Re: [PATCH 3/3] mm: Swap checksum

From: Cesar Eduardo Barros
Date: Sun May 23 2010 - 14:58:22 EST

Em 23-05-2010 12:19, Avi Kivity escreveu:
On 64-bit, we may be able to store the checksum in the pte, if the swap
device is small enough.

Which pte? Correct me if I am wrong, but I do not think all pages written to the swap have exactly one pte pointing to them. And I have not looked at the shmem.c code yet, but does it even use ptes?

It might be possible (find all ptes and write the 32-bit checksum to them, do something else for shmem, have two different code paths for small/large swapfiles), but I do not know if the memory savings are worth the extra complexity (especially the need for two separate code paths).

If we take the trouble to touch the page, we may as well compare it
against zero, and if so drop it instead of swapping it out.

The problem with this is that the page is touched deep inside the crc32c code, which might even be using hardware instructions (crc32c-intel). So we would need to read it two times to compare against zero.

One possibility could be to compare the full page against zero only if its crc is a specific value (the crc32c of a page full of zeros). This would not be too slow (we would be wasting time only when we have a very high probability of saving much more time), and not need to touch the crc32c code at all. I would only have to look at how this messes up the state tracking (i.e. how to make it track the fact that, instead of getting written out, this is now a zeroed page). Other than that, it seems a good idea.

Cesar Eduardo Barros
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