WINDOWS操作系统中可以允许最大的线程数

ProcessesandThreads

AWindowsprocessisessentiallycontainerthathoststheexecutionofanexecutableimagefile.ItisrepresentedwithakernelprocessobjectandWindowsusestheprocessobjectanditsassociateddatastructurestostoreandtrackinformationabouttheimage’sexecution.Forexample,aprocesshasavirtualaddressspacethatholdstheprocess’sprivateandshareddataandintowhichtheexecutableimageanditsassociatedDLLsaremapped.Windowsrecordstheprocess’suseofresourcesforaccountingandquerybydiagnostictoolsanditregisterstheprocess’sreferencestooperatingsystemobjectsintheprocess’shandletable.Processesoperatewithasecuritycontext,calledatoken,thatidentifiestheuseraccount,accountgroups,andprivilegesassignedtotheprocess.

Finally,aprocessincludesoneormorethreadsthatactuallyexecutethecodeintheprocess(technically,processesdon’trun,threadsdo)andthatarerepresentedwithkernelthreadobjects.Thereareseveralreasonsapplicationscreatethreadsinadditiontotheirdefaultinitialthread:processeswithauserinterfacetypicallycreatethreadstoexecuteworksothatthemainthreadremainsresponsivetouserinputandwindowingcommands;applicationsthatwanttotakeadvantageofmultipleprocessorsforscalabilityorthatwanttocontinueexecutingwhilethreadsaretiedupwaitingforsynchronousI/Ooperationstocompletealsobenefitfrommultiplethreads.

ThreadLimits

Besidesbasicinformationaboutathread,includingitsCPUregisterstate,schedulingpriority,andresourceusageaccounting,everythreadhasaportionoftheprocessaddressspaceassignedtoit,calledastack,whichthethreadcanuseasscratchstorageasitexecutesprogramcodetopassfunctionparameters,maintainlocalvariables,andsavefunctionreturnaddresses.Sothatthesystem’svirtualmemoryisn’tunnecessarilywasted,onlypartofthestackisinitiallyallocated,orcommittedandtherestissimplyreserved.Becausestacksgrowdownwardinmemory,thesystemplacesguardpagesbeyondthecommittedpartofthestackthattriggeranautomaticcommitmentofadditionalmemory(calledastackexpansion)whenaccessed.Thisfigureshowshowastack’scommittedregiongrowsdownandtheguardpagemoveswhenthestackexpands,witha32-bitaddressspaceasanexample(notdrawntoscale):





ThePortableExecutable(PE)structuresoftheexecutableimagespecifytheamountofaddressspacereservedandinitiallycommittedforathread’sstack.Thelinkerdefaultstoareserveof1MBandcommitofonepage(4K),butdeveloperscanoverridethesevalueseitherbychangingthePEvalueswhentheylinktheirprogramorforanindividualthreadinacalltoCreateThread.YoucanuseatoollikeDumpbinthatcomeswithVisualStudiotolookatthesettingsforanexecutable.Here’stheDumpbinoutputwiththe/headersoptionfortheexecutablegeneratedbyanewVisualStudioproject:





Convertingthenumbersfromhexadecimal,youcanseethestackreservesizeis1MBandtheinitialcommitis4KandusingthenewSysinternalsVMMaptooltoattachtothisprocessandviewitsaddressspace,youcanclearlyseeathreadstack’sinitialcommittedpage,aguardpage,andtherestofthereservedstackmemory:





Becauseeachthreadconsumespartofaprocess’saddressspace,processeshaveabasiclimitonthenumberofthreadstheycancreatethat’simposedbythesizeoftheiraddressspacedividedbythethreadstacksize.

32-bitThreadLimits

Evenifthethreadhadnocodeordataandtheentireaddressspacecouldbeusedforstacks,a32-bitprocesswiththedefault2GBaddressspacecouldcreateatmost2,048threads.Here’stheoutputoftheTestlimittoolrunningon32-bitWindowswiththe–tswitch(createthreads)confirmingthatlimit:





Again,sincepartoftheaddressspacewasalreadyusedbythecodeandinitialheap,notallofthe2GBwasavailableforthreadstacks,thusthetotalthreadscreatedcouldnotquitereachthetheoreticallimitof2,048.

