From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754501AbaAULw3 (ORCPT ); Tue, 21 Jan 2014 06:52:29 -0500 Received: from cantor2.suse.de ([195.135.220.15]:42803 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754134AbaAULw1 (ORCPT ); Tue, 21 Jan 2014 06:52:27 -0500 Date: Tue, 21 Jan 2014 11:52:23 +0000 From: Mel Gorman To: riel@redhat.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, peterz@infradead.org, mingo@redhat.com, chegu_vinod@hp.com Subject: Re: [PATCH 1/6] numa,sched,mm: remove p->numa_migrate_deferred Message-ID: <20140121115223.GF4963@suse.de> References: <1390245667-24193-1-git-send-email-riel@redhat.com> <1390245667-24193-2-git-send-email-riel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1390245667-24193-2-git-send-email-riel@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 20, 2014 at 02:21:02PM -0500, riel@redhat.com wrote: > From: Rik van Riel > > Excessive migration of pages can hurt the performance of workloads > that span multiple NUMA nodes. However, it turns out that the > p->numa_migrate_deferred knob is a really big hammer, which does > reduce migration rates, but does not actually help performance. > > Now that the second stage of the automatic numa balancing code > has stabilized, it is time to replace the simplistic migration > deferral code with something smarter. > > Cc: Peter Zijlstra > Cc: Mel Gorman > Cc: Ingo Molnar > Cc: Chegu Vinod > Signed-off-by: Rik van Riel When I added a tracepoint to track deferred migration I was surprised how often it triggered for some workloads. I agree that we want to do something better because it was a crutch albeit a necessary one at the time. Note that the knob was not about performance as such, it was about avoiding worst-case behaviour. We should keep an eye out for bugs that look like excessive migration on workloads that are not converging. Reintroducing this hammer would be a last resort for working around the problem. Finally, the sysctl is documented in Documentation/sysctl/kernel.txt and this patch should also remove it. Functionally, the patch looks fine and it's time to reinvestigate if it's necessary so assuming the documentation gets removed; Acked-by: Mel Gorman -- Mel Gorman SUSE Labs