操作系统-精髓与设计原理英文原版Ch虚拟内存

OperatingSystem-EssenceandDesignPrinciplesE目录contentsOverviewofVirtualMemoryTheprincipleofvirtualmemoryManagementofvirtualmemoryTheadvantagesanddisadvantagesofvirtualmemoryApplicationscenariosofvirtualmemory01OverviewofVirtualMemoryVirtualMemoryisamemorymanagementtechniquethatprovidesalogicalviewofthesystemmemory,allowingeachprocesstothinkithassoleaccesstotheentirememoryspace.Itcreatestheillusionofacontinuous,uninterruptedmemoryspaceforeachprocess,maskingtheactualphysicalmemorylayoutandprovidingaseamlessinterfaceformemoryaccess.DefinitionofVirtualMemory123Eachprocessisisolatedfromotherprocesses,preventingunauthorizedaccesstomemory.EnhancessecurityMemorycanbeswappedouttodiskwhennotinuse,freeingupphysicalmemoryforotherprocesses.ImprovesperformanceMemorycanbeassignedspecificaccessprivileges,preventingunauthorizedwritesorreads.EnablesmemoryprotectionTheroleofvirtualmemoryMemorymappingEachprocess'svirtualaddressspaceismappedtothephysicalmemoryandswappedouttodiskwhennecessary.PagetablesAdatastructurethatmapsvirtualaddressestophysicaladdresses,managedbytheoperatingsystem.PagefaultsWhenaprocesstriestoaccessapagethatisnotcurrentlyloadedintophysicalmemory,apagefaultoccurs,andtheOSswapsintherequiredpagefromdisk.ImplementationofVirtualMemory02TheprincipleofvirtualmemoryAddressspaceAddressspaceisasetofuniqueaddressesthatacomputersystemusestoidentifyandaccessdatawithinitselfordatalocatedinexternalstoragedevices.Inthecontextofvirtualmemory,theaddressspaceisalogicalconstructthatallowsprogramstoaccessmemorylocationsusingvirtualaddresses,whicharetranslatedintophysicaladdressesbythememorymanagementunit(MMU).VirtualaddressspaceThevirtualaddressspaceistheillusioncreatedbytheoperatingsystemtomakeeachprocessbelievethatithassoleaccesstothesystem'smemory.Eachprocessisassigneditsownprivatevirtualaddressspace,whichismappedtothephysicalmemoryandothersystemresourcesusingvarioustechniquessuchaspagetablesandsegmentation.addressspacePagereplacementalgorithmPagereplacementalgorithm：Pagereplacementalgorithmsareusedinvirtualmemorymanagementtodeterminewhichpageframestoallocatetoaprocessandwhichonestoswapoutwhenthesystemrunsoutofphysicalmemory.Themostcommonpagereplacementalgorithmsaretheleastrecentlyused(LRU)algorithmandthefirst-in,first-out(FIFO)algorithm.LeastRecentlyUsed(LRU)：TheLRUalgorithmreplacesthepagethatwasleastrecentlyused.Itkeepstrackofthetimesinceeachpagewaslastusedandreplacesthepagethatistheleastrecentlyused.First-In,First-Out(FIFO)：TheFIFOalgorithmreplacesthepagethatwasfirstloadedintomemory.Itkeepstrackoftheorderinwhichpagesareloadedintomemoryandreplacesthepagethatwasfirstloaded.MemoryprotectionmechanismMemoryprotectionmechanism：Memoryprotectionmechanismsareusedtoensurethesecurityandintegrityofthesystembypreventingunauthorizedaccesstomemoryareas.Theyallowdifferentprocessestohaveseparateaddressspacesbutpreventthemfromaccessingeachother'smemoryareas.Memoryprotectionmechanismscanbeimplementedatvariouslevels,includinglevelprotection,segment-levelprotection,andsupervisor-modeprotection.levelprotection：levelprotectionallowstheoperatingsystemtocontrolaccesstospecificmemorypages.Itcanbesettoallowread,write,orexecuteaccesstospecificpagesortodenyaccessentirely.levelprotectionisimplementedusingpagetablesandistypicallyusedinconjunctionwithotherprotectionmechanisms.Segment-levelprotection：Segment-levelprotectiondividesmemoryintosegmentsandassignsdifferentprivilegestoeachsegment.Itallowstheoperatingsystemtocontrolaccesstospecificsegmentsofmemory,suchasallowingreadorwriteaccessonlywithinaspecificsegment.Segment-levelprotectionisimplementedusingsegmentationregistersandistypicallyusedinolderoperatingsystemsorinconjunctionwithotherprotectionmechanisms.Supervisor-modeprotection：Supervisor-modeprotectionisaprivilegedmodeofoperationintheCPUthatallowsaccesstosystemresourcesandimplementsmemoryprotectionmechanisms.Itrunsatahigherprivilegelevelthanusermodeandcanperformoperationsthatarenotallowedinusermode,suchasaccessingprotectedmemoryareasorexecutingprivilegedinstructions.Supervisor-modeprotectionistypicallyusedforkernel-modecodeandothercriticalsystemcomponents.SharingandIsolationSharing：Sharingistheabilityofmultipleprocessestoaccesscommonresourcesordatawithoutexplicitcoordination.Inthecontextofvirtualmemory,sharingcanrefertosharedlibrariesorshareddatastructuresthatcanbeaccessedbymultipleprocesses.Sharingallowsforcodereuseandreducestheoverallsizeoftheprogram,butitalsointroducespotentialsynchronizationissuesandsecurityrisksthatneedtobeaddressedproperly.Isolation：Isolationistheabilityoftheoperatingsystemtoseparatedifferentprocessesfromeachother,ensuringthattheycannotinterferewitheachother'sexecutionorresources.Isolationisachievedthroughvariousmechanismssuchasprocesscreation,resourceallocation,andinterprocesscommunication(IPC)mechanismsthatallowprocessestoshareinformationwhilemaintainingtheirindependenceandsecurity.Isolationensuresthateachprocesshasitsownprivateaddressspace,resources,andexecutionenvironment,reducingthepotentialforconflictsandsecuritybreaches.03ManagementofvirtualmemoryVSDividingthememoryintoseparatesections,eachsectionassignedtoaspecificprocessortask.Thisallowseachprocesstohaveitsowndedicatedmemoryspace,preventingconflictsandensuringsecureoperation.AdvantagesImprovessecurityandfaultisolationbykeepingdifferentprocessesseparate.Also,allowsformoreefficientmemoryusageasdifferentprocessesmayhavedifferentmemoryrequirementsatdifferenttimes.PartitioningMemorypartitionAtechniquethatdividesthephysicalmemoryintoequal-sizedblockscalledpages,andswapspagesinandoutofmemoryasneeded.Thisallowsthesystemtodynamicallyallocatememorytoprocessesbasedontheirneeds.PagingImprovesmemorymanagementbyallowingthesystemtoallocatememoryinsmall,efficientchunks.Italsoallowsforbettermemoryutilizationasunusedpagescanbefreedupforotherprocesses.AdvantagesMemorypagingSegmentationAmethodofdividingthevirtualmemoryintosegments,eachsegmentcontainingthevariables,code,anddataofasingleprocess.Thisallowseachprocesstohaveitsownaddressspaceandkeepsthemseparatefromeachother.AdvantagesImprovessecuritybykeepingdifferentprocessesseparateandpreventingunauthorizedaccess.Italsoallowsforbettermemoryprotectionaseachsegmentcanbeassignedspecificaccesspermissions.Memorysegmentmanagement04TheadvantagesanddisadvantagesofvirtualmemoryadvantageConvenienceformemorymanagement:Virtualmemoryprovidesaconvenientwaytomanagememorybyabstractingthephysicalmemorydetailsfromtheapplication.Itallowstheapplicationtoaccessacontiguousaddressspace,simplifyingmemoryallocationanddeallocation.Improvedsecurity:Withvirtualmemory,eachprocesshasitsownprivateaddressspace,preventingoneprocessfromaccessingormodifyingthememoryofanotherprocess.Thisenhancessecuritybypreventingunauthorizedaccesstosystemresources.Memoryprotection:Virtualmemoryensuresthateachprocessrunsinitsownsecureenvironment,preventingoneprocessfromcorruptingorinterferingwiththeoperationofanother.Thisensuresthestabilityofthesystem.Efficientmemoryutilization:Virtualmemoryallowsthesystemtousetheavailablephysicalmemorymoreefficiently.Whenphysicalmemoryisscarce,thesystemcanswapoutunusedpagestodisk,freeingupmemoryforotherprocesses.Thisimprovesoverallsystemperformance.shortcomingPerformanceoverhead:Virtualmemoryimposesaperformanceoverheadbecauseoftheneedtotranslatevirtualaddressestophysicaladdresses.Thistranslationprocessrequirestimeandresources,whichcanimpactsystemperformance,especiallyinhigh-performanceapplications.Increasedmemoryusage:Virtualmemorydoesn'talwaysleadtoefficientuseofphysicalmemory.Whenprocessesareswappedouttodisk,thecorrespondingmemorypagesaremarkedasunused,eventhoughtheymaybeneededagainsoon.Thiscanleadtoincreasedmemoryusageandpotentialthrashing(frequentswappingofpagesinandoutofmemory).Complexity:Virtualmemoryintroducesadditionalcomplexityintheoperatingsystemdesignandimplementation.Itrequiresmechanismsforpagefaults,pageswapping,andmanagingthetranslationfromvirtualtophysicaladdresses.Thiscomplexitycanleadtoincreaseddevelopmentandmaintenancecosts.Limitedaddressspace:Virtualmemorylimitsthetotaladdressspaceavailabletoprocesses.Whilethislimitistypicallysufficientformostapplications,itcanbecomeaconstraintforverylargeapplicationsorthosethatrequirevastamountsofmemory.05ApplicationscenariosofvirtualmemoryVirtualmemorycanimprovegameperformancebyprovidingaseamlessandlag-freegamingexperience.Itallowsgamedeveloperstoallocatemorememorytothegame,ensuringsmoothandresponsivegameplay.Withvirtualmemory,gamedeveloperscancreatemorecomplexanddetailedgameenvironments,characters,andscenarios.Thisallowsforaricherandmoreimmersivegamingexperience.Virtualmemorycanhelpgamedeveloperscreategamesthatarecompatiblewithmultipleplatforms,asitabstractstheunderlyingmemorymanagementofeachplatform,enablingdeveloperstofocusongamedevelopmentratherthanplatform-specificoptimizations.ImprovedgameperformanceIncreasedgamecontentCross-platformcompatibilityGamedevelopmentEfficientmemorymanagement01Virtualmemoryinmobileappscanhelpdevelopersmanagememorymoreefficiently,preventingmemoryleaksandensuringthatappsrunsmoot