From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932486AbaHGPbt (ORCPT ); Thu, 7 Aug 2014 11:31:49 -0400 Received: from zene.cmpxchg.org ([85.214.230.12]:58481 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932133AbaHGPbs (ORCPT ); Thu, 7 Aug 2014 11:31:48 -0400 Date: Thu, 7 Aug 2014 11:31:41 -0400 From: Johannes Weiner To: Michal Hocko Cc: Andrew Morton , Tejun Heo , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch 1/4] mm: memcontrol: reduce reclaim invocations for higher order requests Message-ID: <20140807153141.GD14734@cmpxchg.org> References: <1407186897-21048-1-git-send-email-hannes@cmpxchg.org> <1407186897-21048-2-git-send-email-hannes@cmpxchg.org> <20140807130822.GB12730@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140807130822.GB12730@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 07, 2014 at 03:08:22PM +0200, Michal Hocko wrote: > On Mon 04-08-14 17:14:54, Johannes Weiner wrote: > > Instead of passing the request size to direct reclaim, memcg just > > manually loops around reclaiming SWAP_CLUSTER_MAX pages until the > > charge can succeed. That potentially wastes scan progress when huge > > page allocations require multiple invocations, which always have to > > restart from the default scan priority. > > > > Pass the request size as a reclaim target to direct reclaim and leave > > it to that code to reach the goal. > > THP charge then will ask for 512 pages to be (direct) reclaimed. That > is _a lot_ and I would expect long stalls to achieve this target. I > would also expect quick priority drop down and potential over-reclaim > for small and moderately sized memcgs (e.g. memcg with 1G worth of pages > would need to drop down below DEF_PRIORITY-2 to have a chance to scan > that many pages). All that done for a charge which can fallback to a > single page charge. > > The current code is quite hostile to THP when we are close to the limit > but solving this by introducing long stalls instead doesn't sound like a > proper approach to me. THP latencies are actually the same when comparing high limit nr_pages reclaim with the current hard limit SWAP_CLUSTER_MAX reclaim, although system time is reduced with the high limit. High limit reclaim with SWAP_CLUSTER_MAX has better fault latency but it doesn't actually contain the workload - with 1G high and a 4G load, the consumption at the end of the run is 3.7G. So what I'm proposing works and is of equal quality from a THP POV. This change is complicated enough when we stick to the facts, let's not make up things based on gut feeling.