Virtual MemoryHardware and Control StructuresExecution of a ProgramSlide 4Advantages of Breaking up a ProcessTypes of MemoryThrashingPrinciple of LocalitySupport Needed for Virtual MemoryPagingModify Bit in Page TablePage Table EntriesSlide 13Two-Level Scheme for 32-bit AddressPage TablesTranslation Lookaside BufferSlide 17Slide 18Slide 19Slide 20Slide 21Page SizeSlide 23Slide 24Slide 25Example Page SizesSegmentationSegment TablesSegment Table EntriesCombined Paging and SegmentationCombined Segmentation and PagingSlide 32Fetch PolicyReplacement PolicySlide 35Basic Replacement AlgorithmsSlide 37Slide 38Slide 39Slide 40Slide 41Slide 42Resident Set SizeVariable Allocation, Global ScopeVariable Allocation, Local ScopeCleaning PolicySlide 47Load ControlProcess SuspensionSlide 50UNIX and Solaris Memory ManagementData StructuresSlide 53Virtual MemoryChapter 8Hardware and Control Structures•Memory references are dynamically translated into physical addresses at run time–A process may be swapped in and out of main memory such that it occupies different regions•A process may be broken up into pieces that do not need to be located contiguously in main memory–All pieces of a process do not need to be loaded in main memory during executionExecution of a Program•Operating system brings into main memory a few pieces of the program•Resident set - portion of process that is in main memory•An interrupt is generated when an address is needed that is not in main memory•Operating system places the process in a blocking stateExecution of a Program•Piece of process that contains the logical address is brought into main memory–Operating system issues a disk I/O Read request–Another process is dispatched to run while the disk I/O takes place–An interrupt is issued when disk I/O complete which causes the operating system to place the affected process in the Ready stateAdvantages of Breaking up a Process•More processes may be maintained in main memory–Only load in some of the pieces of each process–With so many processes in main memory, it is very likely a process will be in the Ready state at any particular time•A process may be larger than all of main memoryTypes of Memory•Real memory–Main memory•Virtual memory–Memory on disk–Allows for effective multiprogramming and relieves the user of tight constraints of main memoryThrashing•Swapping out a piece of a process just before that piece is needed•The processor spends most of its time swapping pieces rather than executing user instructionsPrinciple of Locality•Program and data references within a process tend to cluster•Only a few pieces of a process will be needed over a short period of time•Possible to make intelligent guesses about which pieces will be needed in the future•This suggests that virtual memory may work efficientlySupport Needed forVirtual Memory•Hardware must support paging and/or segmentation •Operating system must be able to management the movement of pages and/or segments between secondary memory and main memoryPaging•Each process has its own page table•Each page table entry contains the frame number of the corresponding page in main memory•A bit is needed to indicate whether the page is in main memory or notModify Bit inPage Table•Another modify bit is needed to indicate if the page has been altered since it was last loaded into main memory•If no change has been made, the page does not have to be written to the disk when it needs to be swapped outPage Table EntriesTwo-Level Scheme for 32-bit AddressPage Tables•The entire page table may take up too much main memory•Page tables are also stored in virtual memory•When a process is running, part of its page table is in main memoryTranslation Lookaside Buffer•Each virtual memory reference can cause two physical memory accesses–one to fetch the page table–one to fetch the data•To overcome this problem a high-speed cache is set up for page table entries–called the TLB - Translation Lookaside BufferTranslation Lookaside Buffer•Contains page table entries that have been most recently used•Functions same way as a memory cacheTranslation Lookaside Buffer•Given a virtual address, processor examines the TLB•If page table entry is present (a hit), the frame number is retrieved and the real address is formed•If page table entry is not found in the TLB (a miss), the page number is used to index the process page tableTranslation Lookaside Buffer•First checks if page is already in main memory –if not in main memory a page fault is issued•The TLB is updated to include the new page entryPage Size•Smaller page size, less amount of internal fragmentation•Smaller page size, more pages required per process•More pages per process means larger page tables•Larger page tables means large portion of page tables in virtual memory•Secondary memory is designed to efficiently transfer large blocks of data so a large page size is betterPage Size•Small page size, large number of pages will be found in main memory•As time goes on during execution, the pages in memory will all contain portions of the process near recent references. Page faults low.•Increased page size causes pages to contain locations further from any recent reference. Page faults rise.Page Size•Multiple page sizes provide the flexibility needed to effectively use a TLB•Large pages can be used for program instructions•Small pages can be used for threads•Most operating system support only one page sizeExample Page SizesSegmentation•May be unequal, dynamic size•Simplifies handling of growing data structures•Allows programs to be altered and recompiled independently•Lends itself to sharing data among processes•Lends itself to protectionSegment Tables•corresponding segment in main memory•Each entry contains the length of the segment•A bit is needed to determine if segment is already in main memory•Another bit is needed to determine if the segment has been modified since it was loaded in main memorySegment Table EntriesCombined Paging and Segmentation•Paging is transparent to the programmer•Paging eliminates external fragmentation•Segmentation is visible to the programmer•Segmentation allows for growing data structures, modularity, and support for sharing and protection•Each segment is broken into fixed-size pagesCombined Segmentation and PagingFetch Policy•Fetch Policy–Determines when a page should be
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