Several such algorithms exist and it is a subject that enjoys a great deal of theoretical
attention and research. Two of the most well-known are first-in first-out and least
recently used. The first-in first-out (FIFO) policy is based on the theory that the best
page to remove is the one that has been in memory the longest. The least recently used
(LRU) policy chooses the page least recently accessed to be swapped out.