day2Jrockit.docx

day2Jrockit.docx

《day2Jrockit.docx》由会员分享，可在线阅读，更多相关《day2Jrockit.docx（20页珍藏版）》请在冰豆网上搜索。

day2Jrockit

Jrockit

时间：

8

1.ExplainOracleJRockitJDK

AbouttheJDK

TheJRockitJVMisahigh-performanceJVMdevelopedtoensurereliability,scalability,manageability,andflexibilityforJavaapplications.TheJRockitJVMprovidesimprovedperformanceforJavaapplicationsdeployedonIntel32-bit（Xeon）and64-bit（XeonandSPARC）architecturesatsignificantlylowercoststotheenterprise.Further,itistheonlyenterprise-classJVMoptimizedforIntelarchitectures,providingseamlessinteroperabilityacrossmultiplehardwareandoperatingsystemconfigurations.TheJRockitJVMenablesyourJavaapplicationstorunoptimallyonWindowsandLinuxoperatingsystems（both32-bitand64-bitarchitectures）.TheJRockitJVMisespeciallywell-suitedforrunningOracleWebLogicServer.

TheJRockitJVMisonecomponentoftheOracleJRockitJavadevelopmentkit（JDK）.InadditiontotheJRockitJVM,theJDKiscomprisedoftheJavaRuntimeEnvironment（JRE）,whichcontainstheJVMandJavaclasslibraries（asspecifiedbytheJavaPlatform,StandardEdition6APISpecification）,aswellasasetofdevelopmenttools,suchasacompiler.

TheJRockitJDKiscompatiblewithJ2SE5.0andJavaSE6.

2.UseJITCompilationandOptimizations

just-in-timecompilation（JIT）,alsoknownasdynamictranslation,isamethodtoimprovetheruntimeperformanceofcomputerprograms.Traditionally,computerprogramshadtwomodesofruntimeoperation,eitherinterpretedorstatic（ahead-of-time）compilation.Interpretedcodeistranslatedfromahigh-levellanguagetoamachinecodecontinuouslyduringeveryexecution,whereasstaticallycompiledcodeistranslatedintomachinecodebeforeexecution,andonlyrequiresthistranslationonce.

JITcompilersrepresentahybridapproach,withtranslationoccurringcontinuously,aswithinterpreters,butwithcachingoftranslatedcodetominimizeperformancedegradation.Italsooffersotheradvantagesoverstaticallycompiledcodeatdevelopmenttime,suchashandlingoflate-bounddatatypesandtheabilitytoenforcesecurityguarantees.

JITbuildsupontwoearlierideasinrun-timeenvironments:

bytecodecompilationanddynamiccompilation.Itconvertscodeatruntimepriortoexecutingitnatively,forexamplebytecodeintonativemachinecode.

Severalmodernruntimeenvironments,suchasMicrosoft's.NETFrameworkandmostimplementationsofJava,relyonJITcompilationforhigh-speedcodeexecution.

Step1:

JITCompilation

Thefirststepinthemachine-codegenerationprocessisjust-in-time（JIT）compilation.JRockitdoesnotincludeaninterpreter;sotheJITcompilationofthebytecodeintonativemachinecodehastooccurbeforeamethodexecutes.TheJITcompilationisperformedthefirsttimeaJavamethodiscalled.

TheJITcompilerisfastandgeneratesmoderatelyefficientcode.ThisisnecessarytoenabletheJavaapplicationtostartandrunquickly.Subsequently,profilingrevealsfrequentlycalledmethods（hotspots）thatrequirefurtheroptimization.TheJRockitapproach—JITcompilationandnointerpreter—resultsinrelativelylongerstartuptimes,buteveniftheJITcompilationresultsinonlymoderatelyefficientcode,thegeneratedcodeisstillsignificantlyfasterthaninterpretedcode.

Step2:

ThreadMonitoring

Duringthesecondstepofthemachine-codegenerationprocess,theJRockitJVMusesasophisticated,low-cost,samplingbasedtechniquetoidentifywhichfunctionsmeritoptimization:

a"samplerthread"wakesupperiodicallyandchecksthestatusofseveralapplicationthreads.Itidentifieswhateachthreadisexecutingandlogssomeexecutionhistory.Thisinformationistrackedforallthemethods;whentheinformationindicatesthatamethodisheavilyused（hot）,thatmethodisearmarkedforoptimization.Usually,aflurryofsuchoptimizationopportunitiesoccurintheapplication'searlyrunstages,withtherateslowingdownasexecutioncontinues.

Step3:

CodeOptimization

Codeoptimizationisaprocessbywhichcommonly-executedcodeisrecompiledtomakeitrunmoreefficiently.

ThefirsttimethattheJRockitJVMrunsamethod,themethodiscompiledintomachinecode.Thiscompilationisquick,buttheresultingcodeisnotasefficientasitcouldbe.Thiscodeisacceptableformethodsthatarerunonceanddiscarded;however,ifamethodisusedrepeatedly,thesystemcangetaperformanceboostifthecodeforthatparticularmethodisregeneratedinamoreefficientway.

TheJRockitJVMoptimizesthesecommonly-executed（hot）methodstomakethecodeasefficientaspossible.Theoptimizationrunsinthebackgroundanddoesnotinterferewiththerunningapplication.

