From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id A8F1E6B003D for ; Thu, 2 Apr 2009 15:48:43 -0400 (EDT) Date: Thu, 2 Apr 2009 21:49:27 +0200 (CEST) From: Jesper Juhl Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux In-Reply-To: <20090402193920.GF10392@x200.localdomain> Message-ID: References: <1238457560-7613-1-git-send-email-ieidus@redhat.com> <20090402193920.GF10392@x200.localdomain> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Chris Wright Cc: Izik Eidus , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org, avi@redhat.com, aarcange@redhat.com, riel@redhat.com, jeremy@goop.org, mtosatti@redhat.com, hugh@veritas.com, corbet@lwn.net, yaniv@redhat.com, dmonakhov@openvz.org List-ID: On Thu, 2 Apr 2009, Chris Wright wrote: > * 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) > Thank you for that explanation, it set my mind at ease :-) -- Jesper Juhl 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org