FIFO

This is the simplest page replacement algorithm. In this algorithm, the operating system keeps track of all pages in the memory in a queue, the oldest page is in the front of the queue. When a page needs to be replaced page in the front of the queue is selected for removal.

Example-1Consider page reference string 1, 3, 0, 3, 5, 6 with 3 page frames.Find number of page faults.

Initially all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots —> 3 Page Faults.
when 3 comes, it is already in  memory so —> 0 Page Faults.
Then 5 comes, it is not available in  memory so it replaces the oldest page slot i.e 1. —>1 Page Fault.
6 comes, it is also not available in memory so it replaces the oldest page slot i.e 3 —>1 Page Fault.
Finally when 3 come it is not avilable so it replaces 0 1 page fault

Belady’s anomaly – Belady’s anomaly proves that it is possible to have more page faults when increasing the number of page frames while using the First in First Out (FIFO) page replacement algorithm.  For example, if we consider reference string 3, 2, 1, 0, 3, 2, 4, 3, 2, 1, 0, 4 and 3 slots, we get 9 total page faults, but if we increase slots to 4, we get 10 page faults.