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LOCKUPFREEINSTRUCTIONFETCH/PREFETCHCACHEORGANIZATIONDAVIDKROFTCONTROLDATACANADA,LTDCANADIANDEVELOPMENTDIVISIONMISSISSAUGA,ONTARIO,CANADAABSTRACTINTHEPASTDECADE,THEREHASBEENMUCHLITERATUREDESCRIBINGVARIOUSCACHEORGANIZATIONSTHATEXPLOITGENERALPROGRAMMINGIDIOSYNCRASIESTOOBTAINMAXIMUMHITRATETHEPROBABILITYTHATAREQUESTEDDATUMISNOWRESIDENTINTHECACHELITTLE,IFANY,HASBEENPRESENTEDTOEXPLOIT1THEINHERENTDUALINPUTNATUREOFTHECACHEAND2THEMANYDATUMREFERENCETYPECENTRALPROCESSORINSTRUCTIONSNOMATTERHOWHIGHTHECACHEHITRATEIS,ACACHEMISSMAYIMPOSEAPENALTYONSUBSEQUENTCACHEREFERENCESTHISPENALTYISTHENECESSITYOFWAITINGUNTILTHEMISSEDREQUESTEDDATUMISRECEIVEDFROMCENTRALMEMORYAND,POSSIBLY,FORCACHEUPDATEFORTHETWOCASESABOVE,THECACHEREFERENCESFOLLOWINGAMISSDONOTREQUIRETHEINFORMATIONOFTHEDATUMNOTRESIDENTINTHECACHE,ANDARETHEREFOREPENALIZEDINTHISFASHIONINTHISPAPER,ACACHEORGANIZATIONISPRESENTEDTHATESSENTIALLYELIMINATESTHISPENALTYTHISCACHEORGANIZATIONALFEATUREHASBEENINCORPORATEDINACACHE/MEMORYINTERFACESUBSYSTEMDESIGN,ANDTHEDESIGNHASBEENIMPLEMENTEDANDPROTOTYPEDANEXISTINGSIMPLEINSTRUCTIONSETMACHINEHASVERIFIEDTHEADVANTAGEOFTHISFEATURE;FUTURE,MOREEXTENSIVEANDSOPHISTICATEDINSTRUCTIONSETMACHINESMAYOBVIOUSLYTAKEMOREADVANTAGEPRIORTOPROTOTYPING,SIMULATIONSVERIFIEDTHEADVANTAGEINTRODUCTIONACACHEBUFFERT,2ISASMALL,FASTMEMORYHOLDINGMOSTRECENTLYACCESSEDDATAANDITSSURROUNDINGNEIGHBORS,BECAUSETHEACCESSTIMEOFTHISBUFFERISUSUALLYANORDEROFMAGNITUDEGREATERTHANMAINORCENTRALMEMORY,ANDTHESTANDARDSOFTWAREPRACTICEISTOLOCALIZEDATA,THEEFFECTIVEMEMORYACCESSTIMEISCONSIDERABLYREDUCEDWHENACACHEBUFFERISINCLUDEDTHECOSTINCREMENTFORTHISWHENCOMPAREDWITHTHECOSTOFCENTRALMEMORYALONGWITHTHEABOVEACCESSTIMEADVANTAGEINFERSCOSTEFFECTIVENESSACACHEMISS,SPACEALLOCATION,ANDREPLACEMENTALGORITHMSTOMAXIMIZEHITRATEANOTHERMETHODPRESENTEDTOINCREASETHEHITRATEWASSELECTIVEPREFETCHINQALLTHESEMETHODSASSUMETHECACHECANHANDLEONLYONEREQUESTATATIME;ONAMISS,THECACHESTAYSBUSYSERVICINGTHEREQUESTUNTILTHEDATAISRECEIVEDFROMMEMORYAND,POSSIBLY,FORCACHEBUFFERUPDATEINTHISPAPER,ACACHEORGANIZATIONISPRESENTEDTHATINCREASESTHEEFFECTIVENESSOFANORMALCACHEINCLUSIONBYUSINGTHEINHERENTDUALINPUTNATUREOFANOVERALLCACHEANDTHEMANYDATAREFERENCEINSTRUCTIONSINOTHERWORDS,ITWOULDBEEXTREMELYUSEFULTOPIPELINETHEREQUESTSINTOTHECACHEATTHECACHEHITTHROUGHPUTRATEREGARDLESSOFANYMISSESIFTHISCOULDBEACCORNPLISHEDTHENALLFETCHANTI/ORPREFETCHOFINSTRUCTIONSCOULDBETOTALLYTRANSPARENTTOTHEEXECUTIONUNITALSO,FORINSTRUCTIONSTHATREQUIREANUMBEROFDATAREFERENCES,THEREQUESTSCOULDBEALMOSTENTIRELYOVERLAPPEDOBVIOUSLY,REQUESTSCOULDNOTBESTREAMEDINTOTHECACHEATTHEHITTHROUGHPUTRATEINDEFINITELYTHEREISALIMITTHISORGANIZATIONSLIMITISIMPOSEDBYTHENUMBEROFMISSESTHATHAVENOTBEENCOMPLETELYPROCESSEDTHATTHECACHEWILLKEEPTRACKOFSIMULTANEOUSLYWITHOUTLOCKUPORGANIZATIONINADDITIONTOTHESTANDARDBLOCKS,THISCACHEORGANIZATIONREQUIRESTHEFOLLOWING1ONEUNRESOLVEDMISSINFORMATION/STATUSHOLDINGREGISTERMSHRFOREACHMISSTHATWILLBEHANDLEDCONCURRENTLYNOW,ACCEPTINGTHEUSEFULNESSOFACACHEBUFFER,ONELOOKSINTOWAYSOFINCREASINGITSEFFECTIVENESS;THATIS,FURTHERDECREASINGTHEEFFECTIVEMEMORYACCESSTIMECONSIDERABLERESEARCHHASBEENDONETOFINETUNEACACHEDESIGNFORVARIOUSREQULRERNENTSPTHISFINETUNINGCONSISTEDOFSELECTINGOPTIMALTOTALCACHEBUFFERSIZE,BLOCKSIZETHENUMBEROFBYTESTOBEREQUESTEDON8101497111/81/0000/008100751981IEEE23ONENWAYCOMPARATOR,INWHICHNISTHENUMBEROFMSHRREGISTERS,FORREGISTERINGHITSONDATAINTRANSITFROMMEMORYANINPUTSTACKTOHOLDTHETOTALNUMBEROFRECEIVEDDATAWORDSPOSSIBLYOUTSTANDINGTHESIZEOFTHISSTACK,CONSEQUENTLY,ISEQUALTOTHEBLOCKSIZEINWORDSTIMESTHENUMBEROFMSHRREGISTERS4MSHRSTATUSUPDATEANDCOLLECTINGNETWORKS5THEAPPROPRIATECONTROLUNITENHANCEMENTTOACCOMMODATE1THROUGH4FIGURE1ISASIMPLIFIEDBLOCKDIAGRAMOFTHECACHEORGANIZATIONASETASSOCIATIVEOPERATIONISASSUMEDINCLUDEDARETHEREQUIREDBLOCKSFORASETASSOCIATIVECACHETAGARRAYSANDCONTROL,CACHEBUFFER,THECENTRALMEMORYINTERFACEBLOCKSMEMORYREQUESTOR,MEMORYRECEIVERANDTHECACHEENHANCEMENTBLOCKSMISSINFOHOLDINGREGISTERS,MISSCOMPARATORANDSTATUSCOLLECTION,INPUTSTACKTHEMISSINFOHOLDINGREGISTERSHOLDALLNECESSARYINFORMATIONTO1HANDLETHECENTRALMEMORYRECEIVEDDATAPROPERLYAND2INFORMTHEMAINCACHECONTROL,THROUGHTHEMISSCOMPARATORANDSTATUSCOLLECTOR,OFALLHITANDOTHERSTATUSOFDATAINTRANSITFROMMEMORYTHEINPUTSTACKISNECESSARYTOLEAVETHEMAINCACHEBUFFERAVAILABLEFOROVERLAPPEDREADSANDWRITESNOTETHATTHISORGANIZATIONALLOWSFORDATAJUSTRECEIVEDFROMMEMORYORINTHEINPUTSTACKTOBESENTIMMEDIATELYTOTHEREQUESTINGCPUUNITSOFCOURSE,THENUMBEROFMSHRREGISTERSISIMPORTANTASWITHSETSIZEBLOCKSPERSETTHEINCREMENTALVALUEDECREASESRAPIDLYWITHTHENUMBEROFREGISTERSTHISISGOOD,BECAUSETHECOSTINCREASESSIGNIFICANTLYWITHTHENUMBEROFREGISTERSFIGURE2PRESENTSAQUALITATIVECURVETHEAVERAGEDELAYTIMEISCAUSEDBYLOCKOUTONOUTSTANDINGMISSESTHISDELAYTIME,OFCOURSE,ISALSODEPENDENTONCACHEINPUTREQUESTANDHITRATESINTHEDEGENERATECASE,1MSHRREGISTEROFREDUCEDSIZEISREQUIRED;2MSHRREGISTERSALLOWFOROVERLAPWHILEONEMISSISOUTSTANDING,BUTSTILLWOULDLOCKUPTHECACHEINPUTONMULTIPLEMISSESOUTSTANDINGOWINGTOCOSTCONSIDERATIONSANDINCREMENTALEFFECTIVENESSGAINEDONINCREASINGTHENUMBEROFMSHRREGISTERS,4REGISTERSAPPEARTOBEOPTIMALFTHENECESSARYINFORMATIONCONTAINEDWITHINONEOFTHESEMSHRREGISTERSINCLUDESTHEFOLLOWINGFIRSTTHECACHEBUFFERADDRESS,ALONGWITHTHEINPUTREQUESTADDRESS,ISREQUIREDTHECACHEBUFFERADDRESSISKEPTTOKNOWWHERETOPLACETHERETURNINGMEMORYDATA;THEINPUTREQUESTADDRESSISSAVEDTODETERMINEIF,ONSUBSEQUENTREQUESTS,THEDATAREQUESTEDISONITSWAYFROMCENTRALMEMORYSECOND,INPUTREQUESTIDENTIFICATIONTAGS,ALONGWITHTHESENDTOCPUSTATUS,ARESTOREDTLIISINFORMATIONPERMITSTHECACHETORETURNTOCPUREQUESTINGUNITSONLYTHEDATAREQUESTEDANDRETURNITWITHITSIDENTIFICATIONTAGTHIRD,ININPUTSTACKINDICATORSAREUSEDTOALLOWFORREADINGDATADIRECTLYFROMTHEINPUTSTACKFOURTH,ACODEFOREXAMPLE,ONEBITPERBYTEFORPARTIALWRITEISHELDFOREACHWORDTOINDICATEWHATBYTESOFTHEWORDHAVEBEENWRITTENTOTHECACHEBUFFERTHISCODECONTROLSTHECACHEBUFFERWRITEUPDATEANDALLOWSDISPENSINGOFDATAFORBUFFERAREASTHATHAVEBEENTOTALLYWRITTENAFTERREQUESTEDTHECACHE,THUS,HASTHECAPABILITYOFPROCESSINGPARTIALWRITEINPUTREQUESTSONTHEFLYWITHOUTPURGINGOFCOURSE,THISPARTIALWRITECODEMAYNOTBEINCORPORATEDIFTHECACHEBLOCKISPURGEDONAPARTIALWRITEREQUESTTOAWORDINABLOCKINTRANSITFROMMEMORYLASTSOMECONTROLINFORMATIONTHEREGISTERCONTAINSVALIDINFORMATIONONLYFORRETURNINGREQUESTEDDATA,BUTNOTFORCACHEBUFFERUPDATEANDTHENUMBEROFWORDSOFTHEBLOCKTHATHAVEBEENRECEIVEDANDWRITTEN,IFREQUIRED,INTOTHECACHEBUFFERISNEEDEDTHEREFORE,EACHMSHRREGISTERCONTAINS1CACHEBUFFERADDRESS2INPUTREQUESTADDRESS3INPUTIDENTIFICATIONTAGSONEPERWORD4SENDTOCPUINDICATORSONEPERWORD5ININPUTSTACKINDICATORSONEPERWORD6PARTIALWRITECODESONEPERWORD7NUMBEROFWORDSOFBLOCKSPROCESSED8VALIDINFORMATIONINDICATOR9OBSOLETEINDICATORINFORMATIONNOTVALIDFORCACHEUPDATEORMSHRHITONDATAINTRANSITOPERATIONTHEOPERATIONCANBESPLITINTOTWOBASICPARTSMEMORYRECEIVER/INPUTSTACKOPERATIONSANDTAGARRAYCONTROLOPERATIONSFORMEMORYRECEIVER/INPUTSTACKOPERATIONS,THEFIELDSOFMSHRINTERROGATEDARETHEFOLLOWING1SENDTOCPUINDICATOR2INPUTIDENTIFICATIONTAGS3CACHEBUFFERADDRESS4PARTIALWRITECODES825OBSOLETEINDICATOR6VALIDINDICATORWHENAWORDISRECEIVEDFROMMEMORY,ITISSENTTOTHECPUREQUESTINGUNITIFTHESENDTOCPUINDICATORISSET;THEAPPROPRIATEIDENTIFICATIONTAGACCOMPANIESTHEDATATHISWORDISALSOWRITTENINTOTHEINPUTSTACKIFTHEWORDSSPACEHASNOTBEENPREVIOUSLYTOTALLYWRITTENINTHECACHEBUFFERORIFMSHRISNOTOBSOLETEINVALIDFORCACHEUPDATETHEWORDSOFDATAAREREMOVEDFROMTHISINPUTSTACKONAFIRSTIN,FIRSTOUTBASISANDAREWRITTENINTOTHECACHEBUFFERUSINGFIELDS3AND4OFCOURSE,MSHRMUSTHOLDVALIDINFORMATIONWHENINTERROGATED,ORANERRORSIGNALWILLBEGENERATEDASLIGHTDIVERSIONISNECESSARYATTHISPOINTTOEXPLAINCACHEDATATAGGINGONAMISS,THECACHEREQUESTSABLOCKOFWORDSALONGWITHEACHWORD,ACACHETAGISSENTTHISTAGPOINTSTOTHEPARTICULARASSIGNEDMSHRANDINDICATESTHEWORDOFTHEBLOCKNOTETHATTHECACHESAVESINMSHRTHEREQUESTINGUNITSIDENTIFICATIONTAGTHISTAGGINGCLOSESTHEREMAININGOPENLINKFORTHEHANDLINGOFDATARETURNEDFROMMEMORYANDREMOVESALLRESTRICTIONSONMEMORYONTHEORDEROFRESPONSESIFAPARTICULARPROCESSOR/MEMORYINTERFACEALLOWSFORADATAWIDTHOFABLOCKOFWORDSFORCACHETOCENTRALMEMORYREQUESTS,THECACHEDATATAGGINGMAYBESIMPLIFIEDBYMERELYPOINTINGTOTHEPARTICULARASSIGNEDMSHRIF,HOWEVER,ALLOTHERDATAPATHSARESTILLONEWORDWIDE,THEMAINOPERATIONSWOULDBEESSENTIALLYUNCHANGEDCONSEQUENTLY,THISEXTENDEDINTERFACEWOULDNOTSIGNIFICANTLYREDUCETHECONTROLCOMPLEXITYORTHEAVERAGELOCKOUTTIMEDELAYPERREQUESTTHEFIELDSOFTHEMSHRUPDATEDDURINGMEMORYRECEIVER/INPUTSTACKOPERATIONSARETHEFOLLOWINGTOTALLYWRITTENORHAVINGANOBSOLETEMSHR,ORISWRITTENINTOTHECACHEBUFFER,THENUMBEROFWORDSPROCESSEDCOUNTERISINCREMENTEDONNUMBEROFWORDSPROCESSEDCOUNTEROVERFLOWALLWORDSFORABLOCKHAVEBEENRECEIVED,THEVALIDORUSEDMSHRINDICATORISCLEAREDFORTAGARRAYCONTROLOPERATIONS,THEFOLLOWINGFIELDSOFMSHRSAREINTERROGATED1INPUTREQUESTADDRESSES2SENDTOCPUINDICATORS3ININPUTSTACKINDICATORS4PARTIALWRITECODES5VALIDINDICATOR6OBSOLETEINDICATORFIELDS1,5,AND6AREUSEDALONGWITHCURRENTINPUTREQUESTADDRESSANDTHENWAYMSHRCOMPARATORTODETERMINEIFTHEREISAHITONPREVIOUSLYMISSEDDATASTILLBEINGHANDLEDPREVIOUSMISSHITFIELDS2,3,AND4PRODUCEONEOFTHEFOLLOWINGSTATESFORTHEPREVIOUSMISSHITPARTIALLYWRITTENPARTIALWRITECODEHASATLEASTONEBITSETTOTALLYWRITTENPARTIALWRITECODEISALLTSININPUTSTACKALREADYASKEDFORSENDTOCPUINDICATORISALREADYSET3NUMBEROFWORDSOFBLOCKPROCESSED12ININPUTSTACKINDICATORSPARTIALWRITECODESFIGURE3INDICATESTHEACTIONSFOLLOWEDBYTHETAGARRAYCONTROLUNDERALLTHEABOVECOMBINATIONSFORAPREVIOUSMISSHITONAMISS,AMSHRISASSIGNED,ANDTHEFOLLOWINGISPERFORMED1VALIDINDICATORSET4VALIDINFORMATIONINDICATORBEINGUSEDINDICATOR2OBSOLETEINDICATORCLEAREDTHEININPUTSTACKINDICATORSARESETWHENTHEDATAWORDISWRITTENINTOTHEINPUTSTACKANDCLEAREDWHENDATAISREMOVEDFROMTHEINPUTSTACKANDWRITTENINTOTHECACHEBUFFERTHEPARTIALWRITECODEISSETTOINDICATETOTALLYWRITTENWHENTHEDATAWORDFROMCENTRALMEMORYINDICATESTHECACHEBUFFERINADDITION,WHENEVERADATAWORDISDISPOSEDOFBECAUSEOFBEING83345CACHEBUFFERADDRESSSAVEDINASSIGNEDMSHRINPUTREQUESTADDRESSSAVEDINASSIGNEDMSHRAPPROPRIATESENDTOCPUINDICATORSETANDOTHERSCLEARED7ALLPARTIALWRITECODESASSOCIATEDWITHASSIGNEDMSHRCLEARED8ALLMSHRSPOINTINGTOSAMECACHEBUFFERADDRESSPURGEDSETPARTIALWRITECODETOALLTSNOTETHATACTIONS5AND6WILLVARYIFTHECACHEFUNCTIONWASAPREFETCHALLSENDTOCPUINDICATORSARECLEARED,ANDNOTAGISSAVED
编号:201401051948186809    类型:共享资源    大小:420.46KB    格式:PDF    上传时间:2014-01-05
  
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