From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 02/35] writeback: safety margin for bdi stat error Date: Thu, 13 Jan 2011 12:14:40 +0800 Message-ID: <20110113041440.GC7840@localhost> References: <20101213144646.341970461@intel.com> <20101213150326.604451840@intel.com> <20110112215949.GD14260@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , 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 To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20110112215949.GD14260@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Jan 13, 2011 at 05:59:49AM +0800, Jan Kara wrote: > 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? Yes. > > 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)... bdi_stat_error() = nr_cpu_ids * BDI_STAT_BATCH = 8 * (8*(1+ilog2(8))) = 8 * 8 * 4 = 256 pages = 1MB > > 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? Good catch! Yeah it may lead to regressions and should be voided. Thanks, Fengguang -- 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