Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux
From: Chris Wright
Date: Thu Apr 02 2009 - 15:42:22 EST
* Jesper Juhl (jj@xxxxxxxxxxxxx) 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
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