How Second Chance page replacement algorithm differs from FIFO page replacement policy? Discuss the concept of Belady’s anomaly with suitable example.

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How Second Chance page replacement algorithm differs from FIFO page replacement policy? Discuss the concept of Belady&#8217;s anomaly with suitable example.

Suresh Chand · Accepted Answer

The Second Chance Page Replacement algorithm is just the modified version of the First In First Out (FIFO) page replacement policy. In FIFO, when a page needs to be replaced, the oldest i.e. first page in the front of the queue is replaced. The 2nd chance algorithm is based on the FIFO algorithm and it even degenerates to FIFI in its worst-case scenario. The main difference between them is that, in the 2nd chance algorithm, a FIFO replacement is implemented along with a reference bit. If the reference bit is o, then we proceed to replace the page, but if the reference bit is 1, then we give the page the 2nd chance. then its reference bit is cleared. Belady's analogy is the phenomenon in which increasing the number of frames (i.e. memory size) results in an increase in the number of faults for certain memory occurs patterns. This phenomenon is commonly experienced when using the FIFO page replacement algorithm. Let's take an example of Belady's anomaly in the page replacement. Let's take a reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Initially, let's take Memory size = 3 Let's find the total page faults when using the FIFO page replacement algorithm. f3 3 3 3 2 2 2 2 2 4 4 f2 2 2 2 1 1 1 1 1 3 3 3 f1 1 1 1 4 4 4 5 5 5 5 5 5 * * * * * * H H * * * H Total page faults = 9 Now, let's increase the memory size to 4 and check on the same reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 f4 4 4 4 4 4 4 3 3 3 f3 3 3 3 3 3 3 2 2 2 2 f2 2 2 2 2 2 2 1 1 1 1 5 f1 1 1 1 1 1 1 5 5 5 5 4 4 * * * * H H * * * * * * Total page fault = 10 Here, we can see, an increasing memory size to 4 from 3. The total page fault increased to 10 from 9 which is called the Belady's anomaly.

How Second Chance page replacement algorithm differs from FIFO page replacement policy? Discuss the concept of Belady’s anomaly with suitable example.

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How Second Chance page replacement algorithm differs from FIFO page replacement policy? Discuss the concept of Belady’s anomaly with suitable example.

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