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=-0.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 46789C04EB8 for ; Wed, 12 Dec 2018 10:17:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0A5E920839 for ; Wed, 12 Dec 2018 10:17:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0A5E920839 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726964AbeLLKRG (ORCPT ); Wed, 12 Dec 2018 05:17:06 -0500 Received: from mx2.suse.de ([195.135.220.15]:38482 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726680AbeLLKRG (ORCPT ); Wed, 12 Dec 2018 05:17:06 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 51CC3AC85; Wed, 12 Dec 2018 10:17:03 +0000 (UTC) Subject: Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression To: David Rientjes , Andrea Arcangeli Cc: Linus Torvalds , mgorman@techsingularity.net, Michal Hocko , ying.huang@intel.com, s.priebe@profihost.ag, Linux List Kernel Mailing , alex.williamson@redhat.com, lkp@01.org, kirill@shutemov.name, Andrew Morton , zi.yan@cs.rutgers.edu References: <64a4aec6-3275-a716-8345-f021f6186d9b@suse.cz> <20181204104558.GV23260@techsingularity.net> <20181205204034.GB11899@redhat.com> <20181205233632.GE11899@redhat.com> <20181210044916.GC24097@redhat.com> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/12/18 1:37 AM, David Rientjes wrote: > > Regarding the role of direct reclaim in the allocator, I think we need > work on the feedback from compaction to determine whether it's worthwhile. > That's difficult because of the point I continue to bring up: > isolate_freepages() is not necessarily always able to access this freed > memory. That's one of the *many* reasons why having free base pages doesn't guarantee compaction success. We can and will improve on that. But I don't think it would be e.g. practical to check the pfns of free pages wrt compaction scanner positions and decide based on that. Also when you invoke reclaim, you can't tell in advance those pfns, so I'm not sure how the better feedback from compaction to reclaim for this specific aspect would be supposed to work? > But for cases where we get COMPACT_SKIPPED because the order-0 > watermarks are failing, reclaim *is* likely to have an impact in the > success of compaction, Yes that's the heuristic we rely on. > otherwise we fail and defer because it wasn't able > to make a hugepage available. Note that THP fault compaction doesn't actually defer itself, which I think is a weakness of the current implementation and hope that patch 3 in my series from yesterday [1] can address that. Because defering is the general feedback mechanism that we have for suppressing compaction (and thus associated reclaim) in cases it fails for any reason, not just the one you mention. Instead of inspecting failure conditions in detail, which would be costly, it's a simple statistical approach. And when compaction is improved to fail less, defering automatically also happens less. > [ If we run compaction regardless of the order-0 watermark check and find > a pageblock where we can likely free a hugepage because it is > fragmented movable pages, this is a pretty good indication that reclaim > is worthwhile iff the reclaimed memory is beyond the migration scanner. ] I don't think that would be a good direction to pursue, to let scanning happen even without having the free pages. Also as I've mentioned above, LRU-based reclaim cannot satisfy your 'iff' condition, unless it inspected the pfn's it freed, and continued reclaiming until enough of those beyond migration scanner were freed. Instead IMHO we should look again into replacing the free scanner with direct allocation from freelists. [1] https://lkml.kernel.org/r/20181211142941.20500-1-vbabka@suse.cz