From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753682Ab1HKUiL (ORCPT ); Thu, 11 Aug 2011 16:38:11 -0400 Received: from cantor2.suse.de ([195.135.220.15]:43918 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753023Ab1HKUiJ (ORCPT ); Thu, 11 Aug 2011 16:38:09 -0400 Date: Thu, 11 Aug 2011 21:38:05 +0100 From: Mel Gorman To: Rik van Riel Cc: Linux-MM , LKML , XFS , Dave Chinner , Christoph Hellwig , Johannes Weiner , Wu Fengguang , Jan Kara , Minchan Kim Subject: Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority Message-ID: <20110811203805.GC4844@suse.de> References: <1312973240-32576-1-git-send-email-mgorman@suse.de> <1312973240-32576-6-git-send-email-mgorman@suse.de> <4E441D0E.6020602@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <4E441D0E.6020602@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 Thu, Aug 11, 2011 at 02:18:54PM -0400, Rik van Riel wrote: > On 08/10/2011 06:47 AM, Mel Gorman wrote: > >It is preferable that no dirty pages are dispatched for cleaning from > >the page reclaim path. At normal priorities, this patch prevents kswapd > >writing pages. > > > >However, page reclaim does have a requirement that pages be freed > >in a particular zone. If it is failing to make sufficient progress > >(reclaiming< SWAP_CLUSTER_MAX at any priority priority), the priority > >is raised to scan more pages. A priority of DEF_PRIORITY - 3 is > >considered to be the point where kswapd is getting into trouble > >reclaiming pages. If this priority is reached, kswapd will dispatch > >pages for writing. > > > >Signed-off-by: Mel Gorman > >Reviewed-by: Minchan Kim > > My only worry with this patch is that maybe we'll burn too > much CPU time freeing pages from a zone. The throttling patch prevents too much CPU being used if pages under writeback are being encountered during scanning. That said, I shared your concern and recorded kswapd CPU usage over time. > However, chances > are we'll have freed pages from other zones when scanning > one zone multiple times (the page cache dirty limit is global, > the clean pages have to be _somewhere_). > > Since the bulk of the allocators are not too picky about > which zone they get their pages from, I suspect this patch > will be an overall improvement pretty much all the time. > This is roughly similar to my own reasoning. I uploaded all the kswapd CPU usage charts to http://www.csn.ul.ie/~mel/postings/riel-20110811 These are smoothened as the raw figures are barely readable. If you go through them, you'll see that kswapd CPU usage is sometimes higher but generally within 2-3%. > Acked-by: Rik van Riel Thanks. -- Mel Gorman SUSE Labs