Pagereplacement algorithm(computional concept)
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This document discusses page replacement techniques used in computer memory management. It covers the need for page replacement due to limited physical memory frames. The basic page replacement process is described as finding the desired page, a free frame if available, or using a replacement algorithm to select a victim frame if necessary. Common replacement algorithms like FIFO, optimal, and LRU are introduced along with their advantages and drawbacks. Belady's anomaly is also mentioned, which is when page fault rate can paradoxically increase as the number of available frames increases under FIFO replacement.
Pagereplacement algorithm(computional concept)
- 2. Overview 1. Why we need page replacement? 2. Basic page replacement technique. 3. Different type of page replacement algorithm and their examples.
- 3. Why???? Limited physical memory --> limited number of frame --> limited number of frame allocated to a process.
- 4. Basic Page Replacement 1. Find the location of the desired page on the disk. 2. Find a free frame If there is a free frame use it. No free frame – use page replacement algorithm to select a victim frame. Write the victim frame to disk, change the frame and page tables accordingly.
- 5. Basic Page Replacement (Contd) 3. Read the desired page into the newly freed frame, change the page and frame tables. 4. Restart the user process.
- 6. Page Replacement Victim Swap desire page in Reset page table for new page Change to invaid Page table Frame Valid invalid bit 2 4 3 0 F I V Swap out victim page 1 Physical memory
- 7. Overhead If no free frames - 2 page transfers. Solution : Modify bit or Dirty bit.
- 8. Replacement Policy Which page to be replaced? Page removed should be the page least likely to be referenced in the near future. Most policies predict the future behavior on the basis of past behavior.
- 9. Page faults versus number of frames Number of frames Number of page faults
- 10. Replacement Algorithm 1. FIFO page replacement 2. Optimal page replacement 3. LRU page replacement
- 11. FIFO Page Replacement Easy to understand and program. Performance is not always good. Belady’s Anomaly
- 12. Drawbacks - FIFO A page which is being accessed quite often may also get replaced because it arrived earlier than those present 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.
- 13. FIFO – An Example 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 7 0 7 7 0 1 2 0 1 2 3 1 2 3 0 4 3 0 4 2 0 4 2 3 0 2 3 0 1 3 0 1 2 7 1 2 7 0 2 7 0 1
- 14. Optimal Page Replacement Lowest page fault rate of all algorithms Free from Belady’s anomaly
- 15. Optimal Page Replacement (contd) “Replace the page that will not be used for the longest period of time.” Requires future knowledge of the reference string. Used for comparison studies.
- 16. OPR – An Example 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 7 0 7 7 0 1 2 0 1 2 0 3 2 4 3 2 0 3 7 0 1 2 0 1
- 17. LRU Page Replacement Free from Belady’s Anomaly. Problem: How to order the frame defined by the time of last use. Solution: Counters Queue
- 18. LRU – An Example 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 7 0 7 7 0 1 2 0 1 2 0 3 4 3 4 2 3 2 0 3 7 0 1 0 4 2 0 2 3 1 2 0 1
- 19. Ans: C 1. Dirty bit is used to show the a. page with corrupted data b. the wrong page in the memory c. page that is modified after being loaded into cache memory d. page that is less frequently accessed.
- 20. Ans: C 2. What replacement policy is used by Windows NT a. LRU b. NFU c. FIFO d. Clock Replacement e. None of the above
- 21. Reference String 4. The string of memory references is called ___________.
- 22. Belady’s Anomaly 5. FIFO page replacement suffers from ________________anomaly.
- 23. Page fault increases with increase in number of frames. 6. What is Belady’s anomaly??
- 24. TRUE 7. Optimal Page Replacement algorithm has the lowest page fault rate. TRUE/FALSE