Generally, on increasing the number of frames to a process’ virtual memory, its execution becomes faster as less number of page faults occur. Sometimes the reverse happens, i.e. more number of page faults occur when more frames are allocated to a process. This most unexpected result is termed as Belady’s Anomaly.

So Belady’s anomaly is the phenomenon where increasing the number of page frames results in an increase in the number of page faults for a given memory access pattern.

This phenomenon occurs in following Page Replacement Algorithms:

1. First In First Out (FIFO)


2. Second Chance Algorithm


3. Random Page Replacement Algorithm