lecture_13_virtual memory

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1、VirtualMemoryZhaoFangOperatingSystems-forthecollagestudentsGoalforTodayBackgroundDemandPagingPageReplacementPoliciesPageAllocationPoliciesWorkingSet/ThrashingUNIXandSolarisMemoryManagementLinuxMemoryManagementWindowsMemoryManagementMemoryManageme

2、ntModernprogramsrequirealotofphysicalmemoryMemorypersystemgrowingfasterthan25%-30%/yearButtheydon’tusealltheirmemoryallofthetime90-10rule:programsspend90%oftheirtimein10%oftheircodeWastefultorequireallofuser’scodetobeinmemoryTheneedformainmemorySol

3、ution:usemainmemoryascachefordiskProcessorControlCachingTertiarySecondMainSecondaryStorageOnLevelMemoryStorage(Tape)Cache-ChipCache(DRAM)(Disk)Datapath(SRAM)IllusionofInfiniteMemoryTLBPageTablePhysicalDiskMemory500GBVirtual512MBMemory4GBKeypointsinMemor

4、yManagement1)MemoryreferencesarelogicaladdressesdynamicallytranslatedintophysicaladdressesatruntimeAprocessmaybeswappedinandoutofmainmemoryoccupyingdifferentregionsatdifferenttimesduringexecution2)Aprocessmaybebrokenupintopiecesthatdonotneedtolocatedcon

5、tiguouslyinmainmemoryBreakthroughinMemoryManagementIfbothofthosetwocharacteristicsarepresent,thenitisnotnecessarythatallofthepagesorallofthesegmentsofaprocessbeinmainmemoryduringexecution.Ifthenextinstruction,andthenextdatalocationareinmemorythenexecu

6、tioncanproceedatleastforatimeExecutionofaProcessOperatingsystembringsintomainmemoryafewpiecesoftheprogramResidentset-portionofprocessthatisinmainmemoryAninterruptisgeneratedwhenanaddressisneededthatisnotinmainmemoryOperatingsystemplacestheprocessina

7、blockingstateExecutionofaProcessPieceofprocessthatcontainsthelogicaladdressisbroughtintomainmemoryOperatingsystemissuesadiskI/OReadrequestAnotherprocessisdispatchedtorunwhilethediskI/OtakesplaceAninterruptisissuedwhendiskI/Ocompletewhichcausestheoper

8、atingsystemtoplacetheaffectedprocessintheReadystateImplicationsofthisnewstrategyMoreprocessesmaybemaintainedinmainmemoryOnlyloadinsomeofthepiecesofeachprocessWithsomanyprocessesinmainmemory,itisverylikelyaprocesswillbei