This document discusses different page replacement algorithms used in operating systems. It begins by explaining the basic concept of page replacement that occurs when memory is full and a page fault happens. It then describes several common page replacement algorithms: FIFO, Optimal, LRU, LRU approximations using reference bits, and Second Chance. The key aspects of each algorithm are summarized, such as FIFO replacing the oldest page, Optimal replacing the page not used for longest time, and LRU approximating this by tracking recently used pages. The document provides an overview of page replacement techniques in computer systems.

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Page replacement algorithms

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  Page ReplacementAlgorithmsPrepared By:Mr. Sangram A. PatilAssistant Professor PVPIT,Budhgaon
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  Need for pagereplacement
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  Basic Page ReplacementPage replacement takes following approach. If no frame is free then page fault occurs and we find a frame that is not currentlybeing used and free it. We can free a space by writing its content to swap space and changing the pagetable (all other table). To indicate page is not in memory. Now we can use the freed frame to hold the page for which the process is faulted. We modify the page table entry for requested page.
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  Basic Page ReplacementFind the location of desired page on disk. Find a free frame.1. if there is free frame use it.2. if there is no free frame use page replacement algorithm to select v victimpage3. write a victim frame to the disk. Change frame and page tables accordingly Read the desired page into the newly freed frame. Change the frame and pagetable accordingly. Restart the process.
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  Page replace algorithmsIf multiple processes are in memory then it decide how many frames allocate toeach process and when page replacement is required. Number of page replacement algorithms are available. Each OS has its own page replacement mechanism To evaluate page replacement algorithm we need string of memory references andwe computing number of page faults. String of memory reference is called reference string Need page size.
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  1.FIFO page replacementSimplest page replacement algorithm FIFO: First In First Out Associate with each page the time when that page was brought into memory When page must be replaced oldest page is chosen. We can create a FIFO queue to hold all pages in memory. We replace page at the head of the queue. When page is brought into memory we replace insert it at the tail of the queue.
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   For ourexample of reference string we consider three frames. These three frames are initially empty. FIFO page replacement works as below
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  2.Optimal Page replacementOptimal page replacement has lowest page fault rate of all algorithms will neversuffer from Baladys Anomaly. “replace the page that will not be used for longest period of time” Difficult to implement because it requires future knowledge of reference string.
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  3. LRU PageReplacement LRU: Least Recently Used FIFO: uses when page is brought into memory. Optimal: Uses time when a page to be used. Replace a page that has not been used from longest period of time. When a page must be replaced, LRU choses a page that has not been used fromlongest period of time. Looking backward in time.
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  4. LRU ApproximationFew computer system provides sufficient hardware support for LRU pagereplacement. Some system provide no hardware support and other page replacement algorithmmust be used. Many system provide reference bits. Reference bit is set by hardware whenever page is referenced. reference bit is associated with each entry in page table. Initially bits are cleared (to 0) by OS. A user process executes the bit associated with each page referenced is set (to 1)by hardware. After some time we determine which page have been used and have not been usedby examining the reference bits.
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  5 .Second chancealgorithm Basic algorithm is FIFO algorithm. When page has been selected we check its reference bit. If it is 0 we proceed to replace this page. If it is 1 we give a page second chance and move on to select the next FIFO page. When page gets second chance its reference bit gets cleared and arrival time set tocurrent time. Thus page that is given a second chance will not be replaced until all other pageshave been replaced Implemented by using circular queue.