Is it a good idea to consider the status of dirty bit in Page Replacements?

Question

In a LRU page replacement policy we just replace the least recently used page, without looking at its dirty bit.

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However If we also look at the dirty bit of page table entries along with the LRU bits, and replace only the pages that has not been modified(i.e. replace only pages that has dirty bit = 0) unless all the table entries have been modified ( I.e. all the entries in the page table  have their dirty bit = 1) then we can postpone the process of write back for some time.

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That is we are going to apply LRU only on the pages that have not been modified untill all the pages gets modified.

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Whether it will be advantageous to do so?

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What could be the disadvantages of this scheme?​

Answer 1

I suppose it will be advantageous. But having a separate list of modified pages would be tricky as any page can be modified at any time and that should be moved to the modified list. Also, some frames might be just read from but frequently and due to this policy such pages might get replaced by less actively accessed write pages. So, depends on the accesses this can benefit or not but I guess it is a good technique. There should be such works already done for page replacements.

Comments

Thank you sir, you are right, this policy will depend on the frequency of read & write instructions. but I don’t think its implementation would be trickier. Instead of just checking LRU bit from the page table before replacement, we just have to NAND(gate) the LRU bit & Dirty bit field & check the output, if it is 1 then replace otherwise don’t. right sir?