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The Second Chance Page Replacement Policy

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• In some books, the Second Chance replacement policy is called the Clock replacement policy...

• In the Second Chance page replacement policy, the candidate pages for removal are consider in a round robin matter, and a page that has been accessed between consecutive considerations will not be replaced.

  The page replaced is the one that - considered in a round robin matter - has not been accessed since its last consideration.

• Implementation:
  • Add a "second chance" bit to each memory frame.

  • Each time a memory frame is referenced, set the "second chance" bit to ONE (1) - this will give the frame a second chance...

  • A new page read into a memory frame has the second chance bit set to ZERO (0)

  • When you need to find a page for removal, look in a round robin manner in the memory frames:
    • If the second chance bit is ONE, reset its second chance bit (to ZERO) and continue.
    • If the second chance bit is ZERO, replace the page in that memory frame.

• The following figure shows the behavior of the program in paging using the Second Chance page replacement policy:

  

  • We can see notably that the bad replacement decisions made by FIFO is not present in Second chance !!!

  • There are a total of 9 page read operations to satisfy the total of 18 page requests - just as good as the more computationally expensive LRU method !!!

  • Note: in this example, the Second Chance method resulted in the same number of page faults as LRU. That is not true in general !