Page replacement algorithms are used in operating systems to manage memory allocation. These algorithms decide which page to remove from memory when a new page needs to be loaded. There are many page replacement algorithms available, including the FIFO (First-In-First-Out), LRU (Least Recently Used), and Optimal algorithms.


Belady's Anomaly is a phenomenon that occurs in some page replacement algorithms. It is named after its discoverer, L. Belady. In this anomaly, increasing the number of frames allocated to a process can actually cause an increase in the number of page faults. This contradicts the common intuition that more memory should reduce the number of page faults.


Some page replacement algorithms suffer from Belady's Anomaly, while others do not.

The algorithms that suffer from Belady's Anomaly are:

• FIFO (First-In-First-Out)


• Second Chance


• Random

The algorithms that do not suffer from Belady's Anomaly are:

• LRU (Least Recently Used)


• Optimal

The reason why some page replacement algorithms suffer from Belady's Anomaly is due to their replacement policies. These algorithms replace pages based on a fixed rule, regardless of the page's usage history. This can lead to situations where a page is removed from memory, only to be immediately needed again.

On the other hand, algorithms like LRU and Optimal take into account the usage history of pages. These algorithms are more intelligent in their replacement policies and are less likely to suffer from Belady's Anomaly.

In conclusion, it is important to choose the appropriate page replacement algorithm based on the system requirements and characteristics. Belady's Anomaly should be taken into account when selecting a page replacement algorithm, especially in systems with a large number of frames.