IlinkedtheTestlimitexecutablewiththelargeaddressspace-awareoption,meaningthatifit’spresentedwithmorethan2GBofaddressspace(forexampleon32-bitsystemsbootedwiththe/3GBor/USERVABoot.inioptionoritsequivalentBCDoptiononVistaandlaterincreaseuserva),itwilluseit.32-bitprocessesaregiven4GBofaddressspacewhentheyrunon64-bitWindows,sohowmanythreadscanthe32-bitTestlimitcreatewhenrunon64-bitWindows?Basedonwhatwe’vecoveredsofar,theanswershouldberoughly4096(4GBdividedby1MB),butthenumberisactuallysignificantlysmaller.Here’s32-bitTestlimitrunningon64-bitWindowsXP:





Thereasonforthediscrepancycomesfromthefactthatwhenyouruna32-bitapplicationon64-bitWindows,itisactuallya64-bitprocessthatexecutes64-bitcodeonbehalfofthe32-bitthreads,andthereforethereisa64-bitthreadstackanda32-bitthreadstackareareservedforeachthread.The64-bitstackhasareserveof256K(exceptthatonsystemspriortoVista,theinitialthread’s64-bitstackis1MB).Becauseevery32-bitthreadbeginsitslifein64-bitmodeandthestackspaceituseswhenstartingexceedsapage,you’lltypicallyseeatleast16KBofthe64-bitstackcommitted.Here’sanexampleofa32-bitthread’s64-bitand32-bitstacks(theonelabeled“Wow64”isthe32-bitstack):





32-bitTestlimitwasabletocreate3,204threadson64-bitWindows,whichgiventhateachthreaduses1MB+256Kofaddressspaceforstack(again,exceptthefirstonversionsofWindowspriortoVista,whichuses1MB+1MB),isexactlywhatyou’dexpect.IgotdifferentresultswhenIran32-bitTestlimiton64-bitWindows7,however:





ThedifferencebetweentheWindowsXPresultandtheWindows7resultiscausedbythemorerandomnatureofaddressspacelayoutintroducedinWindowsVista,AddressSpaceLoadRandomization(ASLR),thatleadstosomefragmentation.RandomizationofDLLloading,threadstackandheapplacement,helpsdefendagainstmalwarecodeinjection.AsyoucanseefromthisVMMapoutput,there’s357MBofaddressspacestillavailable,butthelargestfreeblockisonly128Kinsize,whichissmallerthanthe1MBrequiredfora32-bitstack:





AsImentioned,adevelopercanoverridethedefaultstackreserve.Onereasontodosoistoavoidwastingaddressspacewhenathread’sstackusagewillalwaysbesignificantlylessthanthedefault1MB.TestlimitsetsthedefaultstackreservationinitsPEimageto64Kandwhenyouincludethe–nswitchalongwiththe–tswitch,Testlimitcreatesthreadswith64Kstacks.Here’stheoutputona32-bitWindowsXPsystemwith256MBRAM(Ididthisexperimentonasmallsystemtohighlightthisparticularlimit):





Notethedifferenterror,whichimpliesthataddressspaceisn’ttheissuehere.Infact,64Kstacksshouldallowforaround32,000threads(2GB/64K=32,768).What’sthelimitthat’sbeinghitinthiscase?Alookatthelikelycandidates,includingcommitandpool,don’tgiveanyclues,asthey’reallbelowtheirlimits:





It’sonlyalookatadditionalmemoryinformationinthekerneldebuggerthatrevealsthethresholdthat’sbeinghit,residentavailablememory,whichhasbeenexhausted:





ResidentavailablememoryisthephysicalmemorythatcanbeassignedtodataorcodethatmustbekeptinRAM.Nonpagedpoolandnonpageddriverscountagainstit,forexample,asdoesmemorythat’slockedinRAMfordeviceI/Ooperations.Everythreadhasbothauser-modestack,whichiswhatI’vebeentalkingabout,buttheyalsohaveakernel-modestackthat’susedwhentheyruninkernelmode,forexamplewhileexecutingsystemcalls.Whenathreadisactiveitskernelstackislockedinmemorysothatthethreadcanexecutecodeinthekernelthatcan’tpagefault.

