Lecture #3: Modern Paradigms Of Memory Management - Warwick

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Operating SystemsLecture #3: Modern Paradigms of Memory ManagementWritten by David Goodwinbased on the lecture series of Dr. Dayou Liand the book Understanding Operating Systems 4th ed.by I.M.Flynn and A.McIver McHoes (2006)Department of Computer Science and Technology,University of Bedfordshire.Operating Systems, 201311th February 2013

OutlineLecture #1 Basicconcepts of O/S1 IntroductionDavid GoodwinUniversity ofBedfordshire2 Paged MemoryDisplacementIntroduction3 Demand PagingPaged MemoryPMTHardware instruction processingPage fault handlerReplacement PoliciesDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerFIFOLRUMRULFUReplacement PoliciesFIFOLRUMRULFU4 Segmented MemorySegmented Memory5 Virtual MemoryVirtual Memory6 Cache MemoryCache Memorysummary7 summaryOperating Systems39

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroduction3Paged MemoryIntroductionDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39

ReviewLecture #1 Basicconcepts of O/S Last lecture, we looked at some simple memory allocationDavid GoodwinUniversity ofBedfordshireIntroductionschemes4Paged MemoryDisplacementDemand PagingPMTFixed partitionsFirst-fitBest-fitDeallocation Each of these required the memory manager to store theHardware instructionprocessingentire program in main memory in contigious locationsPage fault handlerReplacement Policies They created problems such as fragmentation, or theFIFOoverheads of relocation/compactionLRUMRU This lecture we will follow memory allocation schemes thatLFUremove the restriction of storing programs contigiously, andremove the requirement that the entire program reside inmemory during executionSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems 39

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged Memory5DisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39Paged Memory

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged Memory6DisplacementDemand Paging Sections of disk where jobs are stored are called sectors (orPMTblocks)Hardware instructionprocessing Sections of main memory are called page framesPage fault handler One sector will hold one page of job instruction and fit intoReplacement Policiesone page frame of memoryFIFO When loading a job into main memory, the Memory managerLRUMRUneeds toLFU1 Calculate the number of pages in the job2 Locate enough free page frames in main memory3 Load all pages into the page framesSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems Concept Each incoming job is divided into pages of equal size Main memory is also divided into a number of page frames In some O/Ss, sizePage sizeDiskSector sizeMemoryPageFrame to ensure efficiency39

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged Memory7 Analysis Advantages: Pages do not need to store in the main memory contiguouslyDisplacement(the free page frame can spread all places in main memory)Demand Paging More efficient use of main memory (comparing to thePMTHardware instructionprocessingapproaches of early memory management) - noexternal/internal fragmentationPage fault handler New Replacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39problem brought:The memory manager has to keep track of all the pagesEnlarging the size and complexity of OS (overhead)When a job is to be executed, the entire job must be loadedinto memory

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/S ExampleDavid GoodwinUniversity ofBedfordshireIntroductionPaged Memory8DisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39

PAGED MEMORY ALLOCATION Mapping jobs with page frames Job table (JT) contains two entities:Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire Job size PMT locationIntroductionPaged Memory9Displacementframe)Demand Paging Memory map table (MMT) Status of each page framePMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems Page map table (PMT) contains Page index (hidden) Page fame memory address (the first address of a page39

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/S Displacement (or offset) of a line: A factor used to locate each lines of a page within its pageframe It is relative to the location of line 0 of a page AlgorithmDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacement10Demand PagingPMTHardware instructionprocessingN – page sizeM – line no to be locatedQuotient – page number (M/N)Remainder – displacement Example:Page fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems 39

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacement11 This gives the location of the line with respect to the job’sPMTHardware instructionprocessingpagesPage fault handler The algorithm needs to be expanded to find the exactReplacement PoliciesFIFOlocation of the line in main memoryLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems The division algorithm is carried out by the hardware, the OSis responsible for maintaining the tablesDemand Paging39

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire. . . we need to correlate each job’s pages with its page framenumber via the Page Map Table1 determine the page number and displacement of lineIntroductionPaged MemoryDisplacement112Demand PagingPMT2Hardware instructionprocessing2Refer to this job’s PMT and find which page frame containsthe job’s page we are interested3Get the address of the beginning of the page frame bymultiplying the page frame number by the page frame size4Now, add the displacement (from step 1) to the startingaddress of the page frame to compute the precise location inmemory of the line of interestPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39Page number integer quotient from division algorithm (jobspace address / page size )Displacement remainder from the page number division

