From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 59420C3F2C6 for ; Tue, 3 Mar 2020 13:02:51 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 23AA1208C3 for ; Tue, 3 Mar 2020 13:02:51 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 23AA1208C3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id C9AD96B0006; Tue, 3 Mar 2020 08:02:50 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C4B7C6B0007; Tue, 3 Mar 2020 08:02:50 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B89306B0008; Tue, 3 Mar 2020 08:02:50 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0227.hostedemail.com [216.40.44.227]) by kanga.kvack.org (Postfix) with ESMTP id 9FC586B0006 for ; Tue, 3 Mar 2020 08:02:50 -0500 (EST) Received: from smtpin10.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 54DC6202FA for ; Tue, 3 Mar 2020 13:02:50 +0000 (UTC) X-FDA: 76554065700.10.water75_52e19a972fa3c X-HE-Tag: water75_52e19a972fa3c X-Filterd-Recvd-Size: 4782 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf25.hostedemail.com (Postfix) with ESMTP for ; Tue, 3 Mar 2020 13:02:49 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 2F724AF68; Tue, 3 Mar 2020 13:02:47 +0000 (UTC) Date: Tue, 3 Mar 2020 13:02:41 +0000 From: Mel Gorman To: "Huang, Ying" Cc: David Hildenbrand , Michal Hocko , Johannes Weiner , Matthew Wilcox , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vlastimil Babka , Zi Yan , Peter Zijlstra , Dave Hansen , Minchan Kim , Hugh Dickins , Alexander Duyck Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating Message-ID: <20200303130241.GE3772@suse.de> References: <20200228033819.3857058-1-ying.huang@intel.com> <20200228034248.GE29971@bombadil.infradead.org> <87a7538977.fsf@yhuang-dev.intel.com> <871rqf850z.fsf@yhuang-dev.intel.com> <20200228094954.GB3772@suse.de> <87h7z76lwf.fsf@yhuang-dev.intel.com> <20200302151607.GC3772@suse.de> <87zhcy5hoj.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <87zhcy5hoj.fsf@yhuang-dev.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 03, 2020 at 09:51:56AM +0800, Huang, Ying wrote: > Mel Gorman writes: > > On Mon, Mar 02, 2020 at 07:23:12PM +0800, Huang, Ying wrote: > >> If some applications cannot tolerate the latency incurred by the memory > >> allocation and zeroing. Then we cannot discard instead of migrate > >> always. While in some situations, less memory pressure can help. So > >> it's better to let the administrator and the application choose the > >> right behavior in the specific situation? > >> > > > > Is there an application you have in mind that benefits from discarding > > MADV_FREE pages instead of migrating them? > > > > Allowing the administrator or application to tune this would be very > > problematic. An application would require an update to the system call > > to take advantage of it and then detect if the running kernel supports > > it. An administrator would have to detect that MADV_FREE pages are being > > prematurely discarded leading to a slowdown and that is hard to detect. > > It could be inferred from monitoring compaction stats and checking > > if compaction activity is correlated with higher minor faults in the > > target application. Proving the correlation would require using the perf > > software event PERF_COUNT_SW_PAGE_FAULTS_MIN and matching the addresses > > to MADV_FREE regions that were freed prematurely. That is not an obvious > > debugging step to take when an application detects latency spikes. > > > > Now, you could add a counter specifically for MADV_FREE pages freed for > > reasons other than memory pressure and hope the administrator knows about > > the counter and what it means. That type of knowledge could take a long > > time to spread so it's really very important that there is evidence of > > an application that suffers due to the current MADV_FREE and migration > > behaviour. > > OK. I understand that this patchset isn't a universal win, so we need > some way to justify it. I will try to find some application for that. > > Another thought, as proposed by David Hildenbrand, it's may be a > universal win to discard clean MADV_FREE pages when migrating if there are > already memory pressure on the target node. For example, if the free > memory on the target node is lower than high watermark? > That is an extremely specific corner case that is not likely to occur. NUMA balancing is not going to migrate a MADV_FREE page under these circumstances as a write cancels MADV_FREE is read attempt will probably fail to allocate a destination page in alloc_misplaced_dst_page so the data gets lost instead of remaining remote. sys_movepages is a possibility but the circumstances of an application delibertly trying to migrate to a loaded node is low. Compaction never migrates cross-node so the state of a remote node under pressure do not matter. Once again, there needs to be a reasonable use case to be able to meaningfully balance between the benefits and risks of changing the MADV_FREE semantics. -- Mel Gorman SUSE Labs