Abasickernelstackis12Kon32-bitWindowsand24Kon64-bitWindows.14,225threadsrequireabout170MBofresidentavailablememory,whichcorrespondstoexactlyhowmuchisfreeonthissystemwhenTestlimitisn’trunning:





Oncetheresidentavailablememorylimitishit,manybasicoperationsbeginfailing.Forexample,here’stheerrorIgotwhenIdouble-clickedonthedesktop’sInternetExplorershortcut:





Asexpected,whenrunon64-bitWindowswith256MBofRAM,Testlimitisonlyabletocreate6,600threads–roughlyhalfwhatitcreatedon32-bitWindowswith256MBRAM–beforerunningoutofresidentavailablememory:





ThereasonIsaid“basic”kernelstackearlieristhatathreadthatexecutesgraphicsorwindowingfunctionsgetsa“large”stackwhenitexecutesthefirstcallthat’s20Kon32-bitWindowsand48Kon64-bitWindows.Testlimit’sthreadsdon’tcallanysuchAPIs,sotheyhavebasickernelstacks.

64-bitThreadLimits

Like32-bitthreads,64-bitthreadsalsohaveadefaultof1MBreservedforstack,but64-bitprocesseshaveamuchlargeruser-modeaddressspace(8TB),soaddressspaceshouldn’tbeanissuewhenitcomestocreatinglargenumbersofthreads.Residentavailablememoryisobviouslystillapotentiallimiter,though.The64-bitversionofTestlimit(Testlimit64.exe)wasabletocreatearound6,600threadswithandwithoutthe–nswitchonthe256MB64-bitWindowsXPsystem,thesamenumberthatthe32-bitversioncreated,becauseitalsohittheresidentavailablememorylimit.However,onasystemwith2GBofRAM,Testlimit64wasabletocreateonly55,000threads,farbelowthenumberitshouldhavebeenabletoifresidentavailablememorywasthelimiter(2GB/24K=89,000):





Inthiscase,it’stheinitialthreadstackcommitthatcausesthesystemtorunoutofvirtualmemoryandthe“pagingfileistoosmall”error.OncethecommitlevelreachedthesizeofRAM,therateofthreadcreationslowedtoacrawlbecausethesystemstartedthrashing,pagingoutstacksofthreadscreatedearliertomakeroomforthestacksofnewthreads,andthepagingfilehadtoexpand.Theresultsarethesamewhenthe–nswitchisspecified,becausethethreadshavethesameinitialstackcommitment.

ProcessLimits

ThenumberofprocessesthatWindowssupportsobviouslymustbelessthanthenumberofthreads,sinceeachprocesshasonethreadandaprocessitselfcausesadditionalresourceusage.32-bitTestlimitrunningona2GB64-bitWindowsXPsystemcreatedabout8,400processes:





Alookinthekerneldebuggershowsthatithittheresidentavailablememorylimit:





Iftheonlycostofaprocesswithrespecttoresidentavailablememorywasthekernel-modethreadstack,Testlimitwouldhavebeenabletocreatefarmorethan8,400threadsona2GBsystem.TheamountofresidentavailablememoryonthissystemwhenTestlimitisn’trunningis1.9GB:





DividingtheamountofresidentmemoryTestlimitused(1.9GB)bythenumberofprocessesitcreated(8,400)yields230Kofresidentmemoryperprocess.Sincea64-bitkernelstackis24K,thatleavesabout206Kunaccountedfor.Where’stherestofthecostcomingfrom?Whenaprocessiscreated,Windowsreservesenoughphysicalmemorytoaccommodatetheprocess’sminimumworkingsetsize.Thisactsasaguaranteetotheprocessthatnomatterwhat,therewillenoughphysicalmemoryavailabletoholdenoughdatatosatisfyitsminimumworkingset.Thedefaultworkingsetsizehappenstobe200KB,afactthat’sevidentwhenyouaddtheMinimumWorkingSetcolumntoProcessExplorer’sdisplay:





Theremainingroughly6Kisresidentavailablememorychargedforadditionalnon-pageablememoryallocatedtorepresentaprocess.Aprocesson32-bitWindowswilluseslightlylessresidentmemorybecauseitskernel-modethreadstackissmaller.

