On Mon, May 24, 2010 at 09:41:22AM +0300, Avi Kivity wrote: > On 05/23/2010 09:58 PM, Cesar Eduardo Barros wrote: > >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? > > All of them. > > >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? > > Well, the ptes need the swap address written into them, so they are > already found and updated somehow. All that's needed is to update > the value written to also include the checksum. > > >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). > > Certainly not at first, but later it may be worthwhile. > > > > >>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. > > The second read is very cheap since the page is already in cache. > Also, we fail early when any word is nonzero, so usually the compare > exits quickly. For a page being written back from pagecache to disk, or for a page being swapped out, the contents are likely cache cold and likely not to be used in future either. Therefore a crc routine for that would do well to minimise cache pollution. > >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). > > Instead of returning a swap pte to be written to the page tables, > return a zeroed pte. A pte_none pte, to be precise. I wonder, though. If we no longer trust block devices to give the correct data back, should we provide a meta block device to do error detection? No production filesystem on Linux has checksums (well, ext4 has a few). Of the ones that add checksumming, I'd say most will not do data checksumming (and for direct IO it is not done). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>