并行處理與體系結(jié)構(gòu)hitsz-lec08-review2

MemoryHierarchyReview2Outline7.4VirtualMemoryAddressMappingKeyDesignIssuesPageTablePageFaultsTranslationlookasidebuffer(TLB)ProtectionwithVirtualMemory7.5CommonFrameworkFourquestionMissrateswithsetassociativeandcachesizeThreemisses7.6Modernmemoryhierarchies7.8ConcludingRemark7.4VirtualMemoryMainmemorycanactasacacheforthesecondarystorage(disk)MotivationToallowefficientandsafesharingofmemoryamongmultipleprograms.AllowasingleuserprogramtoexceedthesizeofprimarymemoryPrevioussolution:programmersdividedprogramsintopieces,calledoverlaysAdvantages:illusionofhavingmorephysicalmemoryprogramrelocationprotection7.4AddressMappingforVirtualMemoryVirtualaddressPhysicaladdressPage:blocksofmemoryPageindisk7.4MappingFromVirtualtoPhysicalPages:virtualmemoryblocksVirtualaddressspace=232=4GBPhysicaladdressspace=230=1GB7.4KeyDesignIssuesinVMPagesshouldbelargeenoughtoamortizethehighaccesstimehugemisspenalty,thuspagesshouldbefairlylargeE.g32-64KBpagesReducethepagefaultratefull-associativeplacementPagefaultpenaltytakesmillionsofcyclescanhandlethefaultsinsoftwareinsteadofhardwareUsecleveralgorithmtoreplacepageswhilepagefaultsoccur.E.g.LRU(leastrecentlyused)Usewrite-backinsteadofwrite-throughusingwrite-throughistooexpensivesoweusewrite-back

7.4PageTablesPagetablesareusedtolocatedpagesbyusingafulltablethatindexesthememoryBecausefull-associativeplacementtolocateaentryisdifficultPagetablesarestoredinmainmemory,eachprogramhasitsownpagetable7.4PageTablesPagetableisindexedwiththevirtualpagenumbertoobtainthecorrespondingportionofthephysicaladdressExample:Pagesize:212=4KB,numberofpageentry:220=1millionentriesPageentryonlyneed19bits,otherbitsareusedtostoreadditionalinformation,tection7.4HandlePageFaultPagefaultsDataisnotinmemory,needtoretrieveitfromdisk,thevalidbitforavirtualpageisoffTheoperatingsystemtakesover,andthetransferisdonewiththeexceptionmechanism.TheOSmustfindthepageinthenextlevelofhierarchy,anddecidewheretoplacetherequestedpageinthemainmemory.LRUpolicyisoftenused.7.4ProcessContainThepagetable,togetherwiththeprogramcounterandtheregisters,specifiesthestateofaprogram.IfwewanttoallowanotherprogramtousetheCPU,wemustsavethisstate.Weoftenrefertothisstateasaprocess.Aprocessisconsideredactivewhenit’sinpossessionoftheCPU.7.4MakingAddressTranslationFastTranslationlookasidebuffer(TLB):AcacheforpagetableTagfield:IfnomatchingentryinTLBforapage,thepagetablemustbeexaminedTypicalTLBvaluesTLBsize:32–4096entriesBlocksize:1-2pagetableentriesHittime:0.5–1cycleMissPenalty:10-30clockcyclesMissrate:0.01%-1%7.4TLBandCachesImplementationImplementinDECStation3100,Cacheisdirectmap,TLBisfullyassociativeFullyassociativeTLBrequireeverytagtobecomparedwithindexIfTLBhit,pagenumberwithpageoffsetareusedtoaccessthecachesVirtualaddress7.4AccessTLBsandCachesProcessingareadorawritethroughthecacheDECStation3100TLBsandCaches7.4ProtectionwithVirtualMemoryVirtualMemoryallowsthesharingofasinglemainmemorybymultipleprocessesOSplacethepagetablesintheaddressspaceofOSIndependentvirtualpagesmaptodisjointphysicalpageUserprocesscannotchangethepagetablemapping7.4ProtectionwithVirtualMemoryThreebasiccapabilitiesforprotectionimplementationSupportatleasttwomodes,toindicatewhetheruseprocess(usermode)orsystemprocess(supervisormode)ProvideaportionCPUstatethatauseprocesscanreadbutnotwriteProvidemechanismwherebytheCPUcanswitchbetweenuserandsupervisormodes7.4HandlePageFaultsandTLBmissesTLBmissesindicate:Pageispresentinmemory,onlyneedtocreateamissingTLBentryPageisnotpresentinmemory,needtotransferthecontroltoOStodealwithpagefaultCauseasystemexceptionOSdealwithpagefaultinthreestepsLookupthepagetableentryusingvirtualaddressandfindthelocationofthereferencepageondiskChooseaphysicalpagetoreplace,ifthepageisdirty,mustwriteouttodiskbeforereadanewreferencepageStartreadareferencedpagefromdiskintothechosenphysicalpage7.4VirtualMemorySummaryVirtualmemoryCachebetweenmemoryanddiskVMallowsAprogramtoexpanditsaddressspaceSharingofmainmemoryamongmultipleprocessesToreducepagefaultLargepages–usespatiallocalityFullassociativemappingOSusecleverreplacementmethod(LRU,…)7.5MissRatesisImprovedastheAssociativelyIncreaseandCacheSize7.5ThreeMissesofMemoryHierarchiesAllmissesareclassifiedintooneofthreecategories:CompulsorymissesMissedbyfirstaccesstoablock,calledclod-startmissesCapacitymissesCachecannotcontainalltheblocksConflictmissesOccurinsetassociativeordirectmappedcachewhenmultipleblockscompeteforthesameset,alsocalledcollisionmissesProcessorspeedscontinuetoincreaseveryfastmuchfasterthaneitherDRAMordiskaccesstimes

Designchallengedealingwiththisgrowingdisp