Chapter4(Threads)–StudyThreadAthreadisabasicunitofCPUIthasathreadID,programcounter,aregistersetandaItshareswithotherthreadsbelongingtothesameprocess,itscodesection,datasection,andotherOSresources,suchasopenfilesandBenefitsofmultithreadedResponsiveness–Lengthyoperations,suchasadifficultcomputationoranotherIOeventsuchasnetworkcommunicationcanbedoneinparallelwithblockingIOorotherlengthyoperations.能處理復雜冗長的與計ResourcesharingThreadssharedataandcodespacethuseachthreadcanrapidlycommunicateandreduceduplicationthatwouldhaveoccurredwithmultipleprocesses.數據，減少Efficiency–Inadditiontoreducedresourceusage,contextswitchingandthreadcreationcanbedonemoreefficientlythantheequivalentlogicwithprocesses,becausefarlessdatamustbeswitchedout.Scalability–Whileasingleprocessmayonlyrunonaprocessoramultithreadedapplicationcanmoreeasilytakeadvantageofmultiprocessorandmulticoresystemswitheachthreadrunningonadifferentprocessor/core.擴展性高UserandKernelthreadsandtheirUserthreads:applicationmanagesthreadmanagementkernelisnotawareoftheexistenceofthreadsKernelthreads:threadmanagementdonebytheKernel.Thereisnothreadmanagementcodeintheapplicationarea.Multicoresystems–dataparallelism&taskDataparallelism–distributessubsetsofthesamedataacrossmultiplecores,sameoperationoneachTaskparallelism–distributingthreadsacrosscores,eachthreadperforminguniqueoperationMultithreadingmodels(many-to-one,one-to-one,many-to-One-to-OneEachuser-levelthreadmapstokernelBenefits:MoreconcurrencythatMany-to-Drawback:Numberofthreadsperprocesssometimesrestrictedduetooverheadofcreatingakernelthreadassociatedwitheachnewuser-threadcreated.Many-to-OneManyuser-levelthreadsmappedtosinglekernelBenefits:LesscostbysharingonekernelamongmultipleDrawbacks:Onethreadblockingcausesblockingofalltheotherthreads.ManytoManyAllowsmanyuserlevelthreadstobemappedmanykernelThreadThreadlibraryprovidesprogrammerwithAPIforcreatingandmanagingImplicit產生的原因：Growinginpopularityasnumbersofthreadsincrease,programcorrectnessmoredifficultwithexplicitthreads。實現方法：Creationandmanagementofthreadsdonebycompilersandrun-timelibrariesratherthanprogrammers。ThreadDefinition:Sincewhenevertheserverreceivesarequest,ithastocreateaseparatethreadtoservicetherequestandwhentheserviceiscompletedthethreadgetsdiscarded,thiswillresultinoverhead.Theideabehindthreadpooliscreateanumberofthreadsatprocessstartupandcethemintoapool,andwaitforworktobeassignedtothem.Threadpoolisusuallyaqueue.Whenevertheserverreceivesarequest,ifathreadisavailableinthepool,willreceivetherequest,processesitandonceit’scompletedthethreadwillreturntothethreadpoolandwaitsforthenextrequesttobeassignedtoit.SettingthenumberofthreadsinapooldependsonfactorssuchasthenumberofCPUsinthesystem,thenumberofphysicalmemory,andtheexpectednumberofconcurrent requests.SomethreadpoolsadjustthenumberofthreadsinthepoolaccordingtotheusagepatternUsuallyslightlyfastertoservicearequestwithanexistingthreadthancreateanewthreadAllowsthenumberofthreadsintheapplication(s)tobeboundtothesizeofthepoolSeparatingtasktobeperformedfrommechanicsofcreatingtaskallowsdifferentstrategiesforrunningtaskThreadcancellation(Asynchronous&Deferredcancellation,Cancellationspoint)Asynchronouscancellation–whereonethreadimmediayterminatesthetargetthread.Problemsassociatedwithasynchronouscancellationscouldinclude,operatingsystemnotbeingreclaimbacksomeoftheresourcesallocatedtocancelledthreads,andtherefore,asystem-wideresourcemaynotbed.Also,athreadmaygetcancelledinthemidstofupdatingdataitissharingwithotherthreads.Deferredcancellation–wherethetargetthreadperiodicallycheckswhetheritshouldterminate,allowingittoterminateitselfinanorderlymanner.Withdeferredcancellation,cancellationonlyoccurswhenthreadreachescancellationpoint.Thatistosay,onethreadindicatesthatatargetthreadisgoingtobecancelled,butcancellationoccursonlyafterthetargetthreadhascheckedaflagtodetermineifitshouldbecancelledornot.Thisallowsthethreadtobecancelledatasafepoint,deferredtoascancellationpoint.評：根據我十萬行的代碼開發(fā)經驗，Deferredcancellation確實threadcancellation還是要依據thread的模式滴。SignalhandlingSignalsareusedtonotifyaprocessthataparticulareventhasoccurred.Therearetwotypesofsignals:Synchronoussignals(ex:illegalmemoryaccessanddivisionbyAsynchronoussignals(ex;terminatingaprocesswhenyou<control><C>orhavingatimeAsignalmaybehandledbyoneoftwopossiblehandlers:AdefaultsignalhandlerAuser-definedsignalEverysignalhasadefaultsignalhandlerthatthekernelrunswhenhandlingthatsignal.Thisdefaultactioncanbeoverriddenbyauser-definedsignalhandlerthatiscalledtohandlethesignal.Insingle-threadedprograms,signalsaredeliveredtotheprocess.Inmultithreadedprograms,therearedifferentoptionsforsignalDeliverthesignaltothethreadtowhichthesignalDeliverthesignaltoeverythreadintheDeliverthesignaltocertainthreadintheAssignaspecificthreadtoreceiveallsignalsfortheThemethodfordeliveringasignaldependsonthetypeofsignalsEx:synchronoussignalsneedtobedeliveredtothethreadcausingthesignalandnottootherthreadsintheprocess.Thread-LocalThreadsbelongingtothesameprocesssharethedataoftheprocess.However,insomecases,eachthreadmightneeditsowncopyofcertaindata.Thisisusefulincaseswhenwedon’thavecontroloverthethread評：TLSthreadsdataTLS，讓threadsprivatedata.DifferentfromlocalLocalvariablesvisibleonlyduringsinglefunctioninvocation.ButTLSvisibleacrossfunctioninvocationsSchedulerBoth,Many-to-ManyandTwo-levelmodelsrequirecommunicationtomaintaintheappropriatenumberofkernelthreadsallocatedtotheapplication.Oneschemeforcommunicationbetweentheuser-threadlibraryandthekernelisknownasscheduleractivation。Light-weightAlightweightprocess(LWP),whichisanintermediatedatastructurecedbetweenuserandkernelthreads,canbeusedasavirtualonwhichaprocesscanscheduleauserthreadto