Re: Thinking aloud.

On Sat, Oct 20, 2012 at 08:20:28AM +1100, Nigel Cunningham wrote:
> 
> My initial thought is to use SHA1 checksums to determine what pages
> have changed, keeping those checksums (and the pfns to which they
> refer) in memory or on disk between hibernation cycles.

Hi Nigel,

One thought --- if you are on a 64-bit machine, where we don't have a
shortage of page flags, you could set a new flag on all pages that
were saved during the last hibernate cycle, and on the restore, mark
all pages in the page table read/only.  As the pages get modified, on
the page fault, clear the flag.  This should be a lot faster than
burning the memory bandwidth and computational resources needed to
calculate SHA1 checksums on all memory pages which are in use.

If you can't do this approach, something else that would probably
worth doing is using a two-pass checksum; first use a very fast
checksum, such as Adler-32 or Fletcher-32, which is memory bandwidth
limited.  If the fast-but-not-entirely checksum doesn't match, there's
no point trying to doing the (much slower) SHA1 checksum over the
page; you'll know that the page has changed.  If the fast checksum
matches, then you can do a second pass with SHA1 to make sure you can
really skip doing the disk write.

Regards,

						- Ted