View Course Path

Page Replacement Algorithms in OS – Simple Explanation

Before we talk about what page replacement is, you should know about paging.

What do you mean by paging?

  • Paging is the process of swapping a page from the virtual memory space into the main memory.
  • Processes are divided into pages that are stored in the virtual memory.
  • These are fixed-size units.
  • The main memory is divided into frames, that are of the same size as the page.
  • Generally, the range of these frames and pages are from 512 bytes to 64 kb.

Different algorithms are used to perform page replacement. We will learn about some of the most common algorithms in this lesson.

Let’s understand the basic terminologies that will be used in this article:

  • Page: It is a fixed-length contiguous block of virtual memory.
  • Logical Address/Virtual Address: It is an address generated by the CPU.
  • Logical Address Space: It is the set of all logical addresses that are generated by a program.
  • Physical Address: It is the address that is available on the memory unit.
  • Physical Address Space: It is the set of all physical addresses that correspond to the logical addresses.
  • Page Fault: When a running program accesses a memory page that is mapped into the virtual address space but is not loaded in physical memory, it is known as a page fault.

 

First in first out (FIFO) page replacement algorithm

This is the simplest page replacement algorithm. In this algorithm, the operating system keeps a track of all the pages present in the memory in a queue fashion. When a page needs to be replaced, the oldest page in the queue is selected and replaced with the new page.

For instance, let’s suppose we have a page reference string as (1, 3, 0, 3, 5, 6, 3) with three-page frames. Initially, all the slots are empty so the first three reference strings (1, 3, 0) will be allocated to the empty slots with 3 page-faults. Next in the reference string is (3) which is already in the memory, so no page fault. Then comes (5). Since there is no available memory space the newly arrived page will replace the oldest residing page in the memory, i.e., (1) with 1 page-fault. Likewise, now (6) will arrive and will replace (3) with another page fault. And then (3) will arrive and will replace (0) with another page fault.

Every time the OS has to replace a page, a page-fault is counted.

Total Page Fault: 6

 

Optimal page replacement algorithm

Optimal Page Replacement algorithm says that the newly arrived page will replace a page in memory which wouldn’t be used for the longest period of time in the future.

Let’s understand this through an example. Let’s consider a page reference string (7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2) with 4 page frames.

Initially all the memory slots will be empty so (7, 0, 1, 2) will be allocated to the memory with 4 page-faults. As (0) is already there, there’s no page fault. Next in the string is (3), it’ll replace (7) as it’s not used for the longest period of time in the future, with one page fault. Again, 0 is already there, so no page fault. 4 will replace 1 with one page fault. And for the rest of the string, there will be no page fault as all the arriving pages are already there in the memory.

Total Page Fault = 6

Now, as we understand this algorithm, we tend to realize that this algorithm can be implemented in practice as an operating system cannot know the future page requests in advance. So, this algorithm is just an instance of what can be the optimal solution.

Least Recently Used (LRU) page replacement algorithm  

In this algorithm, the new page is replaced with the existing page that has been used the least recently. In other words, the page which has not been referred for a long time will be replaced with the newly arrived page.

This algorithm is just opposite to the Optimal Page Replacement algorithm.

So, if we have a page reference string (7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2) with four page frames, then the page replacement will take place as shown below:

Steps:

  1. (7): It’s inserted into the empty frame, as the first four frames are empty initially. +1 page fault.
  2. (0): This page string also finds an empty frame. +1 page fault.
  3. (1): This page string also finds an empty frame. +1 page fault.
  4. (2): This page string also finds an empty frame. +1 page fault.
  5. (0): As this string is already available in the page frames, it hits. Page fault remains the same.
  6. (3): Page string (7) has been used the least recently, so it gets replaced by (3). +1 page fault.
  7. (0): We get another hit as (0) is already in the frame. Page fault remains the same.
  8. (4): Page string (1) has been used the least recently, so it gets replaced by (4). +1 page fault.
  9. (2): This string is already in the frame.
  10. (3): This string is already in the frame.
  11. (0): This string is already in the frame.
  12. (3): This string is already in the frame.
  13. (2): This string is already in the frame.
  14. (3): This string is already in the frame.

Total Page Fault = 6

 

Not Recently Used (NRU) page replacement algorithm

In most computers with virtual memory, each page is associated with two status bits.

  • Referenced (R) bit is set whenever a page is read or written.
  • Modified (M) bit is set whenever the page is written.

In this algorithm, the operating system uses (R) and (M) bits to distinguish between pages. When a process starts, both page bits (R) and (M) are set to 0 by the operating system. If a page has been referenced, a page fault will occur and the (R) bit is set by the OS.

If a page is modified, then another page fault will occur and the OS will set the (M) bit.

Periodically the R bit is cleared by a clock interrupt. When a page fault occurs and there are no empty frames, the operating system inspects all the pages and divides them into 4 categories:

  • Class 0: not referenced, not modified
  • Class 1: not referenced, modified
  • Class 2: referenced, not modified
  • Class 3: referenced, modified

Looking at it like this, it feels class 1 is not possible. How can a page be modified but not referenced? Well, this happens when a clock interrupt resets the R bit to 0. The M bit doesn’t reset by the clock interrupt.

Based on the R and M bits, all the pages are categorized into the above 4 categories. The algorithm removes a random page with the lowest numbered non-empty class. If class 0 is empty, then a random page from class 1 is replaced and so on.

Second Chance page replacement algorithm

Second chance page replacement algorithm is a modified version of FIFO algorithm.

As the name suggests, the pages are given a second chance. The pages that arrived are stored in a linked list. If the oldest page in the linked list gets referenced, a page fault occurs and the R bit is cleared. The oldest page is now cleared from memory and is pushed to the latest clock interval.

When the time arrives to replace a page, the operating system replaces the oldest page that has also not been referenced.

In a rare situation where all the pages have been referenced, the algorithm goes back to its own roots and performs the simple FIFO algorithm.

Clock page replacement algorithm

The Second Chance Page Replacement algorithm has a drawback that it constantly moves the pages around on its list. To solve this, the OS keeps all the page frames on a circular list in the form of a clock. The hand of the clock (pointer) points to the oldest page.

When a page fault takes place, the page being pointed is inspected. If its R bit is 0, the page is evicted and the new page is inserted into its place. The pointer moves one position ahead.

On the other hand, if R is 1, then it’s cleared and the hand is advanced to the next page until a page with a false R bit is found. Since this works the way a clock works, it is called Clock Page Replacement Algorithm.

Working Set page replacement algorithm

In its truest form, the processes are started up with no pages in the memory. One by one, as the pages are required, they are loaded into the memory. This strategy is called demand paging as the pages are loaded when needed and not in advance.

The set of pages required to execute a process is called its working set. Typically, if the entire working set is in memory then the process is executed without any page faults until it moves into a new execution phase.

If the working set is not entirely in the memory, many page faults may occur and this situation is known as thrashing. This is really bad for the system as it wastes considerable resources.

To prevent thrashing, many paging systems keep track of each process’ working set and try to load the working set in memory before the process is run. This approach is called the working set model. This model emphasizes on greatly reducing the page faults. This method is also called as prepaging.

Related courses for this will be up soon!

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Top