Belady;s anomaly is a condition where increasing the number of page frames results in an increase in the number of page faults. This phenomenon is commonly experienced when page replacement algorithm is First in First Out (FIFO) and was first demonstrated by László Bélády in 1969.
When a page fault occurs and all frames are in use, one must be cleared to make room for the new page. A simple algorithm is FIFO. Whichever page has been in the frames the longest is the one that is cleared. Until Belady anomaly was demonstrated, it was believed that an increase in the number of page frames would always result in the same number or fewer page faults.
Source: WiKi
