Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

Chris Wright
2009-04-02T19:42:21Z

* Jesper Juhl (jj@chaosbits.net) wrote:
> Do you rely only on the checksum or do you actually compare pages to check
> they are 100% identical before sharing?

Checksum has absolutely nothing to do w/ finding if two pages match.
It's only used as a heuristic to suggest whether a single page has
changed. If that page is changing we won't bother trying to find a
match for it. Here's an example of the life of a page w.r.t checksum.

1. checksum = uninitialized
2. first time page is found, checksum it (checksum = A).
if checksum has changed (uninitialize != A) don't go any further w/ that page
3. next time page is found, checksum it (checksum = B).
if checksum has change (A != B) don't go any further w/ that page
4. next time page is found, checksum it (checksum = B).
if checksum has changed (B == B)...it hasn't, continue processing the
page

later if a match is found in the tree (which is sorted by _contents_,
i.e. memcmp) we'll attempt to merge the pages which at it's very core
does:

if (pages_identical(oldpage, newpage))
ret = replace_page(vma, oldpage, newpage, orig_pte, newprot);

pages_identical? you guessed it...just does:

r = memcmp(addr1, addr2, PAGE_SIZE)

thanks,
-chris