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CMU CS 15213 - Virtual Memory

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Virtual Memory Oct. 21, 2003Classic Motivations for Virtual MemoryModern Motivations for VMWhy does VM Work?Motivation #1: DRAM a “Cache” for DiskLevels in Memory HierarchyDRAM vs. SRAM as a “Cache”Impact of Properties on DesignLocating an Object in a “Cache”Locating an Object in “Cache” (cont.)A System with Physical Memory OnlyA System with Virtual MemoryPage Faults (like “Cache Misses”)Servicing a Page FaultMotivation #2: Memory ManagementSolution: Separate Virt. Addr. SpacesContrast: Macintosh Memory ModelMacintosh Memory ManagementMac vs. VM-Based Memory MgmtMAC OS XMotivation #3: ProtectionVM Address TranslationVM Address Translation: HitVM Address Translation: MissSlide 25Page TablesAddress Translation via Page TablePage Table OperationSlide 29Slide 30Integrating VM and CacheSpeeding up Translation with a TLBAddress Translation with a TLBSimple Memory System ExampleSimple Memory System Page TableSimple Memory System TLBSimple Memory System CacheAddress Translation Example #1Address Translation Example #2Address Translation Example #3Multi-Level Page TablesMain ThemesVirtual MemoryOct. 21, 2003Virtual MemoryOct. 21, 2003TopicsMotivations for VMAddress translationAccelerating translation with TLBsclass17.ppt15-213“The course that gives CMU its Zip!”– 2 –15-213, F’03Classic Motivations for Virtual MemoryClassic Motivations for Virtual MemoryUse Physical DRAM as a Cache for the DiskAddress space of a process can exceed physical memory sizeSum of address spaces of multiple processes can exceed physical memorySimplify Memory ManagementMultiple processes resident in main memory.Each process has its own address spaceOnly “active” code and data is actually in memoryAllocate more memory to process as needed.Provide ProtectionOne process can’t interfere with another.Because they operate in different address spaces.User process cannot access privileged informationDifferent sections of address spaces have different permissions.– 3 –15-213, F’03Modern Motivations for VMModern Motivations for VMMemory sharing and controlMemory sharing and controlCopy on write: share physical memory among multiple processes until a process tries to write to it. At that point make a copy. For example, this eliminates the need for vfork()Shared librariesProtection (debugging) via Segment-Drivers (Solaris)Sparse address space support (64bit systems)Sparse address space support (64bit systems)Memory as a fast communication deviceMemory as a fast communication devicePart of memory is shared by multiple processesMultiprocessing (beyond the scope of 15-213)Multiprocessing (beyond the scope of 15-213)– 4 –15-213, F’03Why does VM Work?Why does VM Work?It is not used!– 5 –15-213, F’03Motivation #1: DRAM a “Cache” for DiskMotivation #1: DRAM a “Cache” for DiskFull address space is quite large:32-bit addresses: ~4,000,000,000 (4 billion) bytes64-bit addresses: ~16,000,000,000,000,000,000 (16 quintillion) bytesDisk storage is ~500X cheaper than DRAM storage80 GB of DRAM: ~ $25,00080 GB of disk: ~ $50To access large amounts of data in a cost-effective manner, the bulk of the data must be stored on disk1GB: ~$300 160 GB: ~$1004 MB: ~$500DiskDRAMSRAM– 6 –15-213, F’03Levels in Memory HierarchyLevels in Memory HierarchyCPUCPUregsCacheMemoryMemorydiskdiskSize:Latency:$/Mbyte:Line size:32 B< 1 ns8(16) BRegister Cache Memory Disk Memory32 KB-4MB~2 ns$125/MB32(64) B1024 MB> 50 ns$0.20/MB4(64+) KB100 GB>8 ms$0.001/MBlarger, slower, cheaper8 B 32 B 4 KBcache virtual memory– 7 –15-213, F’03DRAM vs. SRAM as a “Cache”DRAM vs. SRAM as a “Cache”DRAM vs. disk is more extreme than SRAM vs. DRAMAccess latencies:DRAM ~10X slower than SRAMDisk ~160,000X slower than DRAMImportance of exploiting spatial locality:First byte is ~160,000X slower than successive bytes on diskvs. ~4X improvement for page-mode vs. regular accesses to DRAMBottom line: Design decisions made for DRAM caches driven by enormous cost of missesDRAMSRAMDisk– 8 –15-213, F’03Impact of Properties on DesignImpact of Properties on Design If DRAM was to be organized similar to an SRAM cache, how would we set the following design parameters?Line size?Large, since disk better at transferring large blocksAssociativity?High, to minimize miss rateWrite through or write back?Write back, since can’t afford to perform small writes to disk What would the impact of these choices be on:miss rateExtremely low. << 1%hit timeMust match cache/DRAM performancemiss latencyVery high. ~20mstag storage overheadLow, relative to block size– 9 –15-213, F’03Locating an Object in a “Cache”Locating an Object in a “Cache”SRAM CacheTag stored with cache lineMaps from cache block to memory blocksFrom cached to uncached formSave a few bits by only storing tagNo tag for block not in cacheHardware retrieves informationcan quickly match against multiple tagsXObject NameTag DataD 243X 17J 105••••••0:1:N-1:= X?“Cache”– 10 –15-213, F’03Locating an Object in “Cache” (cont.)Locating an Object in “Cache” (cont.)Data243 17105•••0:1:N-1:XObject NameLocation•••D:J:X: 10On Disk“Cache”Page TableDRAM CacheEach allocated page of virtual memory has entry in page tableMapping from virtual pages to physical pagesFrom uncached form to cached formPage table entry even if page not in memorySpecifies disk addressOnly way to indicate where to find pageOS retrieves information– 11 –15-213, F’03A System with Physical Memory OnlyA System with Physical Memory OnlyExamples:Most Cray machines, early PCs, nearly all embedded systems, etc.Addresses generated by the CPU correspond directly to bytes in physical memoryCPU0:1:N-1:MemoryPhysicalAddresses– 12 –15-213, F’03A System with Virtual MemoryA System with Virtual MemoryExamples:Workstations, servers, modern PCs, etc.Address Translation: Hardware converts virtual addresses to physical addresses via OS-managed lookup table (page table)CPU0:1:N-1:Memory0:1:P-1:Page TableDiskVirtualAddressesPhysicalAddresses– 13 –15-213, F’03Page Faults (like “Cache Misses”)Page Faults (like “Cache Misses”)What if an object is on disk rather than in memory?Page table entry indicates virtual address not in memoryOS exception handler invoked to move data


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CMU CS 15213 - Virtual Memory

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