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
Let say the page reference string 7 0 1 2 0 3 0 4 2 3 0 3 2 . Initially we have 4 page slots empty. Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 Page faults 0 is already their so —> 0 Page fault. when 3 came it will take the place of 7 because it is least recently used —>1 Page fault 0 is already in memory so —> 0 Page fault. 4 will takes place of 1 —> 1 Page Fault