From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753399AbdC2QGq (ORCPT ); Wed, 29 Mar 2017 12:06:46 -0400 Received: from mx2.suse.de ([195.135.220.15]:47705 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752716AbdC2QGp (ORCPT ); Wed, 29 Mar 2017 12:06:45 -0400 Subject: Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype To: Joonsoo Kim , Johannes Weiner References: <20170307131545.28577-1-vbabka@suse.cz> <20170307131545.28577-8-vbabka@suse.cz> <20170316021403.GC14063@js1304-P5Q-DELUXE> Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mel Gorman , David Rientjes , kernel-team@fb.com, kernel-team@lge.com From: Vlastimil Babka Message-ID: Date: Wed, 29 Mar 2017 18:06:41 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170316021403.GC14063@js1304-P5Q-DELUXE> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/16/2017 03:14 AM, Joonsoo Kim wrote: > On Tue, Mar 07, 2017 at 02:15:44PM +0100, Vlastimil Babka wrote: >> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE >> pageblocks. This is a heuristic intended to reduce latency, based on the >> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages. >> >> However, with the exception of THP's, most high-order allocations are not >> movable. Should the async compaction succeed, this increases the chance that >> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the >> long-term fragmentation worse. > > I agree with this idea but have some concerns on this change. > > *ASYNC* compaction is designed for reducing latency and this change > doesn't fit it. If everything works fine, there is a few movable pages > in non-MOVABLE pageblocks as you noted above. Moreover, there is quite > less the number of non-MOVABLE pageblock than MOVABLE one so finding > non-MOVABLE pageblock takes long time. These two factors will increase > the latency of *ASYNC* compaction. Right. I lately started to doubt the whole idea of async compaction (for non-movable allocations). Seems it's one of the compaction heuristics tuned towards the THP usecase. But for non-movable allocations, we just can't have both the low latency and long-term fragmentation avoidance. I see now even my own skip_on_failure mode in isolate_migratepages_block() as a mistake for non-movable allocations. Ideally I'd like to make async compaction redundant by kcompactd, and direct compaction would mean a serious situation which should warrant sync compaction. Meanwhile I see several options to modify this patch - async compaction for non-movable allocations will stop doing the skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will make sure that also this kind of compaction finishes the whole pageblock - non-movable allocations will skip async compaction completely and go for sync compaction immediately Both options mean we won't clean the unmovable/reclaimable pageblocks as aggressively, but perhaps the tradeoff won't be bad. What do you think? Johannes, would you be able/willing to test such modification? Thanks > And, there is a concern in implementaion side. With this change, there > is much possibilty that compaction scanner's met by ASYNC compaction. > It resets the scanner position and SYNC compaction would start the > scan at the beginning of the zone every time. It would make cached > position useless and inefficient. > > Thanks. > From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id 833F46B0390 for ; Wed, 29 Mar 2017 12:06:45 -0400 (EDT) Received: by mail-wr0-f197.google.com with SMTP id g7so4029456wrd.16 for ; Wed, 29 Mar 2017 09:06:45 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 17si7727483wmj.152.2017.03.29.09.06.43 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 29 Mar 2017 09:06:43 -0700 (PDT) Subject: Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype References: <20170307131545.28577-1-vbabka@suse.cz> <20170307131545.28577-8-vbabka@suse.cz> <20170316021403.GC14063@js1304-P5Q-DELUXE> From: Vlastimil Babka Message-ID: Date: Wed, 29 Mar 2017 18:06:41 +0200 MIME-Version: 1.0 In-Reply-To: <20170316021403.GC14063@js1304-P5Q-DELUXE> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Joonsoo Kim , Johannes Weiner Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mel Gorman , David Rientjes , kernel-team@fb.com, kernel-team@lge.com On 03/16/2017 03:14 AM, Joonsoo Kim wrote: > On Tue, Mar 07, 2017 at 02:15:44PM +0100, Vlastimil Babka wrote: >> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE >> pageblocks. This is a heuristic intended to reduce latency, based on the >> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages. >> >> However, with the exception of THP's, most high-order allocations are not >> movable. Should the async compaction succeed, this increases the chance that >> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the >> long-term fragmentation worse. > > I agree with this idea but have some concerns on this change. > > *ASYNC* compaction is designed for reducing latency and this change > doesn't fit it. If everything works fine, there is a few movable pages > in non-MOVABLE pageblocks as you noted above. Moreover, there is quite > less the number of non-MOVABLE pageblock than MOVABLE one so finding > non-MOVABLE pageblock takes long time. These two factors will increase > the latency of *ASYNC* compaction. Right. I lately started to doubt the whole idea of async compaction (for non-movable allocations). Seems it's one of the compaction heuristics tuned towards the THP usecase. But for non-movable allocations, we just can't have both the low latency and long-term fragmentation avoidance. I see now even my own skip_on_failure mode in isolate_migratepages_block() as a mistake for non-movable allocations. Ideally I'd like to make async compaction redundant by kcompactd, and direct compaction would mean a serious situation which should warrant sync compaction. Meanwhile I see several options to modify this patch - async compaction for non-movable allocations will stop doing the skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will make sure that also this kind of compaction finishes the whole pageblock - non-movable allocations will skip async compaction completely and go for sync compaction immediately Both options mean we won't clean the unmovable/reclaimable pageblocks as aggressively, but perhaps the tradeoff won't be bad. What do you think? Johannes, would you be able/willing to test such modification? Thanks > And, there is a concern in implementaion side. With this change, there > is much possibilty that compaction scanner's met by ASYNC compaction. > It resets the scanner position and SYNC compaction would start the > scan at the beginning of the zone every time. It would make cached > position useless and inefficient. > > Thanks. > -- 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