,andmuchmore.

ThechaptersinPartVofthisbook,“Threading,”explainhowWindowsandtheCLRworktogethertoprovideathreadingarchitecture.Itismyhopethatafterreadingthesechapters,youwilltakeawayafoundationofknowledgethatwillallowyoutoeffectivelyusethreadstodesignandbuildresponsive,reliable,andscalableapplicationsandcomponents.

Backintheearlydaysofcomputers,operatingsystemsdidn’toffertheconceptofathread.Ineffect,therewasjustonethreadofexecutionthatranthroughouttheentiresystem,whichincludedbothoperatingsystemcodeandapplicationcode.Theproblemwithhavingonlyonethreadofexecutionwasthatalong-runningtaskwouldpreventothertasksfromexecuting.

Forexample,inthedaysof16-bitWindows,itwasverycommonforanapplicationthatwasprintingadocumenttostalltheentiremachine,causingtheOSandallotherapplicationstostopresponding.

And,sometimesapplicationswouldhaveabuginthem,resultinginaninfiniteloopthatalsostoppedtheentiremachinefromoperating.Atthispoint,theenduserwouldhavenochoicebuttorebootthecomputerbypressingtheresetbuttonorpowerswitch.Ofcourse,endusershateddoingthis（theystilldo,infact）becauseallrunningapplicationsterminated;

moreimportantly,anydatathattheseapplicationswereprocessingwasthrownoutofmemoryandlost.Microsoftknewthat16-bitWindowswouldnotbeagoodenoughoperatingsystemtokeepMicrosoftrelevant

asthecomputerindustryprogressed,sotheysetouttobuildanewOStoaddresstheneedsofcorporationsandindividuals.ThisnewOShadtoberobust,reliable,scalable,andsecure,andithadtoimprovethemanydeficienciesof16-bitWindows.ThisOSkerneloriginallyshippedinMicrosoftWindowsNT.Overtheyears,thiskernelhashadmanytweaksandfeaturesaddedtoit.ThelatestversionofthiskernelshipsinthelatestversionsoftheMicrosoftclientandserverWindowsoperatingsystems.

WhenMicrosoftwasdesigningthisOSkernel,theydecidedtoruneachinstanceofanapplicationinwhatiscalledaprocess.Aprocessisjustacollectionofresourcesthatisusedbyasingleinstanceofanapplication.Eachprocessisgivenavirtualaddressspace,ensuringthatthecodeanddatausedbyoneprocessisnotaccessibletoanotherprocess.Thismakesapplicationinstancesrobustbecauseoneprocesscannotcorruptcodeordatabeingusedbyanother.Inaddition,theOS’skernelcodeanddataarenotaccessibletoprocesses;

therefore,it’snotpossibleforapplicationcodetocorruptoperatingsystemcodeordata.Sonow,applicationcodecannotcorruptotherapplicationsortheOSitself,andthewholecomputingexperienceismuchbetterforendusers.Inaddition,thesystemismoresecurebecauseapplicationcodecannotaccessusernames,passwords,creditcardinformation,orothersensitiveinformationthatisinusebyanotherapplicationortheoperatingsystemitself.

Thisisallwellandgood,butwhatabouttheCPUitself?

Whatifanapplicationentersaninfiniteloop?

Well,ifthereisonlyoneCPUinthemachine,thenitexecutestheinfiniteloopandcannotexecuteanythingelse,sowhilethedatacannotbecorruptedandismoresecure,thesystemcouldstillstoprespondingtotheenduser.Microsoftneededtofixthisproblem,too,andthreadsweretheanswer.AthreadisaWindowsconceptwhosejobistovirtualizetheCPU.Windowsgiveseachprocessitsveryownthread（whichfunctionssimilartoaCPU）,andifapplicationcodeentersaninfiniteloop,theprocessassociatedwiththatcodefreezesup,butotherprocesses（whichhavetheirownthreads）arenotfrozen;

theykeeprunning!

ThreadOverhead

ThreadsareawesomebecausetheyenableWindowstoberesponsiveevenwhenapplicationsareexecutinglong-runningtasks.Also,threadsallowtheusertouseoneapplication（likeTaskManager）toforciblykillanapplicationthatappearsfrozenbecauseitisexecutingalong-runningtask.Butaswitheveryvirtualizationmechanism,threadshavespace（memoryconsumption）andtime（runtimeexecutionperformance）overheadassociatedwiththem.Let’sexplorethisoverheadinmoredetailnow.Everythreadhasoneofeachofthefollowing:

●ThreadkernelobjectTheOSallocatesandinitializesoneofthesedatastructuresforeachthreadcreatedinthesystem.Thedatastructurecontainsabunchofproperties（discussedlaterinthischapter）thatdescribethethread.Thisdatastructurealsocontainswhatiscalledthethread’scontext.ThecontextisablockofmemorythatcontainsasetoftheCPU’sregisters.WhenWindowsisrunningonamachinewithanx86CPU,thethread’scontextusesabout700bytesofmemory.Forx64andIA64CPUs,thecontextisabout1,240and2,500bytesofmemory,respectively.

