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 :-) -- Jesper Juhl <jj@xxxxxxxxxxxxx> http://www.chaosbits.net/ Plain text mails only, please http://www.expita.com/nomime.html Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
