GATE | GATE-CS-2017 (Set 1) | Question 63

Recall that Belady’s anomaly is that the pages-fault rate may increase as the number of allocated frames increases. Now consider the following statements:

S1: Random page replacement algorithm (where
    a page chosen at random is replaced) 
    suffers from Belady’s anomaly.

S2: LRU page replacement algorithm suffers
    from Belady’s anomaly .

Which of the following is CORRECT?
(A) S1 is true, S2 is true
(B) S1 is true, S2 is false
(C) S1 is false , S2 is true
(D) S1 is false, S2 is false


Answer: (B)

Explanation: Belady’s anomaly proves that it is possible to have more page faults when increasing the number of page frames while using the First in First Out (FIFO) page replacement algorithm. For example, if we consider reference string 3 2 1 0 3 2 4 3 2 1 0 4 and 3 slots, we get 9 total page faults, but if we increase slots to 4, we get 10 page faults.

S1: Random page replacement algorithm (where a page chosen at random is replaced) suffers from Belady’s anomaly.
-> Random page replacement algorithm can be any including FIFO so its true

S2: LRU page replacement algorithm suffers from Belady’s anomaly .
-> LRU does’nt suffer from Belady’s anomaly .

Therefore, option B is correct

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