Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.
Usually what happens is that when we increase the number of pages of a process in the main memory(i.e we allocate more number of frames to a process) the frequency of getting a page fault is reduced and hence the process-execution becomes faster as now there are more chances of a page already being in the main memory when it is demanded by the processor, but sometimes we observe behaviour deviated from this, that is even if we increase the number of pages of a process in the main memory the number of page faults will increase. This phenomenon of unusual behavior is called **"Beladys Anomaly"**. Generally, it is observed **FIFO** page replacement algorithm, that's why it is also known as **FIFO Anomaly**.