From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753454Ab1H2NMj (ORCPT ); Mon, 29 Aug 2011 09:12:39 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43373 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751502Ab1H2NMe convert rfc822-to-8bit (ORCPT ); Mon, 29 Aug 2011 09:12:34 -0400 Subject: Re: [PATCH 2/5] writeback: dirty position control From: Peter Zijlstra To: Vivek Goyal Cc: Wu Fengguang , "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML Date: Mon, 29 Aug 2011 15:12:07 +0200 In-Reply-To: <20110824180058.GC22434@redhat.com> References: <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> <1313154259.6576.42.camel@twins> <20110812142020.GB17781@localhost> <1314027488.24275.74.camel@twins> <20110823034042.GC7332@localhost> <1314093660.8002.24.camel@twins> <20110823141504.GA15949@localhost> <20110823174757.GC15820@redhat.com> <20110824001257.GA6349@localhost> <20110824180058.GC22434@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1314623527.2816.28.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2011-08-24 at 14:00 -0400, Vivek Goyal wrote: > > Ok, I think I am beginning to see your point. Let me just elaborate on > the example you gave. > > Assume a system is completely balanced and a task is writing at 100MB/s > rate. > > write_bw = dirty_rate = 100MB/s, pos_ratio = 1; N=1 > > bdi->dirty_ratelimit = 100MB/s > > Now another tasks starts dirtying the page cache on same bdi. Number of > dirty pages should go up pretty fast and likely position ratio feedback > will kick in to reduce the dirtying rate. (rate based feedback does not > kick in till next 200ms) and pos_ratio feedback seems to be instantaneous. > Assume new pos_ratio is .5 > > So new throttle rate for both the tasks is 50MB/s. > > bdi->dirty_ratelimit = 100MB/s (a feedback has not kicked in yet) > task_ratelimit = bdi->dirty_ratelimit * pos_ratio = 100 *.5 = 50MB/s > > Now lets say 200ms have passed and rate base feedback is reevaluated. > > write_bw > bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- > dirty_bw > > bdi->dirty_ratelimit_(i+1) = 100 * 100/100 = 100MB/s > > Ideally bdi->dirty_ratelimit should have now become 50MB/s as N=2 but > that did not happen. And reason being that there are two feedback control > loops and pos_ratio loops reacts to imbalances much more quickly. Because > previous loop has already reacted to the imbalance and reduced the > dirtying rate of task, rate based loop does not try to adjust anything > and thinks everything is just fine. > > Things are fine in the sense that still dirty_rate == write_bw but > system is not balanced in terms of number of dirty pages and pos_ratio=.5 > > So you are trying to make one feedback loop aware of second loop so that > if second loop is unbalanced, first loop reacts to that as well and not > just look at dirty_rate and write_bw. So refining new balanced rate by > pos_ratio helps. > write_bw > bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- * pos_ratio > dirty_bw > > Now if global dirty pages are imbalanced, balanced rate will still go > down despite the fact that dirty_bw == write_bw. This will lead to > further reduction in task dirty rate. Which in turn will lead to reduced > number of dirty rate and should eventually lead to pos_ratio=1. Ok so this argument makes sense, is there some formalism to describe such systems where such things are more evident?