Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764797AbZDBTmV (ORCPT ); Thu, 2 Apr 2009 15:42:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759087AbZDBTmI (ORCPT ); Thu, 2 Apr 2009 15:42:08 -0400 Received: from mx2.redhat.com ([66.187.237.31]:52031 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756089AbZDBTmF (ORCPT ); Thu, 2 Apr 2009 15:42:05 -0400 Date: Thu, 2 Apr 2009 12:39:20 -0700 From: Chris Wright To: Jesper Juhl Cc: Izik Eidus , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org, avi@redhat.com, aarcange@redhat.com, chrisw@redhat.com, riel@redhat.com, jeremy@goop.org, mtosatti@redhat.com, hugh@veritas.com, corbet@lwn.net, yaniv@redhat.com, dmonakhov@openvz.org Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux Message-ID: <20090402193920.GF10392@x200.localdomain> References: <1238457560-7613-1-git-send-email-ieidus@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1463 Lines: 36 * 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/