PAGED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacement13Demand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39 It can be seen that this is a lengthy process Every time an instruction is executed, or a data value is used;the OS must translate the job space address, which isrelative, into its physical memory, which is absolute This process of resolving the address is called addressresolution (or address translation) The key to success of this scheme is the size of the page too small - very long PMT too large - excessive internal fragmentation

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacementDemand Paging14PMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39Demand Paging

DEMAND PAGINGLecture #1 Basicconcepts of O/S Fist scheme that removed the restriction of having the entireDavid GoodwinUniversity ofBedfordshirejob in memory from beginning to end of processing Takes advantage of the fact that programs are writtenIntroductionsequentially i.e. not all pages are necessary at once ConceptPaged MemoryDisplacementDemand Paging15PMT Some pages in memory need to be removed to give rooms forPage fault handlerother pages that are requestedReplacement Policies PMT contains more entries to support different predefinedFIFOpolicies for replacing pagesLRU1 to determine if the page begin requested is already in memory2 to determine if the page contents have been modified3 to determine if the page has been referenced recentlyMRULFUSegmented Memory The key to successful implementation of this scheme is to useVirtual MemoryCache Memoryhigh-speed direct access storage device (hard drive) that canwork directly with the CPUsummaryOperating Systems Allowing to load only a part of the program into memory forprocessingHardware instructionprocessing39

DEMAND PAGINGLecture #1 Basicconcepts of O/S PMT Example:David GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacementDemand PagingPMT16Hardware instructionprocessingPage fault handler If a page is already in memory, the system will be spared timeReplacement PoliciesFIFOof bringing the page from secondary storageLRU If the page has not been modified, it doesn’t need to beMRULFUrewritten to the secondary storage Page-swapping policy schemes determine which pages shouldremain in memory (relatively active ones) or which should beswapped out (relatively inactive ones) to make room for otherpages being requestedSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39

Hardware instruction processing algortihmLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionAlgorithm 1 Hardware instruction processing algortihmPaged MemoryDisplacementDemand PagingPMTHardware instruction 17processingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39Start processing instructionGenerate data addressesCompute page numberif page is in memory thenget data and finish instructionadvance to next instructionreturn to step 1elsegenerate page interruptcall page fault handlerend if

Page fault handler algortihmLecture #1 Basicconcepts of O/S When the previous test fails, the OS software takes over withDavid GoodwinUniversity ofBedfordshirea section called page fault handlerIntroductionPaged MemoryAlgorithm 2 Page fault handler algortihmDisplacementDemand PagingPMTHardware instructionprocessingPage fault handler18Replacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems39if there is no free page frame thenselect page to be swapped out using page removal algorithmupdate job’s Page Map Tableif content of page has been changed thenwrite page to diskend ifend ifUse page number from step 3 from the Hardware Instruction Processing Algorithm to getdisk address where the requested page is stored (the file manager uses the page numberto get the disk address)Read page into memoryUpdate job’s Page Map TableUpdate Memeory Map TableRestart interrupted instruction

DEMAND PAGINGLecture #1 Basicconcepts of O/S Where there is an excessive amount of page swappingDavid GoodwinUniversity ofBedfordshirebetween main memory and secondary storage, the operationbecomes inefficient. This is called thrashing.Introduction Ideally, a demand paging scheme is most efficient whenPaged MemoryDisplacementpragrammers are aware of the page size used by theiroperating system, and design their programs to keep pagefaults to a minimum (not really feasible).Demand PagingPMTHardware instructionprocessingPage fault handler19Replacement PoliciesFIFOLRUMRUExample: we could have such a situation where a loop is split over twopages:1LFUSegmented MemoryVirtual Memory2Cache MemorysummaryOperating Systems39for(j 1:j 100; j){k j*j;m a*j;}

DEMAND PAGINGLecture #1 Basicconcepts of O/S Replacement policies First-in First-out (FIFO): the page which has been in memorythe longest will be removed to give room for a page requested ExampleDavid GoodwinUniversity ofBedfordshireIntroduction A program consists of 4 pages A, B, C, D These pages are requested in the sequence ofPaged MemoryDisplacementABACABDBACD There are only two free page frames available A page interrupt is denoted by an asterisk (*)Demand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFO20LRUMRULFUSegmented MemoryVirtual MemoryCache MemoryFigure : The success rate of this example is 2/11, or 18%, and thefailure rate is 82%.summaryOperating Systems39

