30-Structural aspects of the System 360 Model 85, Part II The cache.pdf -- 5 元

Thecache,ahighspeedbufferestablishingastoragehierarchyintheModel85,isdiscussedindepthinthispart,sinceitrepresentsthebasicorganizationaldeparturefromotherSYSTEM/BOcomputers.Discussedareorganizationandoperationofthecache,includingthemechanismsusedtolocateandretrievedataneededbytheprocessor.Theinternalperformancestudiesthatledtouseofthecachearedescribed,andsimulatedperformanceofthechosenconfigurationiscomparedwiththatofatheoreticalsystemhavinganentire80nanosecondmainstorage.Finally,theeffectsofvaryingcacheparametersarediscussedandtabulated.StructuralaspectsoftheSystem/360Model8511ThecachebyJ.S.LiptayAmongtheobjectivesoftheModel85isthatofprovidingaSYSTEM/compatibleprocessorwithbothhighperformanceandhighthroughput.OneoftheimportantingredientsofhighthroughputisalargemainstoragecapacityseetheaccompanyingarticleinPartI.However,itisnotfeasibletoprovidealargemainstoragewithanaccesstimecommensuratewiththe80nanosecondprocessorcycleoftheModel85.Alongeraccesstimecanbepartiallycompensatedforbyanincreaseinoverlap,greaterbuffering,deeperstorageinterleaving,moresophisticationinthehandlingofbranches,andotherimprovementsintheprocessor.Allofthesefactorsonlypartiallycompensatefortheslowerstorage,and,therefore,wedecidedtouseastoragehierarchyinstead.Thestoragehierarchyconsistsofa1.04microsecondmainstorageandasmall,faststorecalledacache,lwhichisintegratedintotheCPU.Thecacheisnotaddressablebyaprogram,butratherisusedtoholdthecontentsofthoseportionsofmainstoragethatarecurrentlybeingused.Mostprocessorfetchescanthenbehandledbyreferringtothecache,sothatmostofthetimetheprocessorhasashortaccesstime.Whentheprogramstartsoperatingondatainadifferentportionofmainstorage,thedatainthatportionmustbeloadedintothecacheandthedatafromsomeotherportionremoved.Thisactivitymusttakeplacewithoutprogramassistance,sincetheModel85mustbecompatiblewiththerestofThispaperdiscussesorganizationofthecacheandthestudiesthatledtoitsuseintheModel85andtoselectingofvaluesforitsparameters.theSYSTEM/360line.ZBMSYSTEMSJOURNALVOL.7NO.1196815Figure1AssignmentofcachesectorstomainstoragesectorsMAINSTORAGECacheorganizationThemainstorageunitsthatcanbeusedontheModel85aretheIBM23655andthe2385.Theyhavea1.04microsecondcycletimeandmakeavailablecapacitiesfrom512Kbytesto4096KbytesK1024.Thecacheisa16Kbyteintegratedstorage,whichiscapableofoperatingeveryprocessorcycle.Optionally,itcanbeexpandedto24Kbytesor32Kbytes.Boththecacheandmainstoragearelogicallydividedintosectors,eachconsistingof1Kcontiguousbytesstartingon1Kbyteboundaries.Duringoperation,acorrespondenceissetupbetweencachesectorsandmainstoragesectorsinwhicheachcachesectorisassignedtoasingledifferentmainstoragesector.However,becauseofthelimitednumberofcachesectors,mostmainstoragesectorsdonothaveanycachesectorsassignedtothemseeFigure1.Eachofthecachesectorshasa14bitsectoraddressregister,whichholdstheaddressofthemainstoragesectortowhichitisassigned.Theassignmentofcachesectorsisdynamicallyadjustedduringassigningoperation,sothattheyareassignedtothemainstoragesectorsthatcachearecurrentlybeingusedbytheprogram.Iftheprogramcausesasectorsfetchfromamainstoragesectorthatdoesnothaveacachesectorassignedtoit,oneofthecachesectorsisthenreassignedtothatmainstoragesector.Tomakeagoodselectionofacachesectortoreassign,enoughinformationismaintainedtoorderthecachesectorsintoanactivitylist.Thesectoratthetopofthelististheonethatwasmostrecentlyreferredto,thesecondoneisthenextmostrecentlyreferredto,andsoforth.Whenacachesectorisreferredto,itismovedtothetopofthelist,andtheinterveningonesaremoveddownoneposition.Thisisnotmeanttoimplyanactualmovementofsectorswithinthecache,butratherreferstoalogical16J.8.LIPTAYorderingofthesectors.Whenitisnecessarytoreassignasector,theoneselectedistheoneatthebottomoftheactivitylist.Thiscachesectoristheonethathasgonethelongestwithoutbeingreferredto.Whenacachesectorisassignedtoadifferentmainstoragesector,thecontentsofallofthe1Kbyteslocatedinthatmainstoragesectorarenotloadedintothecacheatonce.Rather,eachsectorisdividedinto16blocksof64bytes,andtheblocksareloadedonademandbasis.Whenacachesectorisreassigned,theonlyblockthatisloadedistheonethatwasreferredto.Iftheyarerequired,theremainingblocksareloadedlater,oneatatime.Eachblockinthecachehasabitassociatedwithittorecordwhetherithasbeenloaded.Thisvaliditybitisturnedonwhentheblockisloadedandoffwhenthesectorisreassigned.Storeoperationsalwayscausemainstoragetobeupdated.Ifthemainstoragesectorbeingchangedhasacachesectorassignedtoit,thecacheisalsoupdatedotherwise,noactivityrelatedtothecachetakesplace.Therefore,storeoperationscannotcauseacachesectortobereassigned,ablocktobeloaded,ortheactivitylisttoberevised.Sinceallofthedatainthecacheisalsoinmainstorage,itisnotnecessaryonacachesectorreassignmenttomoveanydatafromthecachetomainstorage.Allthatisrequiredistochangethesectoraddressregister,resetthevaliditybits,andinitiateIoadingofablock.Theprocessoriscapableofbufferingoneinstructionrequestingthestoringofinformationinmainstorage,sothatitcanproceedwithsubsequentinstructionsevenifexecutionofthestoreinstructioncannotbeinitiatedimmediately.Twoprocessorcyclesarerequiredtofetchdatathatisinthecache.Thefirstcycleisusedtoexaminethesectoraddressregistersandthevaliditybitstodetermineifthedataisinthecache.Thesecondcycleisthenusedtoreadthedataoutofthecache.However,requestscannormallybeoverlapped,sothatonerequestcanbeprocessedeverycycle.Ifthedataisnotpresentinthecache,additionalcyclesarerequiredwhiletheblockisloadedintothecachefrommainstorage.ThestoragewordsizeonwhichtheModel85operatesinternallyis16bytes.Thisisthewidthofthedatapathstoandfromthestorageunits,andistheamounttheprocessorcanstoreorfetchwithasinglerequest.Becauseasingle23655storageunitoperatesonan8bytewideinterface,twounitsarepairedtogetherandoperatedsimultaneously.Exceptforthe512Kconfiguration,mainstorageisinterleavedfourways.Sinceablockis64bytes,fourfetchestomainstoragearerequiredtoloadoneblockintothecache.Withfourwayinterleaving,thismeansonerequesttoeachbasicstoragemodule.Toimproveperformance,thefirstbasicstoragemodulereferredtoduringeachblockloadistheonecontainingthe16byteswantedbytheprocessor.Inadditiontobeingloadedintothecache,thedataissentdirectlytotheprocessor,sothatexecutioncanproceedassoonaspossibleseeFigure2.OntheModel85,channelsstoreandfetchdatabywayoftheMODEL85CACHEFigure2TimingforablockloadMAINSTORAGENttUtUBSMZACCESSTIMEIBSMPIBSMOACCESSTIMEwBSMBASICSTORAGEMODULEprocessor.Channelfetchesareprocessedbygettingtherequireddatafrommainstoragewithoutreferringtothecache.Channelstoresarehandledthesamewayasprocessorstores.Inthisway,ifachannelchangesdatathatisinthecache,thecacheisupdatedbutthechannelsdonothaveanypartofthecachedevotedtothem.PerformancestudiesAmongthequestionsthathadtobeansweredtodeterminewhetherthecacheapproachshouldbetakenwere1howeffectiveisit,and2doesitseffectivenessvarysubstantiallyfromoneprogramtoanotherTheprincipaltoolsusedtoanswerthesequestionsarethetracingandtimingtechniquesreferredtoinPartI.ThetracingtechniqueproducesaninstructionbyinstructiontraceofaprogramoperatingundertheSYSTEM/OOperatingSystem.Theoutputisasequenceoftracetapes,whichcontaineveryinstructionexecuted,whetherintheproblemprogramortheoperatingsystem,andthenecessaryinformationtodeterminehowlongittakestobeexecuted.Thesetracetapescontainabout250,000instructionseachandareusedasinputtoatimingprogram,whichdetermines,cyclebycycle,howtheModel85wouldexecutethatsequenceofinstructions.Thesetechniquesareintendedtodetermineinternalperformanceanddonotprovideanyinformationconcerningthroughput.Anintensiveinvestigationprecededselectionoftheprogramsusedinthisstudy.Inordertomeasuretheeffectivenessofthecache,wepostulatedcacheasystemidenticaltotheModel85exceptthatthestoragehierarchyeffectivenessisreplacedbyasinglelevelstorageoperatingatcachespeed.TheperformanceofsuchasystemisthatwhichwouldbeachievedbytheModel85ifitalwaysfoundthedataitwantedinthecacheandifitneverencounteredinterferenceinmainstorageduetostores.Therefore,itrepresentsanupperlimitontheperformanceoftheModel85howclosetheModel85approachesthisidealcanserveasameasureofhoweffectivethecacheis.NineteentracetapesI18J.s.LIPTAYFigure3Model85performancerelativetosinglelevelstorageoperatingatcachespeedMEAN8175.79808490.94PERCENTAGEOFIDEALPERFORMANCEFlgure4ProbabilityoffindingfetcheddataincachePROBABILITYweretimedforboththeModel85andthepostulatedsystem,andtheperformanceoftheModel85wasexpressedasapercentageoftheperformanceoftheidealsystem.Figure3showsthedistributionofperformancedataobtained,Theaveragewas81percentoftheperformanceoftheidealsystem,witharangebetween66and94percent.Animportantstatisticrelatedtocacheoperationistheprobabilityoffindingthedatawantedforafetchinthecache.Figure4showsthedistributionofthisprobabilityforthesame19tracetapesusedforFigure3.Theaverageprobabilitywas0.968.Itisworthnotingthat,iftheaddressesgeneratedbyaprogramwererandom,theprobabilityoffindingthedatawantedinthecachewouldbemuchlessthan0.01.Therefore,itcanbesaidthatwhatmakesthecacheworkisthefactthatrealprogramsarenotrandomintheiraddressingpatterns.MODEL85CACHETable2AverageperformancerelativetoanidealsystemwithcachesizeandnumberofbytesperblockvariedNumberofsectors16NumberofNumberofbytesperblockcachebytes64128266,8K0.74416K0.8250.8100.78132K0.8910.8850.870Table3Comparativeperformanceusingdifferentcachesectorreplacementalgorithmsalgorithmperformance1partition1.0002partitions0.9904partitions0.9878partitions0.97916partitions0.933usagebits0.931ReplacementalgorithmchosenfortheModel85Table3summarizestheresultsobtained.Thechoiceoftheactivitylistwasmadebecauseitprovidedthebestbalancebetweencostandperformance.SummarycommentTheinclusionofastoragehierarchyrepresentsoneofthemajoradvancesinsystemorganizationpresentintheModel85.Althoughtheconceptofastoragehierarchyisnotnew,thesuccessfulimplementationofananosecond/microsecondlevelofhierarchywasinhibiteduntilnowbythelackofasuitabletechnology.AsimplementedintheModel85,thefastmonolithicstoragephysicallyintegratedwiththeCPUlogicyieldsthedesiredmachinespeed,whilethelargecorestorageyieldsthedesiredstoragecapacity,thecombinationbeingtransparenttotheuser.Itislikelythatwithfutureprogressintechnologythisnanosecond/microsecondhierarchyisnotmerelyaninnovationthatworkedoutwellfortheModel85,butratheritisafundamentalstepforwardthatwillbeincorporatedintomostlargesystemsofthefuture.CITEDREFREENCEANDFOOTNOTE1.Thetermcacheissynonymouswithhighspeedbuffer,asusedinotherModel85documentation.2.D.H.Gibson,Considerationsinblockorientedsystemsdesign,AFZPSConferenceProceedings,SpringJointComputerConference30,AcademicPress,NewYork,NewYork,75801967.MODEL85CACHE21