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Pagereplacement algorithm(computional concept)
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.
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Pagereplacement algorithm(computional concept)
- 2.Overview1. Why weneed page replacement?2. Basic page replacement technique.3. Different type of page replacement algorithm andtheir examples.
- 3.Why????Limited physical memory--> limited number offrame --> limited number of frame allocated to aprocess.
- 4.Basic Page Replacement1.Find the location of the desired page on thedisk.2. Find a free frame If there is a free frame use it. No free frame – use page replacement algorithm toselect a victim frame. Write the victim frame to disk, change the frame andpage 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 ReplacementVictimSwap desirepageinReset pagetable for newpageChange toinvaidPage tableFrameValidinvalid bit2430FIVSwap outvictim page1Physical memory
- 7.OverheadIf no freeframes - 2 page transfers.Solution : Modify bit or Dirty bit.
- 8.Replacement PolicyWhich pageto be replaced?Page removed should be the page least likely to bereferenced in the near future.Most policies predict the future behavior on the basisof past behavior.
- 9.Page faults versusnumber of framesNumber of framesNumber of pagefaults
- 10.Replacement Algorithm1. FIFOpage replacement2. Optimal page replacement3. LRU page replacement
- 11.FIFO Page ReplacementEasyto understand and program.Performance is not always good.Belady’s Anomaly
- 12.Drawbacks - FIFOApage which is being accessed quite often may alsoget replaced because it arrived earlier than thosepresentIgnores locality of reference. A page which wasreferenced last may also get replaced, although thereis high probability that the same page may be neededagain.
- 13.FIFO – AnExample7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1707 701201231230430420423023013012712702701
- 14.Optimal Page ReplacementLowestpage fault rate of all algorithmsFree from Belady’s anomaly
- 15.Optimal Page Replacement(contd)“Replace the page that will not be used for the longestperiod of time.”Requires future knowledge of the reference string.Used for comparison studies.
- 16.OPR – AnExample7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1707 701201203243203701201
- 17.LRU Page ReplacementFreefrom Belady’s Anomaly.Problem: How to order the frame defined by the timeof last use.Solution: Counters Queue
- 18.LRU – AnExample7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1707 701201203434232037010420231201
- 19.Ans: C1. Dirtybit is used to show thea. page with corrupted datab. the wrong page in the memoryc. page that is modified after being loadedinto cache memoryd. page that is less frequently accessed.
- 20.Ans: C2. Whatreplacement policy is used by Windows NTa. LRUb. NFUc. FIFOd. Clock Replacemente. None of the above
- 21.Reference String4. Thestring of memory references is called___________.
- 22.Belady’s Anomaly5. FIFOpage replacement suffers from________________anomaly.
- 23.Page fault increaseswith increase in number offrames.6. What is Belady’s anomaly??
- 24.TRUE7. Optimal PageReplacement algorithm has thelowest page fault rate.TRUE/FALSE