Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753557AbcCBOJi (ORCPT ); Wed, 2 Mar 2016 09:09:38 -0500 Received: from mx2.suse.de ([195.135.220.15]:51956 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751671AbcCBOJh (ORCPT ); Wed, 2 Mar 2016 09:09:37 -0500 Subject: Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking up kcompactd To: Joonsoo Kim References: <1454938691-2197-1-git-send-email-vbabka@suse.cz> <1454938691-2197-5-git-send-email-vbabka@suse.cz> <20160302063322.GB32695@js1304-P5Q-DELUXE> <56D6BACB.7060005@suse.cz> Cc: Joonsoo Kim , Linux Memory Management List , Andrew Morton , LKML , Andrea Arcangeli , "Kirill A. Shutemov" , Rik van Riel , Mel Gorman , David Rientjes , Michal Hocko , Johannes Weiner From: Vlastimil Babka Message-ID: <56D6F41D.9080107@suse.cz> Date: Wed, 2 Mar 2016 15:09:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2353 Lines: 62 On 03/02/2016 02:57 PM, Joonsoo Kim wrote: > 2016-03-02 19:04 GMT+09:00 Vlastimil Babka : >> On 03/02/2016 07:33 AM, Joonsoo Kim wrote: >>> >>> >>> Why you did the test with THP? THP interferes result of main test so >>> it would be better not to enable it. >> >> >> Hmm I've always left it enabled. It makes for a more realistic interference >> and would also show unintended regressions in that closely related area. > > But, it makes review hard because complex analysis is needed to > understand the result. > > Following is the example. > > "The compaction stalls > (direct compaction) in the interfering kernel builds (probably THP's) also > decreased somewhat to kcompactd activity, yet THP alloc successes improved a > bit." > > So, why do we need this comment to understand effect of this patch? If you did > a test without THP, it would not be necessary. I see. Next time I'll do a run with THP disabled. >>> And, this patch increased compaction activity (10 times for migrate >>> scanned) >>> may be due to resetting skip block information. >> >> >> Note that kswapd compaction activity was completely non-existent for reasons >> outlined in the changelog. >>> Isn't is better to disable it >>> for this patch to work as similar as possible that kswapd does and >>> re-enable it >>> on next patch? If something goes bad, it can simply be reverted. >>> >>> Look like it is even not mentioned in the description. >> >> >> Yeah skip block information is discussed in the next patch, which mentions >> that it's being reset and why. I think it makes more sense, as when kswapd > > Yes, I know. > What I'd like to say here is that you need to care current_is_kswapd() in > this patch. This patch unintentionally change the back ground compaction thread > behaviour to restart compaction by every 64 trials because calling > curret_is_kswapd() > by kcompactd would return false and is treated as direct reclaim. Oh, you mean this path to reset the skip bits. I see. But if skip bits are already reset by kswapd when waking kcompactd, then effect of another (rare) reset in kcompactd itself will be minimal? > Result of patch 4 > and patch 5 would be same. It's certainly possible to fold patch 5 into 4. I posted them separately mainly to make review more feasible. But the differences in results are already quite small.