Page Replacement Algorithms in Operating Systems

In an operating system that uses paging for memory management, a page replacement algorithm is needed to decide which page needs to be replaced when a new page comes in. Page replacement becomes necessary when a page fault occurs and no free page frames are in memory. in this article, we will discuss different types of page replacement algorithms.

Page Replacement Algorithms

Page replacement algorithms are techniques used in operating systems to manage memory efficiently when the physical memory is full. When a new page needs to be loaded into physical memory, and there is no free space, these algorithms determine which existing page to replace.

If no page frame is free, the virtual memory manager performs a page replacement operation to replace one of the pages existing in memory with the page whose reference caused the page fault. It is performed as follows: The virtual memory manager uses a page replacement algorithm to select one of the pages currently in memory for replacement, accesses the page table entry of the selected page to mark it as “not present” in memory, and initiates a page-out operation for it if the modified bit of its page table entry indicates that it is a dirty page.

Common Page Replacement Techniques

• First In First Out (FIFO)
• Optimal Page replacement
• Least Recently Used (LRU)
• Most Recently Used (MRU)

First In First Out (FIFO)

This is the simplest page replacement algorithm. In this algorithm, the operating system keeps track of all pages in the memory in a queue, the oldest page is in the front of the queue. When a page needs to be replaced page in the front of the queue is selected for removal.

Example 1: Consider page reference string 1, 3, 0, 3, 5, 6, 3 with 3-page frames. Find the number of page faults using FIFO Page Replacement Algorithm.

Initially, all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots ---> 3 Page Faults.
when 3 comes, it is already in memory so ---> 0 Page Faults. Then 5 comes, it is not available in memory, so it replaces the oldest page slot i.e 1. ---> 1 Page Fault. 6 comes, it is also not available in memory, so it replaces the oldest page slot i.e 3 ---> 1 Page Fault. Finally, when 3 come it is not available, so it replaces 0 1-page fault.

Implementation of FIFO Page Replacement Algorithm

• Program for Page Replacement Algorithm (FIFO)

Optimal Page Replacement

In this algorithm, pages are replaced which would not be used for the longest duration of time in the future.

Example: Consider the page references 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 3 with 4-page frame. Find number of page fault using Optimal Page Replacement Algorithm.

Initially, all slots are empty, so when 7 0 1 2 are allocated to the empty slots ---> 4 Page faults
0 is already there so ---> 0 Page fault. when 3 came it will take the place of 7 because it is not used for the longest duration of time in the future---> 1 Page fault. 0 is already there so ---> 0 Page fault. 4 will takes place of 1 ---> 1 Page Fault.

Now for the further page reference string ---> 0 Page fault because they are already available in the memory. Optimal page replacement is perfect, but not possible in practice as the operating system cannot know future requests. The use of Optimal Page replacement is to set up a benchmark so that other replacement algorithms can be analyzed against it.

Least Recently Used

In this algorithm, page will be replaced which is least recently used.

Example Consider the page reference string 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 3 with 4-page frames. Find number of page faults using LRU Page Replacement Algorithm.

Initially, all slots are empty, so when 7 0 1 2 are allocated to the empty slots ---> 4 Page faults
0 is already there 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
Now for the further page reference string ---> 0 Page fault because they are already available in the memory.

Implementation of LRU Page Replacement Algorithm

• Program for Least Recently Used (LRU) Page Replacement algorithm

Most Recently Used (MRU)

In this algorithm, page will be replaced which has been used recently. Belady's anomaly can occur in this algorithm.

Example 4: Consider the page reference string 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 3 with 4-page frames. Find number of page faults using MRU Page Replacement Algorithm.

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 comes it will take place of 0 because it is most recently used ---> 1 Page fault

when 0 comes it will take place of 3  ---> 1 Page fault

when 4 comes it will take place of 0  ---> 1 Page fault

2 is already in memory so ---> 0 Page fault

when 3 comes it will take place of 2  ---> 1 Page fault

when 0 comes it will take place of 3  ---> 1 Page fault

when 3 comes it will take place of 0  ---> 1 Page fault

when 2 comes it will take place of 3  ---> 1 Page fault

when 3 comes it will take place of 2  ---> 1 Page fault

Conclusion

In summary, page replacement algorithms are essential for managing a computer's memory efficiently. They help ensure that the system runs smoothly by reducing the number of times it needs to fetch data from slower storage. Different algorithms, like FIFO and LRU, have their own pros and cons, and choosing the right one depends on the specific needs of the system. Understanding these algorithms can help improve system performance and make our computers faster and more efficient.

GATE CS Corner Questions

Practicing the following questions will help you test your knowledge. All questions have been asked in GATE in previous years or in GATE Mock Tests. It is highly recommended that you practice them.

1. Memory Management | Question 1
2. Memory Management | Question 10
3. GATE CS 2014 (Set-1), Question 65
4. GATE CS 2012, Question 40
5. GATE CS 2007, Question 56
6. GATE CS 2007, Question 82
7. GATE CS 2007, Question 83
8. GATE CS 2014 (Set-3), Question 65
9. GATE CS 2002 Question 23
10. GATE CS 2001, Question 21