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Short note: Beladys anomaly.

Mumbai University > Computer Engineering > Sem 5 > Operating System

Marks: 10M

Years: May 2015

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  • As the page-fault rate would decrease as the number of frames increases, but Belady’s anomaly says this is not true in all algorithms.

  • Page fault increases with number of page frame increases.

  • The general principle is if the number of frames is increased, the page fault rate will be decreased.

  • For example, consider the following reference string. 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 If we apply the FIFO algorithm with 3 frames, there are 15 page faults occur, if we increase 4 frames, there are 12 page faults. Consider the reference string. 1 2 3 4 1 2 5 1 2 3 4 5 The number of page faults for 4 frames (10) is greater than the number of faults for 3 frames (9). This result is most unexpected and is known as “Belady’s anomaly”.

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Page fault for 3 frames=9/12 Belady’s Anomaly was not always true it is applicable only to FIFO algorithm.

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