GATE CSE 2001 | Question: 1.21

Question

Consider a virtual memory system with FIFO page replacement policy. For an arbitrary page access pattern, increasing the number of page frames in main memory will

• A. always decrease the number of page faults
• B. always increase the number of page faults
• C. sometimes increase the number of page faults
• D. never affect the number of page faults

Comments

Consider the following reference string:

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 

Case-1: If the system has 3 frames, the given reference string on using FIFO page replacement algorithm yields a total of 9 page faults. The diagram below illustrates the pattern of the page faults occurring in the example.

Case-2: If the system has 4 frames, the given reference string on using FIFO page replacement algorithm yields a total of 10 page faults. The diagram below illustrates the pattern of the page faults occurring in the example.

It can be seen from the above example that on increasing the number of frames while using the FIFO page replacement algorithm, the number of page faults increased from 9 to 10.

 

Source : https://www.geeksforgeeks.org/beladys-anomaly-in-page-replacement-algorithms/

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Belady’s anomaly : More physical memory doesn’t always mean fewer faults.

Answer 1

Answer is (C).

Belady anomaly is the name given to the phenomenon in which increasing the number of page frames results in an increase in the number of page faults for certain memory access patterns. This phenomenon is commonly experienced when using the First in First Out (FIFO) page replacement algorithm

Answer 2

ans is C.

Belady's anomaly.