From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756581Ab1ALV75 (ORCPT ); Wed, 12 Jan 2011 16:59:57 -0500 Received: from cantor2.suse.de ([195.135.220.15]:53245 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756508Ab1ALV7y (ORCPT ); Wed, 12 Jan 2011 16:59:54 -0500 Date: Wed, 12 Jan 2011 22:59:49 +0100 From: Jan Kara To: Wu Fengguang Cc: Andrew Morton , Jan Kara , Peter Zijlstra , Christoph Hellwig , Trond Myklebust , Dave Chinner , "Theodore Ts'o" , Chris Mason , Mel Gorman , Rik van Riel , KOSAKI Motohiro , Greg Thelen , Minchan Kim , linux-mm , linux-fsdevel@vger.kernel.org, LKML Subject: Re: [PATCH 02/35] writeback: safety margin for bdi stat error Message-ID: <20110112215949.GD14260@quack.suse.cz> References: <20101213144646.341970461@intel.com> <20101213150326.604451840@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101213150326.604451840@intel.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 13-12-10 22:46:48, Wu Fengguang wrote: > In a simple dd test on a 8p system with "mem=256M", I find all light > dirtier tasks on the root fs are get heavily throttled. That happens > because the global limit is exceeded. It's unbelievable at first sight, > because the test fs doing the heavy dd is under its bdi limit. After > doing some tracing, it's discovered that > > bdi_dirty < bdi_dirty_limit() < global_dirty_limit() < nr_dirty ^^ bdi_dirty is the number of pages dirtied on BDI? I.e. bdi_nr_reclaimable + bdi_nr_writeback? > So the root cause is, the bdi_dirty is well under the global nr_dirty > due to accounting errors. This can be fixed by using bdi_stat_sum(), So which statistic had the big error? I'd just like to understand this (and how come your patch improves the situation)... > however that's costly on large NUMA machines. So do a less costly fix > of lowering the bdi limit, so that the accounting errors won't lead to > the absurd situation "global limit exceeded but bdi limit not exceeded". > > This provides guarantee when there is only 1 heavily dirtied bdi, and > works by opportunity for 2+ heavy dirtied bdi's (hopefully they won't > reach big error _and_ exceed their bdi limit at the same time). > ... > @@ -458,6 +464,14 @@ unsigned long bdi_dirty_limit(struct bac > long numerator, denominator; > > /* > + * try to prevent "global limit exceeded but bdi limit not exceeded" > + */ > + if (likely(dirty > bdi_stat_error(bdi))) > + dirty -= bdi_stat_error(bdi); > + else > + return 0; > + Ugh, so if by any chance global_dirty_limit() <= bdi_stat_error(bdi), you will limit number of unreclaimable pages for that bdi 0? Why? Honza -- Jan Kara SUSE Labs, CR