Belady’s Anomaly in OS

Belady’s Anomaly

If the number of frames increases, the page fault rate will be decreased.

Example:

7 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 that occur, if we increase 4 frames, there are 12-page faults that happened. So here the page fault rate is decreased. This result is most unexpected and it is called Belady’s Anomaly.

Page fault rate = Number of Page faults / Total number of references