1 / 43

Operating Systems

Operating Systems. Operating Systems. Unit 7: Virtual Memory organization. Evolution of memory organization. Virtual Memory concept. Solves problem of limited memory space Creates illusion of more memory than exists Creates 2 types of addresses Virtual addresses Referenced by processes

jsheldon
Download Presentation

Operating Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Operating Systems Operating Systems Unit 7: Virtual Memory organization

  2. Evolution of memory organization COP 5994 - Operating Systems

  3. Virtual Memory concept • Solves problem of limited memory space • Creates illusion of more memory than exists • Creates 2 types of addresses • Virtual addresses • Referenced by processes • Physical addresses • Describes locations in main memory • Memory management unit (MMU) • Dynamic address translation • Translates virtual addresses to physical address COP 5994 - Operating Systems

  4. Two-level Storage • backing store: • maintained on hard drive • contains complete virtual memory • real memory: • portion of virtual memory that is currently in use • Process can only access real memory • Management of mapping is essential COP 5994 - Operating Systems

  5. Two-level Storage COP 5994 - Operating Systems

  6. Address Translation • Mapping virtual addresses to real addresses • need address translation map per process • Address translation maps • Indicate which regions of a process’s virtual address space are currently in main memory and where they are located COP 5994 - Operating Systems

  7. Mapping based on block COP 5994 - Operating Systems

  8. Concept: block map table • address translation map is table with • block number • actual location of block in memory • typical implementation • block number is offset from table origin • table entries contain block location in memory COP 5994 - Operating Systems

  9. Concept: virtual address • structure of virtual address: • b is the block number in virtual memory • d is the displacement from the start of block b at which the referenced item is located COP 5994 - Operating Systems

  10. Block Mapping Process COP 5994 - Operating Systems

  11. Blocks ? • Pages • Blocks are fixed size • Technique is called paging • Segments • Blocks maybe of different size • Technique is called segmentation COP 5994 - Operating Systems

  12. Paging • Paging uses fixed-size block mapping: • Virtual address in paging system: v = (p, d) • p is the number of the page in virtual memory on which the referenced item resides • d is the displacement from the start of page p at which the referenced item is located COP 5994 - Operating Systems

  13. Page Frame: page in real memory • Fixed-size block of main memory • Aligned with page size multiple COP 5994 - Operating Systems

  14. Page to Frame correspondence COP 5994 - Operating Systems

  15. Page Table Entry (PTE) • maps virtual page p to page frame p´ • Contains bit to indicate if page is in real memory • Term: Page Fault • access to memory location on page that is not in real memory COP 5994 - Operating Systems

  16. Page Address Translation • Direct Mapping • Associative Memory • Translation Lookaside Buffer • Concerns: • memory access speed • size of page table COP 5994 - Operating Systems

  17. Address Translation by Direct Mapping COP 5994 - Operating Systems

  18. Page Table size • address size: 4 bytes bytes virtual memory amount: 4GB • Page size: 4k pages in virtual memory • PTE size: 8 bytes bytes maximum page table size: 8MB • Page table will not fit into processor cache COP 5994 - Operating Systems

  19. Page Table size • address size: 8 bytes bytes virtual memory amount • Page size: 4k pages in virtual memory • PTE size: 16 bytes bytes maximum page table size • Page table will not fit into processor cache COP 5994 - Operating Systems

  20. Address Translation by Associative Mapping COP 5994 - Operating Systems

  21. Combination: Direct/Associative Mapping Experience shows: TLB size of 64 or 128 Can reach 90% of associative mapping performance COP 5994 - Operating Systems

  22. Page Address Translation: concerns • memory access speed • consider cache speed • size of page table • 32 bit vs. 64 bit • use of memory space • locality of memory references • sparseness • advanced approaches • multi-level page tables • inverted page table COP 5994 - Operating Systems

  23. Multilevel Page Tables • Hierarchy of page tables • Each level containing a table that stores pointers to tables in the level below • Bottom-most level comprised of tables containing address translations • Can reduce memory overhead compared to direct-mapping system • page table does not have to be contiguous COP 5994 - Operating Systems

  24. Multilevel Page Tables COP 5994 - Operating Systems

  25. Inverted Page Tables • One inverted page table stores one PTE in memory for each page frame in the system • Inverted relative to traditional page tables • Uses hash functions to map virtual page to inverted page table entry COP 5994 - Operating Systems

  26. Inverted Page Tables COP 5994 - Operating Systems

  27. Inverted Page Tables • Hash collisions increase memory accesses • Collisions can be reduced by increasing the range of the hash function • Cannot increase the size of the inverted page table because it must store exactly one PTE for each page frame • trick: hash anchor table • one more level of indirection COP 5994 - Operating Systems

  28. Inverted Page Tables with hash anchor table COP 5994 - Operating Systems

  29. Also: Page Sharing • programs may share common pages • data and/or instructions • pages marked as sharable or nonsharable COP 5994 - Operating Systems

  30. Segmentation • based on variable-size segments • contiguous block of process’ address space • data, text, stack • process can run if current segment(s) is in main memory COP 5994 - Operating Systems

  31. Segmentation COP 5994 - Operating Systems

  32. Segmentation • virtual memory address v = (s, d) • s is the segment number in virtual memory • d is the displacement within segment s at which the referenced item is located COP 5994 - Operating Systems

  33. Address Translation by Direct Mapping COP 5994 - Operating Systems

  34. Segment map table entry • maps segment s to real memory address s´ • has resident bit to indicate segment is in memory • If so, it stores the segment base address • Otherwise, it stores the location of the segment on secondary storage • has segment length field • useful to protect memory outside of segment COP 5994 - Operating Systems

  35. Segment map table entry COP 5994 - Operating Systems

  36. Segment map table entry: protection COP 5994 - Operating Systems

  37. Also: Segment Sharing Caution: segment protection COP 5994 - Operating Systems

  38. Combination: Segmentation/Paging Systems • Segments occupy one or more pages • All pages of segment need not be in main memory at once • Pages contiguous in virtual memory need not be contiguous in main memory COP 5994 - Operating Systems

  39. Virtual memory address • s is segment number • p is page number within segment • d is displacement within page at which desired item located COP 5994 - Operating Systems

  40. Address Translation COP 5994 - Operating Systems

  41. System-wide Table Structure COP 5994 - Operating Systems

  42. Sharing and Protection • Page replacement requires updates to several tables • Protection checking is complex COP 5994 - Operating Systems

  43. Agenda for next week: • next week: • Chapter 11: Virtual Memory Management • Read ahead ! COP 5994 - Operating Systems

More Related