CS355 Sylabus

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The LRU Page Replacement Policy

- In the Least Recently Used (LRU) page replacement policy, the page that is used least recently will be replaced.

- Implementation:

  - Add a register to every page frame - contain the last time that the page in that frame was accessed

  - Use a "logical clock" that advance by 1 tick each time a memory reference is made.

  - Each time a page is referenced, update its register

- The following figure shows the behavior of the program in paging using the LRU page replacement policy:

  - We can see notably that the bad replacement decisions made by FIFO is not present in LRU.

  - There are a total of 9 page read operations to satisfy the total of 18 page requests - that is almost a 20% improvement over FIFO in such a short experiment

  - (I only want to make the point here that page replacement policy can affect the system performance. I do not want to get into the question of "how much better is LRU than FIFO").

- In fact, it has been shown empirically that LRU is the preferred page replacement policy

- Disadvantage of LRU replacement policy:

  - To identify the page to replace, you need to find the minimum time stamp value in all the registers...

    Lots of work...

- There is a more efficient scheme that approximate the behavior of LRU that runs more efficiently...

  That method is "second chance" - next.