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Virtual memory

Virtual memory. What is program size>memory available?. Load only the parts of the memory that is needed Do bookkeeping When accessing, make sure that that part of the program is in memory; if not in memory, replace some part by the needed part This is called overlay. Demand paging.

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Virtual memory

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  1. Virtual memory

  2. What is program size>memory available? • Load only the parts of the memory that is needed • Do bookkeeping • When accessing, make sure that that part of the program is in memory; if not in memory, replace some part by the needed part • This is called overlay

  3. Demand paging • Maintain two tables • Page map table • same as before (add a column: valid) • File map table • For each page i, this table tells where in disk the page is available

  4. Address translation • When converting page number into frame number, check if that valid bit is 1 (means the page is in memory) • If valid bit is 0, it means the page is not in memory (page fault). Now, bring the page from disk into main memory

  5. Policies • Allocation • Fetch • Replacement

  6. Allocation policies • Static allocation • The number of frames allocated to a program does not change when it executes • Dynamic allocation • The number frames allocated to the program changes with time

  7. Fetch policy • Prefetch • Fetch a page from disk before it is needed • Demand fetch: • Fetch a page from disk only when it is needed.

  8. Replacement policy • When no free frame is available and we have a page fault, one of that process’s pages will have to be replaced. • Which page will be replaced?

  9. Three replacement policies • FIFO • LRU • Belady’s optimal

  10. Basis for comparison • Execution time page trace (ETPT) = sequence of pages referenced when a program executes • Reference string w = replace consecutive identical pages numbers by one occurrence • ETPT = 1223343334456 • Corresponding reference string = 12343456

  11. Belady’s optimal rule • Replace the page that will not be needed for the longest time in the future

  12. FIFO • Exhibits anomaly (when the number of frames allocated increases, the page fault rate increases) • Replace the page in the First in First Out manner. (use a queue data structure)

  13. Least Recently Used (LRU) • Replace the page that has not been referenced for the longest time in the past • LRU does not exhibit anomaly • Variations of this are used in practice

  14. Implementation of LRU • Exact implementation is very expensive • Use approximation algorithms

  15. Approximation algorithms • 1. Clock algorithm • Maintain two bits (r,w) per page in mamory • r bit is set to 1 every time is referenced • w bit is set to 1 every time the page is written • periodically, set all r bits to 0 • Replace the page if the two bits are (0,0) • Else look for page with (0,1) or (1,0)

  16. Approximation 2 • Maintain a long bit string (1 bit per page of the program) • Set bit i when page i is referenced • Periodically, save the bit string in a circular buffer and clear all the bits • At time of page fault, choose the page to be replaced based on its bit values in the strings

  17. Thrashing • When we allocate too few frames to a process, the page fault rate is very high (low CPU utilization) • When too many frames are allocated, fewer programs will be accommodated

  18. Demand paging in Unix • Conditions • Memory architecture allows paging • Kernel implements a paging policy • Restartable execution is possible • execute part of an instruction, page fault and restar

  19. Page [map] Table • Contents: • Physical address of page • Protection bits (r/w/x) • 5 bit fields to support demand paging • valid (1=contents valid) • reference (1= recently referenced) • modify (dirty bit) • copy on write • age

  20. Free Frames • The kernel maintains a supply of free frames in two lists (to be allocated to programs when there is a page fault) • a free list (FIFO list) • a hash list (for finding if a process’s page, after page fault on that page, is in free list • Allocate free frames from the free list

  21. Page Stealer Process • Swaps out pages (frames) that are no longer part of the process’s working set • Created during system initialization and invoked whenever the number of free frames is low

  22. Stealer process • Clear the reference bit of each page • Remember how many passes were made since the last time the page was accessed • When a threshold value is reached, make the page ``ready to swap out.’’ and add to free list

  23. Stealer Process • Page stealer is waken up by the kernel when available free frames is lower than a threshold • Page stealer swaps out pages until the number of frames in the free list is larger than a threshold • The threshold is implementation dependent.

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