操作系统原理与设计第9章VM3（虚存3）陈香兰中国科学技术大学计算机学院2009年09月01日......提纲ThrashingCauseoftrashingWorking-SetModelPage-FaultFrequencyMemory-MappedFilesAllocatingKernelMemoryOtherIssuesOperatingSystemExamples小结和作业......OutlineThrashingCauseoftrashingWorking-SetModelPage-FaultFrequencyMemory-MappedFilesAllocatingKernelMemoryOtherIssuesOperatingSystemExamples小结和作业......ThrashingIIIfaprocessdoesnothave’enough’pages,thepage-faultrateisveryhigh.Thisleadsto:IlowCPUutilizationIOSthinksthatitneedstoincreasethedegreeofmultiprogrammingIanotherprocessaddedtothesystem,gettingworse!IThrashingaprocessisbusyswappingpagesinandout......ThrashingIIICauseoftrashing:unreasonabledegreeofmultiprogramming......ThrashingIIIIHowtolimittheeffectsofthrashingIlocalreplacementalgorithm?notentirelysloved.Iwemustprovideaprocesswithasmanyframesasitneeds–localityIHowdoweknowhowmanyframesisneeded?Iworking-setstrategy(LocalitymodelILocalitymodel:Thisisthereasonwhydemandpagingworks?IProcessmigratesfromonelocalitytoanotherILocalitiesmayoverlapIWhydoesthrashingoccur?Σsizeoflocality>totalmemorysizeILocalityInAMemory-ReferencePattern(figure)......ThrashingIV......OutlineThrashingCauseoftrashingWorking-SetModelPage-FaultFrequencyMemory-MappedFilesAllocatingKernelMemoryOtherIssuesOperatingSystemExamples小结和作业......Working-SetModel(工作集模型)IItheworking-setmodelisbasedontheassumptionoflocality.Ilet∆≡working−setwindow≡afixednumberofpagereferencesExample:10,000instructionIworkingset(工作集):thesetofpagesinthemostrecent∆pagereferences.Ianapproximationoftheprogram’slocality.......Working-SetModel(工作集模型)IIIExample:∆=10Iworkingsetsize:WSSi(workingsetofProcessPi)=totalnumberofpagesreferencedinthemostrecent∆Iveriesintime,dependontheselectionof∆Iif∆toosmallwillnotencompassentirelocalityIif∆toolargewillencompassseverallocalitiesIif∆=1)willencompassentireprogram......Working-SetModel(工作集模型)III