If a page fault occurs and all memory frames have been already allocated, then replacement of a page in memory is required on the request of a new page. ... Sometimes the reverse happens, i.e. more page faults occur when more frames are allocated to a process. This most unexpected result is termed Belady's Anomaly.

What is Belady's anomaly and why does it occur?

In computer storage, Bélády's anomaly is the phenomenon in which increasing the number of page frames results in an increase in the number of page faults for certain memory access patterns. This phenomenon is commonly experienced when using the first-in first-out (FIFO) page replacement algorithm.

How often does Belady's anomaly occur?

We found that over a wide range of process sizes and reference string lengths, Belady's anomaly occurred for up to 58.6% of the (random) reference strings under FIFO, and up to 100% of the reference strings for Random Page.