virtual memory n.
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Virtual Memory
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  1. Virtual Memory

  2. Motivation • Motivations • Some programs (or their data sets) are very large • It’s much easier to write these programs assuming everything fits in main memory • A small main memory requires shuffling data with secondary storage • But a big main memory is expensive • Cost-cutting solution: Virtual Memory • Use a small main memory and big cheap secondary storage (e.g. hard disk) • Let the program think the main memory is big – trick it by shuffling data transparently between main and secondary • Implemented by special hardware and the operating system

  3. Addresses and Address Spaces • Phyical vs. virtual • Program sees virtual memory space and generate virtual addresses • Memory chips provide physical memory space and respond to physical addresses • Special hardware translates virtual addresses into physical addresses with page-size granularity • Pages and page frames • Virtual memory space is divided into numbered virtual pages • Physical memory is divided into numbered page frames • Virtual memory translation maps virtual pages (in virtual memory space) into page frames (in physical memory) • Address format. • E.g. 32 bit address, 4 kB page • offset within page: log2(4096)=12 bits • page number: 32 – 12 = 20 bits. 220 pages

  4. Implementation • OS moves pages in and out of memory • Paging in reads it in from disk • Paging out writes it to disk • A mapped virtual page has been allocated a page frame number • Page table holds mapping information, in page table entries (PTE) • Page’s owner (e.g. process ID) • Virtual page number • Page location in memory (page frame number) or on disk (location within swap file) • Valid? • Reference – recently accessed? • Modified – recently written? • page protection – read-only, read-write, etc.

  5. Demand Paging - Initialization • OS loads part of program by paging in the pages it thinks are needed • E.g. beginning of the program instructions, start of stack • OS initializes page table • Set valid bit and initialize page frame number for mapped pages (paged in) • Clear valid bit and initialize swap file offset for unmapped pages (not yet paged in) • OS starts program running

  6. Demand Paging – Operation • As program runs, each virtual memory reference (instruction load, data load or store) is translated to physical address using page table • Search for page table entry with same page as the virtual memory reference • Is page mapped to a page frame? (i.e. is there a PTE with the virtual page number and valid == 1?) • Yes? Create a physical address by replacing the virtual page number with the (physical) page frame number • No? A page fault occurs. OS Page fault handler runs

  7. Demand Paging – Page Fault Handler • Save state of processor (registers, flags, program counter) • Check range of valid addresses for process. If this is not a valid memory address, terminate process • Else is valid address, so… • Find a free page frame (e.g. from a list of free frames) to use • Look in PTE to find where in swap file (on disk) that page is located • Perform disk operation to load that page into the free page frame • Update PTE • Valid = 1 (in memory) • Update page frame number with new one • Restart instruction which caused page fault

  8. TLB - Accelerating Page Table Search • Have to examine page table for each memory access – very slow in software • Could cache the most recent PTE… • Could cache several recent PTEs… in a Translation lookaside buffer (TLB) • Input: virtual page number • Output: hit/miss, physical page frame number and protection information • If TLB misses, then we need to • Find the virtual page number in the full (memory-resident) page table – page table walking • Update an entry in the TLB so we don’t miss on the next access to the same page

  9. Accelerating TLB Misses • Modern processors have massive address spaces • 32 bit address space with 4 kB-long pages has 220 = 1,048,576 pages (and hence potentially that many PTEs) • Too large to put every PTE into the TLB • Searching (walking)the page table in software on TLB misses is really slow • Page table optimizations: • Forward-mapped (hierarchical) page table • Inverse-mapped (inverted) page table