From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756181AbcHZGoS (ORCPT ); Fri, 26 Aug 2016 02:44:18 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:35627 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751029AbcHZGoQ (ORCPT ); Fri, 26 Aug 2016 02:44:16 -0400 Date: Fri, 26 Aug 2016 08:44:13 +0200 From: Michal Hocko To: David Rientjes Cc: Andrew Morton , Mel Gorman , Joonsoo Kim , Vlastimil Babka , Markus Trippelsdorf , linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm: clarify COMPACTION Kconfig text Message-ID: <20160826064406.GB16195@dhcp22.suse.cz> References: <1471939757-29789-1-git-send-email-mhocko@kernel.org> <20160825065424.GA4230@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Thu 25-08-16 15:34:54, David Rientjes wrote: > On Thu, 25 Aug 2016, Michal Hocko wrote: > > > > I don't believe it has been an issue in the past for any archs that > > > don't use thp. > > > > Well, fragmentation is a real problem and order-0 reclaim will be never > > anywhere close to reliably provide higher order pages. Well, reclaiming > > a lot of memory can increase the probability of a success but that > > can quite often lead to over reclaim and long stalls. There are other > > sources of high order requests than THP so this is not about THP at all > > IMHO. > > > > Would it be possible to list the high-order allocations you are concerned > about other than thp that doesn't have fallback behavior like skbuff and > slub allocations? struct task_struct is an order-1 allocation and there > may be order-1 slab bucket usage, but what is higher order or requires > aggressive compaction to allocate? kernel stacks (order-2 on many arches), some arches need higher order pages for page table allocations (at least the upper level AFAIR). > Surely you're not suggesting that order-0 reclaim cannot form order-1 > memory. I haven't seen fragmentation that bad that order-1 would be completely depleted so I wouldn't be all that worried about this. But order-2 can get depleted as our last oom reports show. > I am concerned about kernels that require a small memory footprint and > cannot enable all of CONFIG_COMPACTION and CONFIG_MIGRATION. Embedded > devices are not a negligible minority of kernels. Fair enough. And nobody discourages them from disabling the compaction. I would expect that kernels for those machines are configured by people who know what they are doing. They have to be careful about disabling many other things already and carefully weight the missing functionality vs. code size savings. I also expect that workloads on those machines are also careful to not require large physically contiguous memory blocks very much. Otherwise they would have problems described by the help text. So I am not really sure what you are objecting to. I am not making COMPACTION on unconditionally. I just want to make sure that regular users do not think this is just a THP thing which is not true since the lumpy reclaim is gone. On my laptop I have more than 40 slab caches which have pagesperslab > 2. -- Michal Hocko SUSE Labs