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

[Jesper Juhl]

On Thu, 2 Apr 2009, Chris Wright wrote:

> * 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)
> 

Thank you for that explanation, it set my mind at ease :-)


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Jesper Juhl <jj@xxxxxxxxxxxxx>             http://www.chaosbits.net/
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