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 D89CB900194 for ; Wed, 22 Jun 2011 20:52:06 -0400 (EDT) Received: by vxg38 with SMTP id 38so1428276vxg.14 for ; Wed, 22 Jun 2011 17:52:03 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <4E027A96.3040905@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> <4E027A96.3040905@redhat.com> Date: Thu, 23 Jun 2011 08:52:03 +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 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel Cc: Izik Eidus , Avi Kivity , Andrew Morton , Andrea Arcangeli , Hugh Dickins , Chris Wright , linux-mm , Johannes Weiner , linux-kernel , kvm On Thu, Jun 23, 2011 at 7:28 AM, Rik van Riel wrote: > On 06/22/2011 07:13 PM, Nai Xia wrote: >> >> On Wed, Jun 22, 2011 at 11:39 PM, Rik van Riel =A0wrote= : >>> >>> On 06/22/2011 07:19 AM, Izik Eidus wrote: >>> >>>> So what we say here is: it is better to have little junk in the unstab= le >>>> tree that get flushed eventualy anyway, instead of make the guest >>>> slower.... >>>> this race is something that does not reflect accurate of ksm anyway du= e >>>> to the full memcmp that we will eventualy perform... >>> >>> With 2MB pages, I am not convinced they will get "flushed eventually", >>> because there is a good chance at least one of the 4kB pages inside >>> a 2MB page is in active use at all times. >>> >>> I worry that the proposed changes may end up effectively preventing >>> KSM from scanning inside 2MB pages, when even one 4kB page inside >>> is in active use. =A0This could mean increased swapping on systems >>> that run low on memory, which can be a much larger performance penalty >>> than ksmd CPU use. >>> >>> We need to scan inside 2MB pages when memory runs low, regardless >>> of the accessed or dirty bits. >> >> 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 >> trades-offs always have the possibilities to annoy =A0someone. =A0Just l= ike >> page-bit-relying LRU approximations none of them is perfect too. But I >> think >> it can benefit some people. So maybe we could just provide a generic >> balanced >> solution but provide fine tuning interfaces to make sure tha when it >> really gets >> in the way of someone, he has a way to walk around. >> Do you agree on my argument? :-) > > That's not an argument. > > That is a "if I wave my hands vigorously enough, maybe people > will let my patch in without thinking about what I wrote" > style argument. Oh, NO, this is not what I meant. Really sorry if I made myself look so evil... I actually mean: "Skip or not, we agree on a point that will not harm most people, and provide another interface to let someon who _really_ want to take another way." I am by no means pushing the idea of "skipping" huge pages. I am just not sure about it and want to get a precise idea from you. And now I get it. > > I believe your optimization makes sense for 4kB pages, but > is going to be counter-productive for 2MB pages. > > Your approach of "make ksmd skip over more pages, so it uses > less CPU" is likely to reduce the effectiveness of ksm by not > sharing some pages. > > For 4kB pages that is fine, because you'll get around to them > eventually. > > However, the internal use of a 2MB page is likely to be quite > different. =A0Chances are most 2MB pages will have actively used, > barely used and free pages inside. > > You absolutely want ksm to get at the barely used and free > sub-pages. =A0Having just one actively used 4kB sub-page prevent > ksm from merging any of the other 511 sub-pages is a problem. No, no, I was just not sure about it. I meant we cannot satisfy all people but I was not sure which one is good for most of them. Sorry, again, if I didn't make it clear. Nai > > -- > All rights reversed > -- 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