Optimal Page Replacement in OS

April 22, 2025

About Optimal Page Replacement in Operating System

Optimal Page Replacement in OS is a memory management technique used by the operating system. The system divides memory into small blocks called pages. When the CPU needs a page that isn’t in the main memory (RAM), it brings it from the secondary memory: This process is called page swapping.

Since RAM has limited space, some pages need to be removed to make room for new ones. The Optimal Page Replacement Algorithm chooses to remove the page that won’t be needed for the longest time in the future. Though it’s hard to use in real systems, it’s considered the best possible method for comparison with other page replacement algorithms.

Optimal page replacement in OS

What is Optimal Page Replacement in OS ?

  • Managing memory efficiently is one of the most important responsibilities of an Operating System (OS).
  • When multiple programs run at the same time, the OS must decide which data to keep in memory (RAM) and which to remove.
  • This decision is made using techniques known as Page Replacement Algorithms.
  • One of the most ideal but theoretical methods is the Optimal Page Replacement Algorithm.

What is Paging?

  • The computer divides memory into small fixed-size blocks called pages. These pages can be stored in either primary memory (RAM) or secondary memory (like a hard disk).
  • When a page needed by the CPU is not in RAM, it’s fetched from the secondary memory. This process is called a page fault. To bring in the new page, an old one must be removed. This is where Page Replacement Algorithms come in.

What is Optimal Page Replacement?

Optimal Page Replacement Algorithm replaces the page that will not be used for the longest time in the future. It’s called optimal because it gives the least number of page faults, which means the best performance.

How It Works ?

Let’s say we have 3 memory slots and the following page requests:

Page Reference String:
7, 0, 1, 2, 0, 3, 0, 4, 2, 3

The algorithm looks ahead in the future.

It always removes the page that won’t be needed for the longest time.

Note:

In reality, we can’t look into the future, which is why this algorithm is theoretical. But it’s used to compare how good other algorithms are.

Why Use Optimal Page Replacement?

  • Best case performance: Minimum possible page faults.
  • Benchmarking tool: Used to measure how close other algorithms come to the ideal case.
  • Helps design better real world algorithms like LRU (Least Recently Used).

Limitations

  • Not practical for real systems, because we can’t predict future requests.
  • Only useful for simulation, academic learning, or comparisons.

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Basic Idea of Optimal Page Replacement in OS

  1. To fulfill the demand of the most frequently used pages, the operating systems keep these pages in the primary memory. Pages that not have been used since a long time are replaced with the most frequently used pages under the method called as Optimal Page Replacement technique.
  2. The approach can be best explained by the below reference string and the frame size.
  3. The diagrammatic representation above states the order in which the stack will be filled as per the demands in the primary memory. In the case of a page fault, the least recently used page is replaced with the pager which in immediate demand.

Algorithm for Optimal Page Replacement

Step 1: Push the first page in the stack as per the memory demand.

Step 2: Push the second page as per the memory demand.

Step 3: Push the third page until the memory is full.

Step 4: As the queue is full, the page which is least recently used is popped.

Step 5: repeat step 4 until the page demand continues and until the processing is over.

Step 6: Terminate the program.

Explore More About Operating System:

  1. Memory Management Introduction
  2. Partition Allocation Method
    1. First Fit in OS
      1. Program in C | C++ | Java | Python
    2. Best fit
      1. Program in C | C++ | Java | Python
    3. Worst fit
      1. Program in C | C++ | Java | Python
    4. Next fit
      1. Program in C | C++ | Java | Python
    5. First fit best fit worst fit in OS Example
  3. Buddy- System Allocator
  4. Paging
    1. Types of Paging
  5. Fragmentation
    1. internal fragmentation
    2. external fragmentation
    3. internal fragmentation vs external fragmentation
  6. Mapping Virtual address to Physical Address.
  7. Virtual Memory 
  8. Demand Paging
  9. Implementation of Demand paging and page fault
  10. Segmentation
  11. Page Replacement Algorithms
    1. LRU
    2. FIFO
    3. Optimal Page Replacement algorithm
  12. Thrashing
  13. Belady’s Anomaly
  14. Static vs Dynamic Loading
  15. Static vs Dynamic Linking
  16. Swapping
  17. Translational Look Aside Buffer
  18. Process Address Space
  19. Difference between Segmentation and Paging
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