From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx148.postini.com [74.125.245.148]) by kanga.kvack.org (Postfix) with SMTP id 7B2706B0068 for ; Wed, 5 Dec 2012 04:54:25 -0500 (EST) Message-ID: <50BF198D.3030509@parallels.com> Date: Wed, 05 Dec 2012 13:53:17 +0400 From: Pavel Emelyanov MIME-Version: 1.0 Subject: Re: [RFC PATCH 0/2] mm: Add ability to monitor task's memory changes References: <50B8F2F4.6000508@parallels.com> <20121203144310.7ccdbeb4.akpm@linux-foundation.org> <50BD86DE.6050700@parallels.com> <20121204152121.e5c33938.akpm@linux-foundation.org> <1354666628.6733.227.camel@calx> <20121204162411.700d4954.akpm@linux-foundation.org> <1354667937.6733.233.camel@calx> In-Reply-To: <1354667937.6733.233.camel@calx> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Matt Mackall , Andrew Morton Cc: Hugh Dickins , KAMEZAWA Hiroyuki , Michal Hocko , Mel Gorman , Johannes Weiner , Linux MM , Rik van Riel , Wu Fengguang On 12/05/2012 04:38 AM, Matt Mackall wrote: > On Tue, 2012-12-04 at 16:24 -0800, Andrew Morton wrote: >> On Tue, 04 Dec 2012 18:17:08 -0600 >> Matt Mackall wrote: >> >>> On Tue, 2012-12-04 at 15:21 -0800, Andrew Morton wrote: >>>> On Tue, 04 Dec 2012 09:15:10 +0400 >>>> Pavel Emelyanov wrote: >>>> >>>>> >>>>>> Two alternatives come to mind: >>>>>> >>>>>> 1) Use /proc/pid/pagemap (Documentation/vm/pagemap.txt) in some >>>>>> fashion to determine which pages have been touched. >>> >>> [momentarily coming out of kernel retirement for old man rant] >>> >>> This is a popular interface anti-pattern. >>> >>> You shouldn't use an interface that gives you huge amount of STATE to >>> detect small amounts of CHANGE via manual differentiation. >> >> I'm not sure that's what checkpoint-restart will be doing. If we want >> to determine "which pages have been touched since the last checkpoint >> ten minutes ago" then that set of touched pages *is* state. And it's >> not "small"! > > Yeah, there is definitely a middle-ground here between "I want > high-frequency updates" and "I want to see the whole picture". > The filesystem analogy is backups: we don't have any good way to say > "find me all files changed since yesterday" short of "find all files". > The closest thing is explicit snapshotting. For what is required for checkpoint-restore is -- we want to query the kernel for "what pages has been written to since moment X". But this "moment X" is a little bit more tricky than just "mark all pages r/o". Consider we're doing this periodically. So when defining the moment X for the 2nd time we should query the "changed" state and remap the respective page r/o atomically. Full snapshot is actually not required, since we don't need to keep the old copy of a page that is written to. Just a sign, that this page was modified is OK. Thanks, Pavel -- 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