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

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

THECACHE,AHIGHSPEEDBUFFERESTABLISHINGASTORAGEHIERARCHYINTHEMODEL85,ISDISCUSSEDINDEPTHINTHISPART,SINCEITREPRESENTSTHEBASICORGANIZATIONALDEPARTUREFROMOTHERSYSTEM/BOCOMPUTERSDISCUSSEDAREORGANIZATIONANDOPERATIONOFTHECACHE,INCLUDINGTHEMECHANISMSUSEDTOLOCATEANDRETRIEVEDATANEEDEDBYTHEPROCESSORTHEINTERNALPERFORMANCESTUDIESTHATLEDTOUSEOFTHECACHEAREDESCRIBED,ANDSIMULATEDPERFORMANCEOFTHECHOSENCONFIGURATIONISCOMPAREDWITHTHATOFATHEORETICALSYSTEMHAVINGANENTIRE80NANOSECONDMAINSTORAGEFINALLY,THEEFFECTSOFVARYINGCACHEPARAMETERSAREDISCUSSEDANDTABULATEDSTRUCTURALASPECTSOFTHESYSTEM/360MODEL8511THECACHEBYJSLIPTAYAMONGTHEOBJECTIVESOFTHEMODEL85ISTHATOFPROVIDINGASYSTEM/COMPATIBLEPROCESSORWITHBOTHHIGHPERFORMANCEANDHIGHTHROUGHPUTONEOFTHEIMPORTANTINGREDIENTSOFHIGHTHROUGHPUTISALARGEMAINSTORAGECAPACITYSEETHEACCOMPANYINGARTICLEINPARTIHOWEVER,ITISNOTFEASIBLETOPROVIDEALARGEMAINSTORAGEWITHANACCESSTIMECOMMENSURATEWITHTHE80NANOSECONDPROCESSORCYCLEOFTHEMODEL85ALONGERACCESSTIMECANBEPARTIALLYCOMPENSATEDFORBYANINCREASEINOVERLAP,GREATERBUFFERING,DEEPERSTORAGEINTERLEAVING,MORESOPHISTICATIONINTHEHANDLINGOFBRANCHES,ANDOTHERIMPROVEMENTSINTHEPROCESSORALLOFTHESEFACTORSONLYPARTIALLYCOMPENSATEFORTHESLOWERSTORAGE,AND,THEREFORE,WEDECIDEDTOUSEASTORAGEHIERARCHYINSTEADTHESTORAGEHIERARCHYCONSISTSOFA104MICROSECONDMAINSTORAGEANDASMALL,FASTSTORECALLEDACACHE,LWHICHISINTEGRATEDINTOTHECPUTHECACHEISNOTADDRESSABLEBYAPROGRAM,BUTRATHERISUSEDTOHOLDTHECONTENTSOFTHOSEPORTIONSOFMAINSTORAGETHATARECURRENTLYBEINGUSEDMOSTPROCESSORFETCHESCANTHENBEHANDLEDBYREFERRINGTOTHECACHE,SOTHATMOSTOFTHETIMETHEPROCESSORHASASHORTACCESSTIMEWHENTHEPROGRAMSTARTSOPERATINGONDATAINADIFFERENTPORTIONOFMAINSTORAGE,THEDATAINTHATPORTIONMUSTBELOADEDINTOTHECACHEANDTHEDATAFROMSOMEOTHERPORTIONREMOVEDTHISACTIVITYMUSTTAKEPLACEWITHOUTPROGRAMASSISTANCE,SINCETHEMODEL85MUSTBECOMPATIBLEWITHTHERESTOFTHISPAPERDISCUSSESORGANIZATIONOFTHECACHEANDTHESTUDIESTHATLEDTOITSUSEINTHEMODEL85ANDTOSELECTINGOFVALUESFORITSPARAMETERSTHESYSTEM/360LINEZBMSYSTEMSJOURNALVOL7NO1196815FIGURE1ASSIGNMENTOFCACHESECTORSTOMAINSTORAGESECTORSMAINSTORAGECACHEORGANIZATIONTHEMAINSTORAGEUNITSTHATCANBEUSEDONTHEMODEL85ARETHEIBM23655ANDTHE2385THEYHAVEA104MICROSECONDCYCLETIMEANDMAKEAVAILABLECAPACITIESFROM512KBYTESTO4096KBYTESK1024THECACHEISA16KBYTEINTEGRATEDSTORAGE,WHICHISCAPABLEOFOPERATINGEVERYPROCESSORCYCLEOPTIONALLY,ITCANBEEXPANDEDTO24KBYTESOR32KBYTESBOTHTHECACHEANDMAINSTORAGEARELOGICALLYDIVIDEDINTOSECTORS,EACHCONSISTINGOF1KCONTIGUOUSBYTESSTARTINGON1KBYTEBOUNDARIESDURINGOPERATION,ACORRESPONDENCEISSETUPBETWEENCACHESECTORSANDMAINSTORAGESECTORSINWHICHEACHCACHESECTORISASSIGNEDTOASINGLEDIFFERENTMAINSTORAGESECTORHOWEVER,BECAUSEOFTHELIMITEDNUMBEROFCACHESECTORS,MOSTMAINSTORAGESECTORSDONOTHAVEANYCACHESECTORSASSIGNEDTOTHEMSEEFIGURE1EACHOFTHECACHESECTORSHASA14BITSECTORADDRESSREGISTER,WHICHHOLDSTHEADDRESSOFTHEMAINSTORAGESECTORTOWHICHITISASSIGNEDTHEASSIGNMENTOFCACHESECTORSISDYNAMICALLYADJUSTEDDURINGASSIGNINGOPERATION,SOTHATTHEYAREASSIGNEDTOTHEMAINSTORAGESECTORSTHATCACHEARECURRENTLYBEINGUSEDBYTHEPROGRAMIFTHEPROGRAMCAUSESASECTORSFETCHFROMAMAINSTORAGESECTORTHATDOESNOTHAVEACACHESECTORASSIGNEDTOIT,ONEOFTHECACHESECTORSISTHENREASSIGNEDTOTHATMAINSTORAGESECTORTOMAKEAGOODSELECTIONOFACACHESECTORTOREASSIGN,ENOUGHINFORMATIONISMAINTAINEDTOORDERTHECACHESECTORSINTOANACTIVITYLISTTHESECTORATTHETOPOFTHELISTISTHEONETHATWASMOSTRECENTLYREFERREDTO,THESECONDONEISTHENEXTMOSTRECENTLYREFERREDTO,ANDSOFORTHWHENACACHESECTORISREFERREDTO,ITISMOVEDTOTHETOPOFTHELIST,ANDTHEINTERVENINGONESAREMOVEDDOWNONEPOSITIONTHISISNOTMEANTTOIMPLYANACTUALMOVEMENTOFSECTORSWITHINTHECACHE,BUTRATHERREFERSTOALOGICAL16J8LIPTAYORDERINGOFTHESECTORSWHENITISNECESSARYTOREASSIGNASECTOR,THEONESELECTEDISTHEONEATTHEBOTTOMOFTHEACTIVITYLISTTHISCACHESECTORISTHEONETHATHASGONETHELONGESTWITHOUTBEINGREFERREDTOWHENACACHESECTORISASSIGNEDTOADIFFERENTMAINSTORAGESECTOR,THECONTENTSOFALLOFTHE1KBYTESLOCATEDINTHATMAINSTORAGESECTORARENOTLOADEDINTOTHECACHEATONCERATHER,EACHSECTORISDIVIDEDINTO16BLOCKSOF64BYTES,ANDTHEBLOCKSARELOADEDONADEMANDBASISWHENACACHESECTORISREASSIGNED,THEONLYBLOCKTHATISLOADEDISTHEONETHATWASREFERREDTOIFTHEYAREREQUIRED,THEREMAININGBLOCKSARELOADEDLATER,ONEATATIMEEACHBLOCKINTHECACHEHASABITASSOCIATEDWITHITTORECORDWHETHERITHASBEENLOADEDTHIS“VALIDITYBIT”ISTURNEDONWHENTHEBLOCKISLOADEDANDOFFWHENTHESECTORISREASSIGNEDSTOREOPERATIONSALWAYSCAUSEMAINSTORAGETOBEUPDATEDIFTHEMAINSTORAGESECTORBEINGCHANGEDHASACACHESECTORASSIGNEDTOIT,THECACHEISALSOUPDATED;OTHERWISE,NOACTIVITYRELATEDTOTHECACHETAKESPLACETHEREFORE,STOREOPERATIONSCANNOTCAUSEACACHESECTORTOBEREASSIGNED,ABLOCKTOBELOADED,ORTHEACTIVITYLISTTOBEREVISEDSINCEALLOFTHEDATAINTHECACHEISALSOINMAINSTORAGE,ITISNOTNECESSARYONACACHESECTORREASSIGNMENTTOMOVEANYDATAFROMTHECACHETOMAINSTORAGEALLTHATISREQUIREDISTOCHANGETHESECTORADDRESSREGISTER,RESETTHEVALIDITYBITS,ANDINITIATEIOADINGOFABLOCKTHEPROCESSORISCAPABLEOFBUFFERINGONEINSTRUCTIONREQUESTINGTHESTORINGOFINFORMATIONINMAINSTORAGE,SOTHATITCANPROCEEDWITHSUBSEQUENTINSTRUCTIONSEVENIFEXECUTIONOFTHESTOREINSTRUCTIONCANNOTBEINITIATEDIMMEDIATELYTWOPROCESSORCYCLESAREREQUIREDTOFETCHDATATHATISINTHECACHETHEFIRSTCYCLEISUSEDTOEXAMINETHESECTORADDRESSREGISTERSANDTHEVALIDITYBITSTODETERMINEIFTHEDATAISINTHECACHETHESECONDCYCLEISTHENUSEDTOREADTHEDATAOUTOFTHECACHEHOWEVER,REQUESTSCANNORMALLYBEOVERLAPPED,SOTHATONEREQUESTCANBEPROCESSEDEVERYCYCLEIFTHEDATAISNOTPRESENTINTHECACHE,ADDITIONALCYCLESAREREQUIREDWHILETHEBLOCKISLOADEDINTOTHECACHEFROMMAINSTORAGETHESTORAGEWORDSIZEONWHICHTHEMODEL85OPERATESINTERNALLYIS16BYTESTHISISTHEWIDTHOFTHEDATAPATHSTOANDFROMTHESTORAGEUNITS,ANDISTHEAMOUNTTHEPROCESSORCANSTOREORFETCHWITHASINGLEREQUESTBECAUSEASINGLE23655STORAGEUNITOPERATESONAN8BYTEWIDEINTERFACE,TWOUNITSAREPAIREDTOGETHERANDOPERATEDSIMULTANEOUSLYEXCEPTFORTHE512KCONFIGURATION,MAINSTORAGEISINTERLEAVEDFOURWAYSSINCEABLOCKIS64BYTES,FOURFETCHESTOMAINSTORAGEAREREQUIREDTOLOADONEBLOCKINTOTHECACHEWITHFOURWAYINTERLEAVING,THISMEANSONEREQUESTTOEACHBASICSTORAGEMODULETOIMPROVEPERFORMANCE,THEFIRSTBASICSTORAGEMODULEREFERREDTODURINGEACHBLOCKLOADISTHEONECONTAININGTHE16BYTESWANTEDBYTHEPROCESSORINADDITIONTOBEINGLOADEDINTOTHECACHE,THEDATAISSENTDIRECTLYTOTHEPROCESSOR,SOTHATEXECUTIONCANPROCEEDASSOONASPOSSIBLESEEFIGURE2ONTHEMODEL85,CHANNELSSTOREANDFETCHDATABYWAYOFTHEMODEL85CACHEFIGURE2TIMINGFORABLOCKLOADMAINSTORAGENTTUTUBSMZACCESSTIMEIBSMPIBSMOACCESSTIMEWBSMBASICSTORAGEMODULEPROCESSORCHANNELFETCHESAREPROCESSEDBYGETTINGTHEREQUIREDDATAFROMMAINSTORAGEWITHOUTREFERRINGTOTHECACHECHANNELSTORESAREHANDLEDTHESAMEWAYASPROCESSORSTORESINTHISWAY,IFACHANNELCHANGESDATATHATISINTHECACHE,THECACHEISUPDATEDBUTTHECHANNELSDONOTHAVEANYPARTOFTHECACHEDEVOTEDTOTHEMPERFORMANCESTUDIESAMONGTHEQUESTIONSTHATHADTOBEANSWEREDTODETERMINEWHETHERTHECACHEAPPROACHSHOULDBETAKENWERE1HOWEFFECTIVEISIT,AND2DOESITSEFFECTIVENESSVARYSUBSTANTIALLYFROMONEPROGRAMTOANOTHERTHEPRINCIPALTOOLSUSEDTOANSWERTHESEQUESTIONSARETHETRACINGANDTIMINGTECHNIQUESREFERREDTOINPARTITHETRACINGTECHNIQUEPRODUCESANINSTRUCTIONBYINSTRUCTIONTRACEOFAPROGRAMOPERATINGUNDERTHESYSTEM/OOPERATINGSYSTEMTHEOUTPUTISASEQUENCEOF‘TRACETAPES,”WHICHCONTAINEVERYINSTRUCTIONEXECUTED,WHETHERINTHEPROBLEMPROGRAMORTHEOPERATINGSYSTEM,ANDTHENECESSARYINFORMATIONTODETERMINEHOWLONGITTAKESTOBEEXECUTEDTHESETRACETAPESCONTAINABOUT250,000INSTRUCTIONSEACHANDAREUSEDASINPUTTOATIMINGPROGRAM,WHICHDETERMINES,CYCLEBYCYCLE,HOWTHEMODEL85WOULDEXECUTETHATSEQUENCEOFINSTRUCTIONSTHESETECHNIQUESAREINTENDEDTODETERMINEINTERNALPERFORMANCEANDDONOTPROVIDEANYINFORMATIONCONCERNINGTHROUGHPUTANINTENSIVEINVESTIGATIONPRECEDEDSELECTIONOFTHEPROGRAMSUSEDINTHISSTUDYINORDERTOMEASURETHEEFFECTIVENESSOFTHECACHE,WEPOSTULATEDCACHEASYSTEMIDENTICALTOTHEMODEL85EXCEPTTHATTHESTORAGEHIERARCHYEFFECTIVENESSISREPLACEDBYASINGLELEVELSTORAGEOPERATINGATCACHESPEEDTHEPERFORMANCEOFSUCHASYSTEMISTHATWHICHWOULDBEACHIEVEDBYTHEMODEL85IFITALWAYSFOUNDTHEDATAITWANTEDINTHECACHEANDIFITNEVERENCOUNTEREDINTERFERENCEINMAINSTORAGEDUETOSTORESTHEREFORE,ITREPRESENTSANUPPERLIMITONTHEPERFORMANCEOFTHEMODEL85;HOWCLOSETHEMODEL85APPROACHESTHISIDEALCANSERVEASAMEASUREOFHOWEFFECTIVETHECACHEISNINETEENTRACETAPESI18JSLIPTAYFIGURE3MODEL85PERFORMANCERELATIVETOSINGLELEVELSTORAGEOPERATINGATCACHESPEEDMEAN81757980849094PERCENTAGEOFIDEALPERFORMANCEFLGURE4PROBABILITYOFFINDINGFETCHEDDATAINCACHEPROBABILITYWERETIMEDFORBOTHTHEMODEL85ANDTHEPOSTULATEDSYSTEM,ANDTHEPERFORMANCEOFTHEMODEL85WASEXPRESSEDASAPERCENTAGEOFTHEPERFORMANCEOFTHEIDEALSYSTEMFIGURE3SHOWSTHEDISTRIBUTIONOFPERFORMANCEDATAOBTAINED,THEAVERAGEWAS81PERCENTOFTHEPERFORMANCEOFTHEIDEALSYSTEM,WITHARANGEBETWEEN66AND94PERCENTANIMPORTANTSTATISTICRELATEDTOCACHEOPERATIONISTHEPROBABILITYOFFINDINGTHEDATAWANTEDFORAFETCHINTHECACHEFIGURE4SHOWSTHEDISTRIBUTIONOFTHISPROBABILITYFORTHESAME19TRACETAPESUSEDFORFIGURE3THEAVERAGEPROBABILITYWAS0968ITISWORTHNOTINGTHAT,IFTHEADDRESSESGENERATEDBYAPROGRAMWERERANDOM,THEPROBABILITYOFFINDINGTHEDATAWANTEDINTHECACHEWOULDBEMUCHLESSTHAN001THEREFORE,ITCANBESAIDTHATWHATMAKESTHECACHEWORKISTHEFACTTHATREALPROGRAMSARENOTRANDOMINTHEIRADDRESSINGPATTERNSMODEL85CACHETABLE2AVERAGEPERFORMANCERELATIVETOANIDEALSYSTEMWITHCACHESIZEANDNUMBEROFBYTESPERBLOCKVARIEDNUMBEROFSECTORS16NUMBEROFNUMBEROFBYTESPERBLOCKCACHEBYTES64128266,8K074416K08250810078132K089108850870TABLE3COMPARATIVEPERFORMANCEUSINGDIFFERENTCACHESECTORREPLACEMENTALGORITHMSALGORITHMPERFORMANCE1PARTITION10002PARTITIONS09904PARTITIONS09878PARTITIONS097916PARTITIONS0933USAGEBITS0931REPLACEMENTALGORITHMCHOSENFORTHEMODEL85TABLE3SUMMARIZESTHERESULTSOBTAINEDTHECHOICEOFTHEACTIVITYLISTWASMADEBECAUSEITPROVIDEDTHEBESTBALANCEBETWEENCOSTANDPERFORMANCESUMMARYCOMMENTTHEINCLUSIONOFASTORAGEHIERARCHYREPRESENTSONEOFTHEMAJORADVANCESINSYSTEMORGANIZATIONPRESENTINTHEMODEL85ALTHOUGHTHECONCEPTOFASTORAGEHIERARCHYISNOTNEW,THESUCCESSFULIMPLEMENTATIONOFANANOSECOND/MICROSECONDLEVELOFHIERARCHYWASINHIBITEDUNTILNOWBYTHELACKOFASUITABLETECHNOLOGYASIMPLEMENTEDINTHEMODEL85,THEFASTMONOLITHICSTORAGEPHYSICALLYINTEGRATEDWITHTHECPULOGICYIELDSTHEDESIREDMACHINESPEED,WHILETHELARGECORESTORAGEYIELDSTHEDESIREDSTORAGECAPACITY,THECOMBINATIONBEINGTRANSPARENTTOTHEUSERITISLIKELYTHATWITHFUTUREPROGRESSINTECHNOLOGYTHISNANOSECOND/MICROSECONDHIERARCHYISNOTMERELYANINNOVA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