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Page replacement

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Operating System - Page replacement

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Page replacement

  1. 1. Page Replacement Sashikanta Taorem MTech – CSE 1 st Semester Presented by:
  2. 2. Overview <ul><li>Why we need page replacement? </li></ul><ul><li>Basic page replacement technique. </li></ul><ul><li>Different type of page replacement algorithm and their examples. </li></ul>
  3. 3. Why???? <ul><li>Limited physical memory --> limited number of frame --> limited number of frame allocated to a process. </li></ul>
  4. 4. Basic Page Replacement <ul><li>Find the location of the desired page on the disk. </li></ul><ul><li>Find a free frame </li></ul><ul><ul><ul><li>If there is a free frame use it. </li></ul></ul></ul><ul><ul><ul><li>No free frame – use page replacement algorithm to select a victim frame. </li></ul></ul></ul><ul><ul><ul><li>Write the victim frame to disk, change the frame and page tables accordingly. </li></ul></ul></ul>
  5. 5. Basic Page Replacement (Contd) <ul><li>3. Read the desired page into the newly freed frame, change the page and frame tables. </li></ul><ul><li>4. Restart the user process. </li></ul>
  6. 6. Page Replacement
  7. 7. Overhead <ul><li>If no free frames - 2 page transfers. </li></ul><ul><li>Solution : Modify bit or Dirty bit. </li></ul>
  8. 8. Replacement Policy <ul><li>Which page to be replaced? </li></ul><ul><li>Page removed should be the page least likely to be referenced in the near future. </li></ul><ul><li>Most policies predict the future behavior on the basis of past behavior. </li></ul>
  9. 9. Page faults versus number of frames <ul><li>Number of frames </li></ul>Number of page faults
  10. 10. Replacement Algorithm <ul><li>FIFO page replacement </li></ul><ul><li>Optimal page replacement </li></ul><ul><li>LRU page replacement </li></ul><ul><li>LRU-Approximation page replacement </li></ul><ul><li>Counting-Based page replacement </li></ul>
  11. 11. FIFO Page Replacement <ul><li>Easy to understand and program. </li></ul><ul><li>Performance is not always good. </li></ul><ul><li>Belady’s Anomaly </li></ul>
  12. 12. Drawbacks - FIFO <ul><li>A page which is being accessed quite often may also get replaced because it arrived earlier than those present </li></ul><ul><li>Ignores locality of reference. A page which was referenced last may also get replaced, although there is high probability that the same page may be needed again. </li></ul>
  13. 13. FIFO – An Example
  14. 14. Optimal Page Replacement <ul><li>Lowest page fault rate of all algorithms </li></ul><ul><li>Free from Belady’s anomaly </li></ul>
  15. 15. Optimal Page Replacement (contd) <ul><li>“Replace the page that will not be used for the longest period of time.” </li></ul><ul><li>Requires future knowledge of the reference string. </li></ul><ul><li>Used for comparison studies. </li></ul>
  16. 16. OPR – An Example
  17. 17. LRU Page Replacement <ul><li>Free from Belady’s Anomaly. </li></ul><ul><li>Problem: How to order the frame defined by the time of last use. </li></ul><ul><li>Solution: </li></ul><ul><ul><ul><li>Counters </li></ul></ul></ul><ul><ul><ul><li>Queue </li></ul></ul></ul>
  18. 18. LRU – An Example
  19. 19. LRU – Approximation Page Replacement <ul><li>Additional-Reference-Bits Algorithm </li></ul><ul><li>Second-Chance Algorithm </li></ul><ul><li>Enhanced Second-Chance Algorithm </li></ul>
  20. 20. Additional-Reference-Bits Algorithm <ul><li>Uses reference bit, initially at 0 ,after reference h/w updates it to 1. </li></ul><ul><li>We can use 8-bit byte as reference bits history. </li></ul><ul><li>At regular interval </li></ul><ul><ul><li>Shift the reference bit into the high-order bit of 8-bit byte, shifting the other bits right by 1 bit and discarding the low-order bit. </li></ul></ul>
  21. 21. Additional-Reference-Bits Algorithm <ul><li>Example: </li></ul><ul><ul><li>00000000 – page has not been use for eight time period. </li></ul></ul><ul><ul><li>11111111 – use at least once in each time period. </li></ul></ul><ul><ul><li>11000100 is use more recently than 01110111 </li></ul></ul>
  22. 22. Second-Chance Algorithm <ul><li>Uses FIFO and a reference bit. </li></ul><ul><li>Implement using a circular queue. </li></ul><ul><li>Refer as CLOCK algorithm. </li></ul>
  23. 24. Enhanced Second-Chance Algorithm <ul><li>Considers the Reference bit and the modify bit as an ordered pair, (Reference bit, Modify bit). </li></ul><ul><li>Four possible cases: </li></ul><ul><ul><ul><li>(0,0) – neither recently used nor modified. </li></ul></ul></ul><ul><ul><ul><li>(0,1) </li></ul></ul></ul><ul><ul><ul><li>(1,0) </li></ul></ul></ul><ul><ul><ul><li>(1,1) </li></ul></ul></ul>
  24. 25. Counting-Based Page Replacement <ul><li>Uses COUNTERS to count the number of references that have been made to each page. </li></ul><ul><li>Least Frequently used </li></ul><ul><li>Most frequently Used </li></ul>
  25. 26. Page-Buffering algorithm <ul><li>Uses POOL of frames </li></ul><ul><li>When a page fault occurs, a victim is chosen but the page is read into a free frame from the pool before the victim is written out. </li></ul><ul><li>When a page fault occurs, we first check whether the desire page is in the free-frame pool. If it is not, we must select a free frame and read into it. </li></ul>
  26. 27. Objective Questions!!!
  27. 28. Ans: C <ul><li>Dirty bit is used to show the </li></ul><ul><li>a. page with corrupted data </li></ul><ul><li>b. the wrong page in the memory </li></ul><ul><li>c. page that is modified after being loaded into cache memory </li></ul><ul><li>d. page that is less frequently accessed. </li></ul>
  28. 29. Ans: C <ul><li>2. What replacement policy is used by Windows NT </li></ul><ul><li>a. LRU </li></ul><ul><li>b. NFU </li></ul><ul><li>c. FIFO </li></ul><ul><li>d. Clock Replacement </li></ul><ul><li>e. None of the above </li></ul>
  29. 30. CLOCK algorithm <ul><li>3. Second-Chance page replacement algorithm is also known as ……………… </li></ul>
  30. 31. Reference String <ul><li>4. The string of memory references is called </li></ul><ul><li>___________. </li></ul>
  31. 32. Belady’s Anomaly <ul><li>5. FIFO page replacement suffers from </li></ul><ul><li>________________anomaly. </li></ul>
  32. 33. Page fault increases with increase in number of frames. <ul><li>6. What is Belady’s anomaly?? </li></ul>
  33. 34. TRUE <ul><li>7. Optimal Page Replacement algorithm has the lowest page fault rate. </li></ul><ul><li>TRUE/FALSE </li></ul>
  34. 35. The reference bit and the modify bit <ul><li>8. Enhanced Second-Chance algorithm uses </li></ul><ul><li>_____________and ___________ as an ordered pair. </li></ul>

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