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Chapter 8 Virtual Memory

Chapter 8 Virtual Memory. Operating Systems: Internals and Design Principles. Video tutorial. http:// www.youtube.com/watch?v=iqfjWIYyQ9k http://www.youtube.com/watch?v=4px3RrDBa6w. Real and Virtual Memory. Operating System Software. Policies for Virtual Memory. Key issue: Performance

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Chapter 8 Virtual Memory

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  1. Chapter 8Virtual Memory Operating Systems:Internals and Design Principles Seventh Edition William Stallings

  2. Video tutorial • http://www.youtube.com/watch?v=iqfjWIYyQ9k • http://www.youtube.com/watch?v=4px3RrDBa6w

  3. Real and Virtual Memory

  4. Operating System Software

  5. Policies for Virtual Memory • Key issue: Performance • minimize page faults (desired page is not in main memory)

  6. Replacement Policy • Deals with the selection of a page in main memory to be replaced when a new page must be brought in • objective is that the page that is removed be the page least likely to be referenced in the near future • The more elaborate the replacement policy the greater the hardware and software overhead to implement it

  7. Basic Algorithms

  8. Optimal Policy • Selects the page for which the time to the next reference is the longest • Produces three page faults after the frame allocation has been filled 1 is replaced as 1 will be not appeared after 5

  9. Least Recently Used (LRU) • Replaces the page that has not been referenced for the longest time • By the principle of locality, this should be the page least likely to be referenced in the near future • Difficult to implement • one approach is to tag each page with the time of last reference • this requires a great deal of overhead

  10. LRU Example

  11. First-in-First-out (FIFO) • Treats page frames allocated to a process as a circular buffer • Pages are removed in round-robin style • simple replacement policy to implement • Page that has been in memory the longest is replaced

  12. FIFO Example

  13. Clock Policy • Requires the association of an additional bit with each frame • referred to as the use bit • Uses and additional bit called a “use bit” • When a page is first loaded in memory or referenced, the use bit is set to 1 • When it is time to replace a page, the OS scans the set flipping all 1’s to 0 • The first frame encountered with the use bit already set to 0 is replaced.

  14. Clock Policy Example

  15. Clock Policy

  16. Clock Policy

  17. Combined Examples

  18. Summary • Desirable to: • maintain as many processes in main memory as possible • free programmers from size restrictions in program development • With virtual memory: • all address references are logical references that are translated at run time to real addresses • a process can be broken up into pieces • two approaches are paging and segmentation • management scheme requires both hardware and software support

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