Subject: Operating System

Topic: MEMORY MANAGEMENT

Difficulty: Hard

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”.

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