DEMAND PAGINGLecture #1 Basicconcepts of O/S Least recent used (LRU): the page that shows the least recentused will be removed to give room for a page requestedDavid GoodwinUniversity ofBedfordshire Referenced column of PMT is checked for this purpose Example A program consists of 4 pages A, B, C, D These pages are requested in the sequence ofIntroductionPaged MemoryDisplacementABACABDBACDDemand Paging There are only two free page frames availablePMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRU21MRULFUSegmented MemoryVirtual MemoryCache MemoryFigure : The success rate of this example is 3/11, or 27%, and thefailure rate is 73%.summaryOperating Systems39

DEMAND PAGINGLecture #1 Basicconcepts of O/S Most recent used (MRU): the page that shows the mostDavid GoodwinUniversity ofBedfordshirerecent active will be removed to give room for a pagerequested Last referenced time is checked for this purpose Example A program consists of 4 pages A, B, C, D These pages are requested in the sequence ofIntroductionPaged MemoryDisplacementDemand PagingABACABDBACDPMT There are only two free page frames availableHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRU22LFUSegmented MemoryVirtual MemoryCache MemoryFigure : The success rate of this example is 4/11, or 36%, and thefailure rate is 66%.summaryOperating Systems39

DEMAND PAGING Least frequently used (LFU): the page that shows the leastLecture #1 Basicconcepts of O/Sfrequently used will be removed to give room for a pagerequestedDavid GoodwinUniversity ofBedfordshire Time when loaded is checked for this purpose Example A program consists of 4 pages A, B, C, D These pages are requested in the sequence ofIntroductionPaged MemoryDisplacementABACABDBACDDemand Paging There are only two free page frames availablePMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFU23Segmented MemoryVirtual MemoryCache MemoryFigure : The success rate of this example is 3/11, or 27%, and thefailure rate is 73%.summaryOperating Systems39

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemorySegmented MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented Memory24Virtual MemoryCache MemorysummaryOperating Systems39

SEGMENTED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire Problem in paging Pages have the same and fixed size, like fixed partitions, canstill cause memory waste via internal fragmentation Concept of segmented memory allocation A job is divided into segments according program’s structuralmodules (logical groupings of code) A segment map table (SMT) is generated for each job These segments can have different size Memory is allocated in a dynamic manner (something likedynamic partitions)IntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRU designed to reduce page faults that result from having aLFUSegmented Memory25Cache MemorysummaryOperating Systemssegment’s loop split over two or more pages a subroutine is an example of one such logical groupVirtual Memory39

SEGMENTED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshirePMT Segment Map Table contains:1 segment numbers2 segement lengths3 access rights4 status5 location in memoryHardware instructionprocessing The memory manager needs to keep track of of segments inIntroductionPaged MemoryDisplacementDemand Pagingmemory.Page fault handlerReplacement Policies This is done with three tables combining the aspects of bothFIFOdynamic partitions and demand paging:LRUMRU1LFUSegmented Memory263Virtual MemoryCache MemorysummaryOperating Systems239Job Table lists every job in processThe Segment Map Table lists details about each segementThe Memory Map Table monitors the allocation of mainmemory

SEGMENTED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/S To access a specific location within a segment we canperform an operation similar to the one used for pagedmemory management The only difference is we work with segemnts instead of pages ExampleDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented Memory27Virtual MemoryCache MemorysummaryOperating Systems39

SEGMENTED MEMORY ALLOCATIONLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire The disadvantage of any allocation scheme in which memoryIntroductionis partitioned dynamically is the return of externalfragmentation.Paged MemoryDisplacement Therefore, the relocation/compaction algorithms must beDemand PagingPMTused at predefined timesHardware instructionprocessing The difference between paging and segmentation isPage fault handlerconceptual:Replacement Policies Pages are physical units that are invisible to the user’sFIFOLRUprogram, and are fixed sizesMRU Segements are logical units that are visible to the user’sLFUSegmented Memory28Virtual MemoryCache MemorysummaryOperating Systems39program, and are variable sizes

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryVirtual MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual Memory29Cache MemorysummaryOperating Systems39

