From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966133AbcIWMKD (ORCPT ); Fri, 23 Sep 2016 08:10:03 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:33687 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965835AbcIWMKC (ORCPT ); Fri, 23 Sep 2016 08:10:02 -0400 Date: Fri, 23 Sep 2016 14:09:59 +0200 From: Michal Hocko To: Vlastimil Babka Cc: Andrew Morton , Arkadiusz Miskiewicz , Ralf-Peter Rohbeck , Olaf Hering , linux-kernel@vger.kernel.org, Linus Torvalds , linux-mm@kvack.org, David Rientjes , Joonsoo Kim , Mel Gorman , Rik van Riel Subject: Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Message-ID: <20160923120958.GM4478@dhcp22.suse.cz> References: <20160906135258.18335-1-vbabka@suse.cz> <20160921171830.GH24210@dhcp22.suse.cz> <56f2c2ed-8a58-cf9c-dd00-c0d0e274607a@suse.cz> <20160923082627.GE4478@dhcp22.suse.cz> <9194950c-06b5-31d7-de17-1f8710dd5682@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9194950c-06b5-31d7-de17-1f8710dd5682@suse.cz> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 23-09-16 12:55:23, Vlastimil Babka wrote: > On 09/23/2016 10:26 AM, Michal Hocko wrote: > >> include/linux/compaction.h | 5 +++-- > >> mm/compaction.c | 44 +++++++++++++++++++++++--------------------- > >> mm/internal.h | 1 + > >> mm/vmscan.c | 6 ++++-- > >> 4 files changed, 31 insertions(+), 25 deletions(-) > > > > This is much more code churn than I expected. I was thiking about it > > some more and I am really wondering whether it actually make any sense > > to check the fragidx for !costly orders. Wouldn't it be much simpler to > > just put it out of the way for those regardless of the compaction > > priority. In other words does this check makes any measurable difference > > for !costly orders? > > I've did some stress tests and sampling > /sys/kernel/debug/extfrag/extfrag_index once per second. The lowest > value I've got for order-2 was 0.705. The default threshold is 0.5, so > this would still result in compaction considered as suitable. > > But it's sampling so I might not got to the interesting moments, most of > the time it was -1.000 which means the page should be just available. > Also we would be changing behavior for the user-controlled > vm.extfrag_threshold, so I'm not entirely sure about that. Does anybody depend on that or even use it out there? I strongly suspect this is one of those dark corners people even do not know they exist... > I could probably reduce the churn so that compaction_suitable() doesn't > need a new parameter. We could just skip compaction_suitable() check > from compact_zone() on the highest priority, and go on even without > sufficient free page gap? Whatever makes the code easier to understand. Please do not take me wrong I do not want to push back on this too hard I just always love to get rid of an obscure heuristic which even might not matter. And as your testing suggests this might really be the case for !costly orders AFAIU. -- Michal Hocko SUSE Labs