January 2014 Archive


PAGE REPLACEMENT ALGORITHMS
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PAGE REPLACEMENT ALGORITHMS

When a page fault takes place, the operating system has to choose a page to remove (remove from memory) to make room for the incoming page. If the page to be removed has been customized while in memory, it must be rewritten to the disk to bring the disk copy up to date. If,

The Not Recently Used Page Replacement Algorithm
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The Not Recently Used Page Replacement Algorithm

In order to allow the operating system to collect useful page usage statistics, most computers with virtual memory have two status bits associated with each page. R is set whenever the page is referenced (read or written). M is set when the page is written to (i.e., modified).

The Least Recently Used (LRU) Page Replacement Algorithm
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The Least Recently Used (LRU) Page Replacement Algorithm

A good estimation to the best algorithm is based on the observation that pages that have been heavily used in the last few instructions will possibly be heavily used again in the next few. On the other hand, pages that have not been used for ages will possibly remain unused for a

Simulating LRU in Software
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Simulating LRU in Software

Although both of the previous LRU algorithms are (in principle) realizable, few, if any, machines have the required hardware. Instead, a solution that can be implemented in software is required One possibility is called the NFU (Not Frequently Used) algorithm. It requires a

The Working Set Page Replacement Algorithm
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The Working Set Page Replacement Algorithm

In the purest form of paging, processes are started up with none of their pages in memory. As soon as the CPU tries to get the first instruction, it gets a page fault, causing the operating system to bring in the page containing the first instruction. Other page faults for global

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