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Virtual Memory Memory Management VirtualMemory CS217 MemoryManagement • Problem1: Twoprogramscan’tcontrolallofmemorysimultaneously • Problem2: Oneprogramshouldn’tbeallowedtoaccess/change thememoryofanotherprogram 0 OS • Problem3: 0x2000 Machinemayhaveonly256MBofmemory, Text whilevirtualaddressspaceis4GB Data BSS Heap OOppeerraattiinnggssyysstteemmmmuussttmmaannaaggee sshhaarriinnggooffpphhyyssiiccaallmmeemmoorryy bbeettwweeeennmmaannyypprroocceesssseess Stack 0xffffffff 1 VirtualMemory • Basicidea Programsdon’t(andcan’t)namephysicaladdresses Instead,theynamevirtualaddresses (eachprocesshasownaddressspace) Thekerneltranslateseachvirtualaddressintoaphysicaladdress beforetheoperationiscarriedout • Advantages Canrunmanyprogramsatonce, withoutthemworryingthattheywillusethesamephysicalmemory Kernelcontrolsaccesstophysicalmemory,sooneprogramcan’t accessormodifythememoryofanother Canrunaprogramthatusesmorevirtualmemorythanthe computerhasavailableinphysicalmemory Segmentation • Allocatememoryforsegments Providemappingfromaddressesinsegmentstophysicalmemory • Usebaseandlimitregisterstotranslatevirtualaddresses tophysicaladdresses 1 limit base Baseregister Virtual Physical Address Address cpu < + 2 Limitregister Physical Memory 2 Segmentation • Allocatememoryforsegments Providemappingfromaddressesinsegmentstophysicalmemory • Problems: Segmentsmaygrow Physical Disk Memory Storage Fragmentation Largeprocesses Swappingefficiency 1 Baseregister 3 2 Limitregister Paging • Motivation Mappingentiresegmentsistoocoarsegranularity Mappingindividualbytesistoofinegranularity • Pages Divideupmemoryintoblocks,calledpages(~4KB) Eachvirtualpagecanbemappedtoanyphysicalpage Eachtranslationinvolvestwosteps: – Decidewhichphysicalpage holdsthevirtualaddress – Decideawhatoffset thevirtualaddressisinsidethepage Thephysicaladdressisformedbygluingtogether thephysicalpagenumberandtheoffsetwithinthepage 3 Paging • Pagetablemapsvirtualaddressestophysicaladdresses Silberschatz &Peterson Paging(cont) Silberschatz &Peterson 4 PagedSegmentation Silberschatz &Peterson Swapping • Whathappensifcumulativesizesofsegments exceedsphysical memory? 5 SwappingtoDisk • Ifallthevirtualmemorycan’tfitinphysicalmemory, theOScantemporarilystashsomepagesondisk Cansupportvirtualmemorybiggerthanphysicalmemory Silberschatz &Peterson PageTable • TheOSstoresforeachpage... Physicalpagenumber(24bits) Cacheablebit(C) Modifiedbit(M) Referencedbit(R) Accesspermissions(Readonly,Read/write) Valid/invalid(V) 6 PageFaults • Ifprocessaccesses virtualaddressthat mapstoapage notinmemory, thentheOSmust fetchthatpage fromdisk • Sincemost referencesfollow othersonsame page,thecostof readingfromdisk isamortizedacross Silberschatz manyreferences &Peterson PageReplacement • Whenreadonepagefromdisk, anotherpagemustbeevicted? • Whichpageshouldbereplaced? Ideal: – Onethatwillbeaccessedfurthestinfuture Practicalheuristics: – Leastrecentlyused – Leastfrequentlyused – Etc. void StringArray_read(StringArray_Ts,FILE*fp) { charstring[MAX_STRING_LENGTH]; s->nstrings =0; while(fgets(string,MAX_STRING_LENGTH, fp)){ StringArray_grow(nstrings+1); s->strings[(s->nstrings)++]= strdup(string); } } 7 PageReplacement(cont) Silberschatz &Peterson WorkingSets • Localityofreference Mostmemoryreferencesarenearbypreviousones • Workingset Atanypointinaprogram’sexecution,usually asmallregionofmemoryisaccessedfrequently Theregionofmemory(workingset)changesduring thecourseofexecution int main() { Array_T*strings; strings= ReadStrings(stdin); SortStrings(strings); WriteStrings(strings,stdout); return0; } 8 Thrashing • Whathappenswhencumulativesizeofworkingsets exceedscapacityofphysicalmemory? StorageHierarchy • Registers ~128,1-5nsaccesstime(CPUcycletime) • Cache 1KB– 4MB,20-100ns(multiplelevels) • Memory 64MB– 2GB,200ns • Disk 1GB– 100GB,10ms • Long-termStorage 1TB,1-10s 9 StorageHierarchyLatency Jim Gray Summary • Memorymanagement Importantfunctionofoperatingsystem Understandinghowitworksiscriticalto effectivesystemdevelopment • Virtualmemory OS&Hardwaresupportformapping virtualaddressestophysicaladdresses Mappingisusuallyatpagegranularity,whichfacilitates... – Relocation – Swappingtodisk – Protection – Fragmentation – Sharing 10.
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