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Virtual Memory Management

Virtual Memory Management. demand paging page fault. Virtual Memory. Definition : A technique that allows the execution of processes that may not be completely in memory. Advantage : Programs can be larger than physical memory. Disadvantage : not easy to implement

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Virtual Memory Management

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  1. Virtual Memory Management demand paging page fault SunMoon University

  2. Virtual Memory • Definition : • A technique that allows the execution of processes that may not be completely in memory. • Advantage : • Programs can be larger than physical memory. • Disadvantage : • not easy to implement • decrease performance if it is used carelessly SunMoon University

  3. Motivation(1) • Exam. of real programs • the entire program is not needed  p.238 • not needed at the same time  overlay • Advantages of partial allocation • fig 8.1 • large VAS, simplifying the programming task • increase the multiprogramming degree • less I/O  run faster SunMoon University

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  5. Motivation(2) • Virtual Memory • implemented by (demand) paging or • by segmentation or • by paged segmentation scheme • Several Storage Management Strategies for Virtual Storage System  problems of partial allocation in V.M. 1) Fetch Demand Fetch Anticpatory Fetch SunMoon University

  6. Motivation(3) 2) Placement Paging Segmentation Paged Segmentation 3) Replacement SunMoon University

  7. Demand Paging(1) - similar to a paging system with swapping fig 8.2 - pager vs. swapper (lazy) swapper swapping the entire process into memory never swaps if that page will not be needed pager individual pages of a process decreasing the swap time and the amount of physical memory needed SunMoon University

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  9. Demand Paging(2) • Page Table • fig 8.3 • valid-invalid bit • Step in Handling a Page Fault • fig 8.4 • In the extreme case, -Start executing a process with no page in memory • “Pure Demand Paging” : never bring a page into memory until it is required SunMoon University

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  12. Demand Paging(3) • The H/W to support Demand Paging • Page Table • The ability to mark an entry ( Invalid / Valid ) • Secondary Memory • holds those pages not in main memory • high-speed disks • swap device/swap space SunMoon University

  13. Performance of Paging(1) • Three Major Components of the Page-Fault Service Time • Service the Page-Fault Interrupt. • Read in the Page. • Restart the Process.  Disk I/O is do expensive. SunMoon University

  14. Performance of Paging(2) • 페이지 부재 발생하지 않은 경우 : • 유효접근시간 = 메모리접근시간(Ma) • 페이지 부재 확률 : p • 0≤p≤1 ⇒ 0에 매우 가깝기를 기대(약간의 페이지 부재율) • 유효 접근 시간 = (1-p) × Ma + p ×( 페이지 부재 시간) • 평균 페이지 부재 서비스 시간 : 10ms • 메모리 접근 시간 : 1 sec • 유효 접근 시간 = (1-p) × (1 sec) + p × (10 ms) = (1-p) + 10000 * P sec = 1+9999 * P sec SunMoon University

  15. Performance of Paging(3) • [예] 페이지 부재율 p = 1/1000 1 ㎲ + 9999 P ㎲ = 1 ㎲ + (9999 / 1000) ㎲ = 10.9 ㎲ (약 11 ㎲ ) • 페이지 부재가 전혀 없을 때(P = 0)의 접근 시간 (1 ㎲) 보다 11배 정도 느림 SunMoon University

  16. Performance of Paging(4) • 유효 접근 시간 : 페이지 부재율(page fault rate)에 비례 • 1,000개 중에 1 개 : 페이지 부재 발생 • 유효 접근 시간 : 11 sec(요구 페이징으로 11배 감속) • 10% 감속보다 적게 하려면 다음 조건이 필요 • 1.10 > 1+ 9999 × P • .10 > 9999 × P • P < .00001 SunMoon University

  17. Performance of Paging(5) • 10,000번 중에서 1번의 페이지 부재 발생 비율보다 더 낮게 비율 유지 필요 • 요구 페이징 시스템 • 페이지 부재율을 낮게 유지시키는 것이 중요 • 유효 접근 시간이 증가할수록 프로세스 수행 시간은 늦어짐 SunMoon University

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