VIRTUAL MEMORYLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire Concept Virtual memory is a technique which allow pages or segmentsmoving between main memory and secondary storage Virtual memory is based on paging and/or segmentation Information sharing can be implemented without a large I/OoverheadIntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handler Multiple users, each has a copy of shared information With virtual memory, a single copy is loaded on demandReplacement PoliciesFIFO Virtual memory works well in a multiprogrammingLRUMRUenvironmentLFUSegmented MemoryVirtual Memory30Cache MemorysummaryOperating Systems39

VIRTUAL MEMORYLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireVirtual memory with pagingAllows internal fragmentationNo external fragmentationequal sized pagesAbsolute address neededRequires PMTIntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingVirtual memory with segmentationNo internal fragmentationAllows external fragmentationunequal sized segementsAbsolute address neededRequires SMTPage fault handler There are some disadvantages, although outweighed by the advantages.Replacement PoliciesFIFOThe disadvantages are:LRUMRU1LFU2Segmented MemoryVirtual Memory331Cache MemorysummaryOperating Systems39Increased processor hardware costsIncreased overhead for handling page interruptsIncreased software complexity to prevent thrashing

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryCache MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache Memory32summaryOperating Systems39

CACHE MEMORYLecture #1 Basicconcepts of O/S Concept Traditionally, when a program is executed, instructions of theprogram are loaded to a register in CPU one after anotherfrom main memory In most programs, the same instructions are used repeatedlyand loading these instructions again and again wastes CPUtime Cache memory stores these instructions within CPU StructureDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache Memory33summaryOperating Systems39

CACHE MEMORYLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshire Measure of efficiency Hit ratio found in cacheh requeststotal requests Hit ratio is usually expressed as a percentage (multiplied by100 above gives the hit ratio as a percentage) (1 h) m, miss ratioIntroductionPaged MemoryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handler Average memory access timeReplacement PoliciesFIFOta tc (1 h) tmtc average cache access timetm average memory access timeLRUMRULFUSegmented MemoryVirtual MemoryCache Memory34summaryOperating Systems39

main memory transfer algorithmLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionAlgorithm 3 main memory transfer algorithmPaged MemoryCPU puts the address of a memory location in the memory address register and requestsdata or an instruction to be retrieved from that addressA test is performed to determine if the block containing this address is already in a cacheslot:if address is already in cache slot thentransfer the information to the CPU register - DONEelseAccess main memory for the block containing the requested addressAllocate a free cache slot to the blockperform these in parallel: 1) transfer the information to CPU 2) load the block intoslot; - DONEend ifDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache Memory35summaryOperating Systems39

Lecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemorysummaryDisplacementDemand PagingPMTHardware instructionprocessingPage fault handlerReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems3639

SUMMARYLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireIntroductionPaged MemoryDisplacement PagingDemand Paging SegmentPMTHardware instructionprocessing Virtual memoryPage fault handler CacheReplacement PoliciesFIFOLRUMRULFUSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems3739

Key termsLecture #1 Basicconcepts of O/SDavid GoodwinUniversity ofBedfordshireaddress resolutionIntroductionassociative memoryPaged MemoryDisplacementcache memoryDemand Pagingdemand pagingPMTHardware instructionprocessingdisplacementPage fault handlerFIFO policyReplacement PoliciesFIFOJob TableLRUMRULRU policyLFUMemory Map TableSegmented MemoryVirtual MemoryCache MemorysummaryOperating Systems3839

Key termsLecture #1 Basicconcepts of O/Spagepage fault handlerDavid GoodwinUniversity ofBedfordshirepage frameIntroductionPage Map TablePaged MemoryDisplacementpage replacement policyDemand Pagingpage swappingPMTHardware instructionprocessingpaged memory allocationPage fault handlersectorReplacement PoliciesFIFOsegementLRUMRUSegment Map TableLFUsegmented memory allocationSegmented MemoryVirtual MemorysubroutineCache MemorysummaryOperating Systems39thrashingvirtual memory39

Virtual Memory Cache Memory summary Operating Systems PAGED MEMORY ALLOCATION Analysis Advantages: Pages do not need to store in the main memory contiguously (the free page frame can spread all places in main memory) More e cient use of main memory (comparing to the approaches of early memory management) - no external/internal fragmentation

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