From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta7.messagelabs.com (mail6.bemta7.messagelabs.com [216.82.255.55]) by kanga.kvack.org (Postfix) with ESMTP id 1B71A900194 for ; Wed, 22 Jun 2011 21:30:35 -0400 (EDT) Received: by vxg38 with SMTP id 38so1444485vxg.14 for ; Wed, 22 Jun 2011 18:30:33 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20110622232532.GA20843@redhat.com> References: <201106212055.25400.nai.xia@gmail.com> <201106212132.39311.nai.xia@gmail.com> <4E01C752.10405@redhat.com> <4E01CC77.10607@ravellosystems.com> <4E01CDAD.3070202@redhat.com> <4E01CFD2.6000404@ravellosystems.com> <4E020CBC.7070604@redhat.com> <20110622232532.GA20843@redhat.com> Date: Thu, 23 Jun 2011 09:30:33 +0800 Message-ID: Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking From: Nai Xia Content-Type: text/plain; charset=ISO-8859-1 Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: Rik van Riel , Izik Eidus , Avi Kivity , Andrew Morton , Hugh Dickins , Chris Wright , linux-mm , Johannes Weiner , linux-kernel , kvm On Thu, Jun 23, 2011 at 7:25 AM, Andrea Arcangeli wrote: > On Thu, Jun 23, 2011 at 07:13:54AM +0800, Nai Xia wrote: >> I agree on this point. Dirty bit , young bit, is by no means accurate. Even >> on 4kB pages, there is always a chance that the pte are dirty but the contents >> are actually the same. Yeah, the whole optimization contains trade-offs and > > Just a side note: the fact the dirty bit would be set even when the > data is the same is actually a pros, not a cons. If the content is the > same but the page was written to, it'd trigger a copy on write short > after merging the page rendering the whole exercise wasteful. The > cksum plays a double role, it both "stabilizes" the unstable tree, so > there's less chance of bad lookups, but it also avoids us to merge > stuff that is written to frequently triggering copy on writes, and the > dirty bit would also catch overwrites with the same data, something > the cksum can't do. Good point. I actually have myself another version of ksm(off topic, but if you want to take a glance: http://code.google.com/p/uksm/ :-) ) that did do statistics of the ratio of the pages in a VMA that really got COWed. due to KSM merging on each scan round basis. It's complicated to deduce a precise information only from the dirty and cksum. Thanks, Nai > -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org