From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread Date: Sat, 19 Jun 2010 09:29:39 +1000 Message-ID: <20100618232939.GG6590@dastard> References: <1276797878-28893-1-git-send-email-jack@suse.cz> <20100618060901.GA6590@dastard> <1276852260.27822.1598.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org, akpm@linux-foundation.org, wfg@mail.ustc.edu.cn To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <1276852260.27822.1598.camel@twins> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Jun 18, 2010 at 11:11:00AM +0200, Peter Zijlstra wrote: > On Fri, 2010-06-18 at 16:09 +1000, Dave Chinner wrote: > > > + bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written; > > > > The resolution of the percpu counters is an issue here, I think. > > percpu counters update in batches of 32 counts per CPU. wc->written > > is going to have a value of roughly 8 or 32 depending on whether > > bdi->dirty_exceeded is set or not. I note that you take this into > > account when checking dirty threshold limits, but it doesn't appear > > to be taken in to here. > > The BDI stuff uses a custom batch-size, see bdi_stat_error() and > related. The total error is in the order of O(n log n) where n is the > number of CPUs. #define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids))) A dual socket server I have here: [ 0.000000] NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:32 nr_node_ids:2 Which means that the bdi per-cpu counter batch size on it would be 48 and the inaccuracy would be even bigger than I described. ;) > But yeah, the whole dirty_exceeded thing makes life more > interesting. I suspect the 8 vs 32 pages could go away without too much impact with the Jan's mechanism... Another thing to consider is the impact of mulipage writes on the incoming value to balance_dirty_pages - if we get 1024 page writes running, then that may impact on the way throttling behaves, too. Cheers, Dave. -- Dave Chinner david@fromorbit.com -- 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/ . Don't email: email@kvack.org