From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 3C24E6B00F1 for ; Wed, 12 Jan 2011 16:59:55 -0500 (EST) Date: Wed, 12 Jan 2011 22:59:49 +0100 From: Jan Kara 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> Sender: owner-linux-mm@kvack.org 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 List-ID: 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org