This document discusses different page replacement algorithms used in operating systems. It begins by explaining the basic concept of page replacement that occurs when memory is full and a page fault happens. It then describes several common page replacement algorithms: FIFO, Optimal, LRU, LRU approximations using reference bits, and Second Chance. The key aspects of each algorithm are summarized, such as FIFO replacing the oldest page, Optimal replacing the page not used for longest time, and LRU approximating this by tracking recently used pages. The document provides an overview of page replacement techniques in computer systems.

1. Page Replacement
Algorithms
Prepared By: Mr. Sangram A. Patil
Assistant Professor PVPIT,Budhgaon

2. Need for page replacement

3. Basic Page Replacement
 Page replacement takes following approach.
 If no frame is free then page fault occurs and we find a frame that is not currently
being used and free it.
 We can free a space by writing its content to swap space and changing the page
table (all other table). To indicate page is not in memory.
 Now we can use the freed frame to hold the page for which the process is faulted.
 We modify the page table entry for requested page.

4. Basic Page Replacement
 Find the location of desired page on disk.
 Find a free frame.
1. if there is free frame use it.
2. if there is no free frame use page replacement algorithm to select v victim
page
3. write a victim frame to the disk. Change frame and page tables accordingly
 Read the desired page into the newly freed frame. Change the frame and page
table accordingly.
 Restart the process.

6. Page replace algorithms
 If multiple processes are in memory then it decide how many frames allocate to
each process and when page replacement is required.
 Number of page replacement algorithms are available.
 Each OS has its own page replacement mechanism
 To evaluate page replacement algorithm we need string of memory references and
we computing number of page faults.
 String of memory reference is called reference string
 Need page size.

7. 1.FIFO page replacement
 Simplest page replacement algorithm
 FIFO: First In First Out
 Associate with each page the time when that page was brought into memory
 When page must be replaced oldest page is chosen.
 We can create a FIFO queue to hold all pages in memory.
 We replace page at the head of the queue.
 When page is brought into memory we replace insert it at the tail of the queue.

8.  For our example of reference string we consider three frames.
 These three frames are initially empty.
 FIFO page replacement works as below

9. 2.Optimal Page replacement
 Optimal page replacement has lowest page fault rate of all algorithms will never
suffer from Baladys Anomaly.
 “replace the page that will not be used for longest period of time”
 Difficult to implement because it requires future knowledge of reference string.

11. 3. LRU Page Replacement
 LRU: Least Recently Used
 FIFO: uses when page is brought into memory.
 Optimal: Uses time when a page to be used.
 Replace a page that has not been used from longest period of time.
 When a page must be replaced, LRU choses a page that has not been used from
longest period of time.
 Looking backward in time.

13. 4. LRU Approximation
 Few computer system provides sufficient hardware support for LRU page
replacement.
 Some system provide no hardware support and other page replacement algorithm
must be used.
 Many system provide reference bits.
 Reference bit is set by hardware whenever page is referenced.
 reference bit is associated with each entry in page table.
 Initially bits are cleared (to 0) by OS.
 A user process executes the bit associated with each page referenced is set (to 1)
by hardware.
 After some time we determine which page have been used and have not been used
by examining the reference bits.

14. 5 .Second chance algorithm
 Basic algorithm is FIFO algorithm.
 When page has been selected we check its reference bit.
 If it is 0 we proceed to replace this page.
 If it is 1 we give a page second chance and move on to select the next FIFO page.
 When page gets second chance its reference bit gets cleared and arrival time set to
current time.
 Thus page that is given a second chance will not be replaced until all other pages
have been replaced
 Implemented by using circular queue.