# Page Replacement Algorithms

- Feb 8, 2002

- The Optimal Page Replacement Algorithm
- The Not Recently Used Page Replacement Algorithm
- The First-In, First-Out (FIFO) Page Replacement Algorithm
- The Second Chance Page Replacement Algorithm
- The Clock Page Replacement Algorithm
- The Least Recently Used (LRU) Page Replacement Algorithm
- Simulating LRU in Software
- The Working Set Page Replacement Algorithm
- The WSClock Page Replacement Algorithm
- Summary of Page Replacement Algorithms

## The Second Chance Page Replacement Algorithm

A simple modification to FIFO that avoids the problem of throwing out
a heavily used page is to inspect the *R *bit of the oldest page. If it is
0, the page is both old and unused, so it is replaced immediately. If the *R
*bit is 1, the bit is cleared, the page is put onto the end of the list of
pages, and its load time is updated as though it had just arrived in memory.
Then the search continues.

The operation of this algorithm, called **second chance**, is
shown in Fig. 4-1. In Fig. 4-1(a) we see pages *A *through *H *kept on
a linked list and sorted by the time they arrived in memory.

**Figure 4-1. Operation of second chance. (a) Pages sorted in FIFO order.
(b) Page list if a page fault occurs at time 20 and A has its R bit set. The
numbers above the pages are their loading times.**

Suppose that a page fault occurs at time 20. The oldest page is
*A*, which arrived at time 0, when the process started. If *A *has the
*R *bit cleared, it is evicted from memory, either by being written to the
disk (if it is dirty), or just abandoned (if it is clean). On the other hand, if
the *R *bit is set, *A *is put onto the end of the list and its
"load time" is reset to the current time (20). The *R
*bit is also cleared. The search for a suitable page continues with *B*.

What second chance is doing is looking for an old page that has not
been referenced in the previous clock interval. If all the pages have been
referenced, second chance degenerates into pure FIFO. Specifically, imagine that
all the pages in Fig. 4-1(a) have their *R *bits set. One by one, the
operating system moves the pages to the end of the list, clearing the *R
*bit each time it appends a page to the end of the list. Eventually, it comes
back to page *A*, which now has its *R *bit cleared. At this point
*A *is evicted. Thus the algorithm always terminates.