In operating systems that use paging for memory management, page replacement algorithm are needed to decide which page needed to be replaced when new page comes in. Whenever a new page is referred and not present in memory, page fault occurs and Operating System replaces one of the existing pages with newly needed page. Different page replacement algorithms suggest different ways to decide which page to replace. The target for all algorithms is to reduce number of page faults.

In Least Recently Used (LRU) algorithm is a Greedy algorithm where the page to be replaced is least recently used. The idea is based on locality of reference, the least recently used page is not likely.

Example:

•Consider the page references 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, with 4 page frame. Find number of page fault.

• Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 Page faults
• 0 is already there so —> 0 Page fault.
• when 3 came it will take the place of 7 because it is not used for the longest duration of time in the future.—>1 Page fault.
• 0 is already there so —> 0 Page fault.
• 4 will takes place of 1 —> 1 Page Fault.
• Now for the further page reference string —> 0 Page fault because they are already available in the memory.