Tutorial 9 Page Replacement Algorithms In the following

Tutorial 9 Page Replacement Algorithms

In the following examples • A frame with a value of 0 is empty. A frame shown with an “=” sign holds the same value as it held in the previous time period

First in first out (FIFO) • The reference string of pages is 123472512645 • When a page must be replaced, the oldest page is chosen 1 2 3 4 7 2 5 1 2 6 4 5 1 = = = 7 = = 6 = = 0 2 = = 5 = = = 4 = 0 = 3 = = 1 = = = 5 0 = = 4 = = 2 = = = – page faults: 11

Optimal • The reference string of pages is 123472512645 • Replace the page that will not be used for the longest period of time 1 2 3 4 7 2 5 1 2 6 4 5 1 = = = = 6 = = 0 2 = = = = = 0 = 3 = 7 = 5 = = = 0 = = 4 = = = = – page faults: 7

Least recently used (LRU) • • • The reference string of pages is 123472512645 When a page must be replaced , LRU choses the page that has not been used for the largest period of time 1 2 3 4 7 2 5 1 2 6 4 5 1 = = = 7 = = 6 = = 0 2 = = = = = 0 = 3 = = = 5 = = = 4 = 0 = = 4 = = = 1 = = = 5 page faults: 10

Second chance When a page is used its use bit is set to 1. We also use a pointer to the next victim which is initialized to the first page/frame. When a page is loaded, it is set to point to the next frame. The list of pages is considered as a circular queue. When a page is considered for replacement, its use bit is examined. If it is zero [that page is replaced] otherwise [the use bit is set to zero, the next victim pointer is advanced, and the process repeated until a page is found with a zero use bit].

Second chance – 9 page faults.