3.ManageMemory

Mnumberof"static"collectorsthatcanbestartedeitherbyspecifyingtheactualcollector（-Xgc:

）orbydefault,asdeterminedbytheJVMmodeyouselectatstartup.

Commandlineoption:

-Xns:

Thesizeofthenurseryhasanimpactonallocationspeed,garbagecollectionfrequencyandgarbagecollectiontimes.Asmallnurserywillbecomefullquicklyandmustbegarbagecollectedmoreoften,whilegarbagecollectionofalargenurserytakesslightlylongertime.Anurserythatissosmallthatfewornoobjectshavediedbeforeayoungcollectionisstartedisofverylittleuse,andneitherisanurserythatissolargethatnoyoungcollectionsareperformedbetweengarbagecollectionsofthewholeheapthataretriggeredduetoallocationoflargeobjectsinoldspace.

SettingaPauseTargetforDeterministicMode

Commandlineoption:

-XpauseTarget:

Thedeterministicmodeusesapausetargetforoptimizingthepausetimes.Thepausetargetimpactstheapplicationthroughput,asalowerpausetargetwillinflictmoreoverheadonthememorymanagementsystem.Setthepausetargetashighasyourapplicationcantolerate.

Thegarbagecollectorwillaimonkeepingthegarbagecollectionpausesbelowthegivenpausetarget.

SettingaPauseTargetforPausetimeMode

Commandlineoption:

-XpauseTarget:

Thepausetimemodeusesapausetargetforoptimizingthepausetimes.Thepausetargetimpactstheapplicationthroughput,asalowerpausetargetwillinflictmoreoverheadonthememorymanagementsystem.Setthepausetargetashighasyourapplicationcantolerate.

4.ManageThreadsandLocks

UnderstandingThreads

AjavaapplicationconsistsofoneormorethreadsthatrunJavacode.TheentireJVMprocessconsistsoftheJavathreadsandsomeJVMinternalthreads,forexampleoneormoregarbagecollectionthreads,acodeoptimizerthreadandoneormorefinalizerthreads.

Fromtheoperatingsystem’spointofviewtheJavathreadsarejustlikeanyapplicationthreads.Schedulingofthethreadsishandledbytheoperatingsystem,aswellasthreadpriorities.

WithinJava,theJavathreadsarerepresentedbythreadobjects.Eachthreadalsohasastack,usedforstoringruntimedata.Thethreadstackhasaspecificsize.Ifathreadtriestostoremoreitemsonthestackthanthestacksizeallows,thethreadwillthrowastackoverflowerror.

UnderstandingLocks

Whenthreadsinaprocessshareandupdatethesamedata,theiractivitiesmustbesynchronizedtoavoiderrors.InJava,thisisdonewiththesynchronizedkeyword,orwithwaitandnotify.Synchronizationisachievedbytheuseoflocks,eachofwhichisassociatedwithanobjectbytheJVM.Forathreadtoworkonanobject,itmusthavecontroloverthelockassociatedwithit,itmust“hold”thelock.Onlyonethreadcanholdalockatatime.Ifathreadtriestotakealockthatisalreadyheldbyanotherthread,thenitmustwaituntilthelockisreleased.Whenthishappens,thereissocalled“contention”forthelock.

Therearefourdifferentkindsoflocks:

•Fatlocks:

Afatlockisalockwithahistoryofcontention（severalthreadstryingtotakethelocksimultaneously）,oralockthathasbeenwaitedon（fornotification）.

•Thinlocks:

Athinlockisalockthatdoesnothaveanycontention.

●Recursivelocks:

Arecursivelockisalockthathasbeentakenbyathreadseveraltimeswithouthavingbeenreleased.

•Lazylocks:

Alazylockisalockthatisnotreleasedwhenacriticalsectionisexited.Oncealazylockisacquiredbyathread,otherthreadsthattrytoacquirethelockhavetoensurethatthelockis,orcanbe,released.LazylocksareusedbydefaultinOracleJRockitJVM27.6.Inolderreleases,lazylocksareonlyusedifyouhavestartedtheJVMwiththe-XXlazyUnlockingoption.

Athinlockcanbeinflatedtoafatlockandafatlockcanbedeflatedtoathinlock.TheJRockitJVMusesacomplexsetofheuristicstodeterminewhentoinflateathinlocktoafatlockandwhentodeflateafatlocktoathinlock.

LockChains

Severalthreadscanbetiedupinwhatiscalledlockchains.Althoughtheyappearsomewhatcomplex,lockchainsarefairlystraightforward.Theycanbedefinedasfollows:

●ThreadsAandBformalockchainifthreadAholdsalockthatthreadBistryingtotake.IfAisnottryingtotakealock,thenthelockchainis“open.”

●IfA->Bisalockchain,andB->Cisalockchain,thenA->B->Cisamorecompletelockchain.

●IfthereisnoadditionalthreadwaitingforalockheldbyC,thenA->B->Cisacompleteandopenlockchain.

LockChainTypes

TheJRockitJVManalyzesthethreadsandformscompletelockchains.Therearethreepossiblekindsoflockchains:

Open,DeadlockedandBlockedlockchains.

OpenChains

Openlockchainsrepresentastraightdependency,threadA