●Threadenvironmentblock（TEB）TheTEBisablockofmemoryallocatedandinitializedinusermode（addressspacethatapplicationcodecanquicklyaccess）.TheTEBconsumes1pageofmemory（4KBonx86andx64CPUs,8KBonanIA64CPU）.TheTEBcontainstheheadofthethread’sexception-handlingchain.Eachtryblockthatthethreadentersinsertsanodeintheheadofthischain;

thenodeisremovedfromthechainwhenthethreadexistsinthetryblock.Inaddition,theTEBcontainsthethread’sthread-localstoragedataaswellassomedatastructuresforusebyGraphicsDeviceInterface（GDI）andOpenGLgraphics.

●User-modestackTheuser-modestackisusedforlocalvariablesandargumentspassedtomethods.Italsocontainstheaddressindicatingwhatthethreadshouldexecutenextwhenthecurrentmethodreturns.Bydefault,Windowsallocates1MBofmemoryforeachthread’suser-modestack.

●Kernel-modestackThekernel-modestackisalsousedwhenapplicationcodepassesargumentstoakernel-modefunctionintheoperatingsystem.Forsecurityreasons,Windowscopiesanyargumentspassedfromuser-modecodetothekernelfromthethread’suser-modestacktothethread’skernel-modestack.Oncecopied,thekernelcanverifythearguments’values,andsincetheapplicationcodecan’taccessthekernelmodestack,theapplicationcan’tmodifythearguments’valuesaftertheyhavebeenvalidatedandtheOSkernelcodebeginstooperateonthem.Inaddition,thekernelcallsmethodswithinitselfandusesthekernel-modestacktopassitsownarguments,tostoreafunction’slocalvariables,andtostorereturnaddresses.Thekernel-modestackis12KBwhenrunningona32-bitWindowssystemand24KBwhenrunningona64-bitWindowssystem.

●DLLthread-attachandthread-detachnotificationsWindowshasapolicythatwheneverathreadiscreatedinaprocess,allDLLsloadedinthatprocesshavetheirDllMainmethodcalled,passingaDLL_THREAD_ATTACHflag.Similarly,wheneverathreaddies,allDLLsintheprocesshavetheirDllMainmethodcalled,passingitaDLL_THREAD_DETACHflag.SomeDLLsneedthesenotificationstoperformsomespecialinitializationorcleanupforeachthreadcreated/destroyedintheprocess.Forexample,theC-RuntimelibraryDLLallocatessomethread-localstoragestatethatisrequiredshouldthethreadusefunctionscontainedwithintheC-Runtimelibrary.

IntheearlydaysofWindows,manyprocesseshadmaybe5or6DLLsloadedintothem,buttoday,someprocesseshaveseveralhundredDLLsloadedintothem.Rightnow,onmymachine,MicrosoftOfficeOutlookhasabout250DLLsloadedintoitsprocessaddressspace!

ThismeansthatwheneveranewthreadiscreatedinOfficeOutlook,250DLLfunctionsmustgetcalledbeforethethreadisallowedtodowhatitwascreatedtodo.Andthese250functionsmustbecalledagainwheneverathreadinOutlookdies.Wow—thiscanseriouslyaffecttheperformanceofcreatinganddestroyingthreadswithinaprocess.

Sonow,youseeallthespaceandtimeoverheadthatisassociatedwithcreatingathread,lettingitsitaroundinthesystem,anddestroyingit.Butthesituationgetsevenworse—nowwe’regoingtostarttalkingaboutcontextswitching.AcomputerwithonlyoneCPUinitcandoonlyonethingatatime.Therefore,WindowshastosharetheactualCPUhardwareamongallthethreads（logicalCPUs）thataresittingaroundinthesystem.

Atanygivenmomentintime,WindowsassignsonethreadtoaCPU.Thatthreadisallowedtorunforatime-slice（sometimesreferredtoasaquantum）.Whenthetime-sliceexpires,Windowscontextswitchestoanotherthread.EverycontextswitchrequiresthatWindowsperformsthef