Astheycanforuser-modethreadstacks,processescanoverridetheirdefaultworkingsetsizewiththeSetProcessWorkingSetSizefunction.Testlimitsupportsa–nswitch,thatwhencombinedwith–p,causeschildprocessesofthemainTestlimitprocesstosettheirworkingsettotheminimumpossible,whichis80K.Becausethechildprocessesmustruntoshrinktheirworkingsets,Testlimitsleepsafteritcan’tcreateanymoreprocessesandthentriesagaintogiveitschildrenachancetoexecute.Testlimitexecutedwiththe–nswitchonaWindows7systemwith4GBofRAMhitalimitotherthanresidentavailablememory:thesystemcommitlimit:





Hereyoucanseethekerneldebuggerreportingnotonlythatthesystemcommitlimithadbeenhit,butthattherehavebeenthousandsofmemoryallocationfailures,bothvirtualandpagedpoolallocations,followingtheexhaustionofthecommitlimit(thesystemcommitlimitwasactuallyhitseveraltimesasthepagingfilewasfilledandthengrowntoraisethelimit):





ThebaselinecommitmentbeforeTestlimitranwasabout1.5GB,sothethreadshadconsumedabout8GBofcommittedmemory.Eachprocessthereforeconsumedroughly8GB/6,600,or1.2MB.Theoutputofthekerneldebugger’s!vmcommand,whichshowstheprivatememoryallocatedbyeachactiveprocess,confirmsthatcalculation:





Theinitialthreadstackcommitment,describedearlier,hasanegligibleimpactwiththerestcomingfromthememoryrequiredfortheprocessaddressspacedatastructures,pagetableentries,thehandletable,processandthreadobjects,andprivatedatatheprocesscreateswhenitinitializes.

HowManyThreadsandProcessesareEnough?

Sotheanswertothequestions,“howmanythreadsdoesWindowssupport?”and“howmanyprocessescanyourunconcurrentlyonWindows?”depends.Inadditiontothenuancesofthewaythatthethreadsspecifytheirstacksizesandprocessesspecifytheirminimumworkingsets,thetwomajorfactorsthatdeterminetheansweronanyparticularsystemincludetheamountofphysicalmemoryandthesystemcommitlimit.Inanycase,applicationsthatcreateenoughthreadsorprocessestogetanywhereneartheselimitsshouldrethinktheirdesign,astherearealmostalwaysalternatewaystoaccomplishthesamegoalswithareasonablenumber.Forinstance,thegeneralgoalforascalableapplicationistokeepthenumberofthreadsrunningequaltothenumberofCPUs(withNUMAchangingthistoconsiderCPUspernode)andonewaytoachievethatistoswitchfromusingsynchronousI/OtousingasynchronousI/OandrelyonI/OcompletionportstohelpmatchthenumberofrunningthreadstothenumberofCPUs.

Win7,Server2008R2最大线程数限制

最近在做压力测试时发现Win7和Server2008R2系统内线程数设为1500则无法创建线程池，深入分析发现32位和64位程序存在很大性能差异。

最大线程数：

32bit：1450

64bit：100000

测试代码如下:

[cpp]viewplaincopy

#include"stdafx.h"

#include<stdio.h>

#include<windows.h>

DWORDCALLBACKThreadProc(void*)

{

Sleep(INFINITE);

return0;

}

int__cdeclmain(intargc,constchar*argv[])

{

inti;

for(i=0;i<100000;i++)

{

DWORDid;

HANDLEh=CreateThread(NULL,4096,ThreadProc,NULL,STACK_SIZE_PARAM_IS_A_RESERVATION,&id);

if(!h)

break;

CloseHandle(h);

printf("%d\n",i);

}

//default1413[3/18/2012WangJinhui]

printf("Created%dthreads\n",i);return0;

}