From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755779AbcCPFmc (ORCPT ); Wed, 16 Mar 2016 01:42:32 -0400 Received: from gum.cmpxchg.org ([85.214.110.215]:46590 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755212AbcCPFmH (ORCPT ); Wed, 16 Mar 2016 01:42:07 -0400 Date: Tue, 15 Mar 2016 22:41:57 -0700 From: Johannes Weiner To: Vladimir Davydov Cc: Andrew Morton , Michal Hocko , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below usage Message-ID: <20160316054157.GB11006@cmpxchg.org> References: <1457643015-8828-1-git-send-email-hannes@cmpxchg.org> <20160311083440.GI1946@esperanza> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160311083440.GI1946@esperanza> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 11, 2016 at 11:34:40AM +0300, Vladimir Davydov wrote: > On Thu, Mar 10, 2016 at 03:50:13PM -0500, Johannes Weiner wrote: > > When setting memory.high below usage, nothing happens until the next > > charge comes along, and then it will only reclaim its own charge and > > not the now potentially huge excess of the new memory.high. This can > > cause groups to stay in excess of their memory.high indefinitely. > > > > To fix that, when shrinking memory.high, kick off a reclaim cycle that > > goes after the delta. > > I agree that we should reclaim the high excess, but I don't think it's a > good idea to do it synchronously. Currently, memory.low and memory.high > knobs can be easily used by a single-threaded load manager implemented > in userspace, because it doesn't need to care about potential stalls > caused by writes to these files. After this change it might happen that > a write to memory.high would take long, seconds perhaps, so in order to > react quickly to changes in other cgroups, a load manager would have to > spawn a thread per each write to memory.high, which would complicate its > implementation significantly. While I do expect memory.high to be adjusted every once in a while, I can't see anybody doing it by a significant fraction of the cgroup every couple of seconds - or tighter than the workingset; and dropping use-once cache is cheap. What kind of usecase would that be? But even if we're wrong about it and this becomes a scalability issue, the knob - even when reclaiming synchroneously - makes no guarantees about the target being met once the write finishes. It's a best effort mechanism. What would break if we made it async later on?