计算机操作系统原理5

Chapter5:ThreadsOverviewMultithreadingModelsThreadingIssues*Pthreads*Solaris2Threads*Windows2000Threads*LinuxThreads*JavaThreads*OperatingSystemConcepts5.1OverviewThread

–sometimescalledlightweightprocess(LWP),isabasicunitofCPUutilization;ComprisesathreadID,aprogramcounter,aregisterset,andastack.Shareswithotherthreadsbelongingtothesameprocessitscodesection,datasection,andotherOSresources,suchasopenfilesandsignals.Traditionalprocess–orheavyweightprocess;hasasinglethreadofcontrol.Aprocesshavingmultiplethreadsofcontrolcandomorethanonetaskatatime.OperatingSystemConceptsSingleandMultithreadedProcessesOperatingSystemConceptsManysoftwarepackagesrunningonmoderndesktopPCsaremultithreaded.Anapplicationtypicallyisimplementedasaseparate

process

with

severalthreadsofcontrol.Suchaswebbrowser,webserver,wordprocessor.Solutionsforasingleapplication(suchaswebserver)toperformseveralsimilartasks:Haveserverrunasasingleprocesstoacceptrequests.Whenreceivingarequest,itcreatesaprocesstoservicethatrequest.Amoreefficientwayistohaveoneprocessthatcontainsmultiplethreadstoservethesamepurpose.5.1Overview(1)

--MotivationOperatingSystemConcepts5.1Overview(2)

--BenefitsResponsiveness–Multithreadinganinteractiveapplicationmayallowaprogramtocontinuerunningevenifpartofitisblockedorisperformingalengthyoperation,soincreasingresponsiveness.ResourceSharing–threadssharememoryandtheresourcesoftheprocesstowhichtheybelong.Codesharingallowsanapplicationtohaveseveraldifferentthreadofactivityallwithinthesameaddressspace.Economy–allocatingmemoryandresourcesforprocesscreationiscostly.Threadsshareresourcesoftheprocess.Itismoreeconomicaltocreateandcontextswitchthreadsthanprocess.UtilizationofMPArchitectures–inaMParchitecture,eachthreadofmultithreadingcanruninparallelonadifferentprocessor.Itincreasesconcurrency.OperatingSystemConcepts5.1Overview(3)Userthread–managementdonebyuser-levelthreadslibraryandwithnosupportfromthekernel.threadscreationandschedulingaredoneinuserspacewithoutinterventionofkernel,sotheyarefasttocreateandmanage.Examples--POSIXPthreads,MachC-threads,SolaristhreadsKernelthreads–supporteddirectlybyoperatingsystem.Kernelperformsthreadcreation,scheduling,andmanagementinkernelspace.Ifathreadperformsblockingsystemcall,thekernelcanscheduleanotherthreadintheapplicationforexecution.Inmultiprocessorenvironment,kernelcanschedulethreadsondifferentprocessors.Examples--Windows95/98/NT/2000,Solaris,Tru64UNIX,Linux–UserandkernelThreadsOperatingSystemConcepts5.2MultithreadingModelsMany-to-OneManyuser-levelthreadsmapstosinglekernelthread.EntireprocesswillblockifathreadmakesablockingsystemcallOnlyonecanaccessthekernelatatime,sothreadsdon’truninparallelonmulti-CPUsmachine.Usedonsystemsthatdonotsupportkernelthreads.OperatingSystemConcepts5.2MultithreadingModels(1)One-to-One--Eachuser-levelthreadmapstoakernelthread.allowinganotherthreadtorunwhenathreadmakesblocking.AllowsmultiplethreadstoruninparallelonmultiprocessorsButeachuserthreadneedscreatingacorrespondingkernelthread,mayburdenperformanceofanapplication.Requiretorestrictthenumberofthreadssupportedbysystem.Examples--Windows95/98/NT/2000,OS/2OperatingSystemConcepts5.2MultithreadingModels(2)Many-to-ManyModel--Allowsmanyuserlevelthreadstobemappedtomanykernelthreads.Allowstheoperatingsystemtocreateasufficientnumberofkernelthreads.KernelthreadscanruninparallelonamultiprocessorsIfathreadperformsablockingsystemcall,kernelcanscheduleanotherthreadforexecution.Overcomesshortcomingsofmany-to-oneandone-to-onemodels.Examples--Solaris2,HP-UX,andTru64UNIX.OperatingSystemConceptsMany-to-ManyModelOperatingSystemConcepts5.3ThreadingIssuesSeveralissuestoconsiderwithmultithreadedprogramsSemanticsoffork()andexec()systemcalls.Threadcancellation.SignalhandlingThreadpoolsThreadspecificdataOperatingSystemConcepts5.4PthreadsaPOSIXstandard(IEEE1003.1c)APIforthreadcreationandsynchronization.APIspecifiesbehaviorofthethreadlibrary,implementationisuptodevelopmentofthelibrary.CommoninUNIXoperatingsystems.OperatingSystemConcepts5.5Solaris2ThreadsOperatingSystemConcepts5.5Solaris2Threads(1)SolarisProcessOperatingSystemConcepts5.6Windows2000ThreadsImplementstheone-to-onemapping.Eachthreadcontains -athreadid -registerset -separateuserandkernelstacks -privatedatastorageareaOperatingSystemConcepts5.7LinuxThreadsLinuxreferstothemastasksratherthanthreads.Threadcreationisdonethroughclone()systemcall.Clone()allowsachildtas