From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753390AbaGJMSf (ORCPT ); Thu, 10 Jul 2014 08:18:35 -0400 Received: from zene.cmpxchg.org ([85.214.230.12]:53645 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752681AbaGJMSe (ORCPT ); Thu, 10 Jul 2014 08:18:34 -0400 Date: Thu, 10 Jul 2014 08:18:30 -0400 From: Johannes Weiner To: Mel Gorman Cc: Andrew Morton , Linux Kernel , Linux-MM , Linux-FSDevel Subject: Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy Message-ID: <20140710121830.GN29639@cmpxchg.org> References: <1404893588-21371-1-git-send-email-mgorman@suse.de> <1404893588-21371-7-git-send-email-mgorman@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1404893588-21371-7-git-send-email-mgorman@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 09, 2014 at 09:13:08AM +0100, Mel Gorman wrote: > The fair zone allocation policy round-robins allocations between zones > within a node to avoid age inversion problems during reclaim. If the > first allocation fails, the batch counts is reset and a second attempt > made before entering the slow path. > > One assumption made with this scheme is that batches expire at roughly the > same time and the resets each time are justified. This assumption does not > hold when zones reach their low watermark as the batches will be consumed > at uneven rates. Allocation failure due to watermark depletion result in > additional zonelist scans for the reset and another watermark check before > hitting the slowpath. > > On UMA, the benefit is negligible -- around 0.25%. On 4-socket NUMA > machine it's variable due to the variability of measuring overhead with > the vmstat changes. The system CPU overhead comparison looks like > > 3.16.0-rc3 3.16.0-rc3 3.16.0-rc3 > vanilla vmstat-v5 lowercost-v5 > User 746.94 774.56 802.00 > System 65336.22 32847.27 40852.33 > Elapsed 27553.52 27415.04 27368.46 > > However it is worth noting that the overall benchmark still completed > faster and intuitively it makes sense to take as few passes as possible > through the zonelists. > > Signed-off-by: Mel Gorman Acked-by: Johannes Weiner