CODEWITHSUNDEEP
algorithmmemorymemory-managementoperating-system
Ski Mask
Ski Mask

Reputation: 353

Practical example of the Optimal Page Replacement Algorithm, with 4 Tiles

I'm doing some theoretical examples with different page replacement algorithms, in order to get a better understanding for when I actually write the code. I'm kind of confused about this example.

Given below is a physical memory with 4 tiles (4 sections?). The following pages are visited one after the other:

R = 1, 2, 3, 2, 4, 5, 3, 6, 1, 4, 2, 3, 1, 4

Run the optimal page replacement algorithm on R with 4 tiles.

I know that when a page needs to be swapped in, the operating system swaps out the page whose next use will occur farthest in the future. In practice I'll have:

Time    1 2 3 4 5 6 7 8 9 10 11 12 13 14
Page    1 2 3 2 4 5 3 6 1 4  2  3  1  4
Tile 1  1 1 1
Tile 2    2 2
Tile 3      3
Tile 4

I'm not sure what happens at time 4 because we get page 2, but thats already present in the memory. Normally, if it was another number like 6, then it would go in Tile 4 but I'm lost in this case.

Upvotes: 0

Views: 329

Answers (1)

Phenomenal One
Phenomenal One

Reputation: 2587

At time t=4, page 2 is already present, so there is no need to do anything. You can just skip it and move to the next time interval.

If there was a another number like 6, if there is a free slot available, you move it there, or else find the page that won't be used for the longest duration in the future and swap it.

Upvotes: 1

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