From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752288Ab1HSCyM (ORCPT ); Thu, 18 Aug 2011 22:54:12 -0400 Received: from mga02.intel.com ([134.134.136.20]:7535 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750959Ab1HSCyK (ORCPT ); Thu, 18 Aug 2011 22:54:10 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.67,351,1309762800"; d="scan'208";a="39279712" Date: Fri, 19 Aug 2011 10:54:06 +0800 From: Wu Fengguang To: Vivek Goyal Cc: "linux-fsdevel@vger.kernel.org" , Peter Zijlstra , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML Subject: Re: [PATCH 5/5] writeback: IO-less balance_dirty_pages() Message-ID: <20110819025406.GA13365@localhost> References: <20110816022006.348714319@intel.com> <20110816022329.190706384@intel.com> <20110819020637.GA13597@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110819020637.GA13597@redhat.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 Hi Vivek, > > + base_rate = bdi->dirty_ratelimit; > > + pos_ratio = bdi_position_ratio(bdi, dirty_thresh, > > + background_thresh, nr_dirty, > > + bdi_thresh, bdi_dirty); > > + if (unlikely(pos_ratio == 0)) { > > + pause = MAX_PAUSE; > > + goto pause; > > } > > + task_ratelimit = (u64)base_rate * > > + pos_ratio >> RATELIMIT_CALC_SHIFT; > > Hi Fenguaang, > > I am little confused here. I see that you have already taken pos_ratio > into account in bdi_update_dirty_ratelimit() and wondering why to take > that into account again in balance_diry_pages(). > > We calculated the pos_rate and balanced_rate and adjusted the > bdi->dirty_ratelimit accordingly in bdi_update_dirty_ratelimit(). Good question. There are some inter-dependencies in the calculation, and the dependency chain is the opposite to the one in your mind: balance_dirty_pages() used pos_ratio in the first place, so that bdi_update_dirty_ratelimit() have to use pos_ratio in the calculation of the balanced dirty rate, too. Let's return to how the balanced dirty rate is estimated. Please pay special attention to the last paragraphs below the "......" line. Start by throttling each dd task at rate task_ratelimit = task_ratelimit_0 (1) (any non-zero initial value is OK) After 200ms, we measured dirty_rate = # of pages dirtied by all dd's / 200ms write_bw = # of pages written to the disk / 200ms For the aggressive dd dirtiers, the equality holds dirty_rate == N * task_rate == N * task_ratelimit == N * task_ratelimit_0 (2) Or task_ratelimit_0 = dirty_rate / N (3) Now we conclude that the balanced task ratelimit can be estimated by balanced_rate = task_ratelimit_0 * (write_bw / dirty_rate) (4) Because with (2) and (3), (4) yields the desired equality (1): balanced_rate == (dirty_rate / N) * (write_bw / dirty_rate) == write_bw / N ............................................................................. Now let's revisit (1). Since balance_dirty_pages() chooses to execute the ratelimit task_ratelimit = task_ratelimit_0 = dirty_ratelimit * pos_ratio (5) Put (5) into (4), we get the final form used in bdi_update_dirty_ratelimit() balanced_rate = (dirty_ratelimit * pos_ratio) * (write_bw / dirty_rate) So you really need to take (dirty_ratelimit * pos_ratio) as a single entity. Thanks, Fengguang From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 5/5] writeback: IO-less balance_dirty_pages() Date: Fri, 19 Aug 2011 10:54:06 +0800 Message-ID: <20110819025406.GA13365@localhost> References: <20110816022006.348714319@intel.com> <20110816022329.190706384@intel.com> <20110819020637.GA13597@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "linux-fsdevel@vger.kernel.org" , Peter Zijlstra , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML To: Vivek Goyal Return-path: Content-Disposition: inline In-Reply-To: <20110819020637.GA13597@redhat.com> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org Hi Vivek, > > + base_rate = bdi->dirty_ratelimit; > > + pos_ratio = bdi_position_ratio(bdi, dirty_thresh, > > + background_thresh, nr_dirty, > > + bdi_thresh, bdi_dirty); > > + if (unlikely(pos_ratio == 0)) { > > + pause = MAX_PAUSE; > > + goto pause; > > } > > + task_ratelimit = (u64)base_rate * > > + pos_ratio >> RATELIMIT_CALC_SHIFT; > > Hi Fenguaang, > > I am little confused here. I see that you have already taken pos_ratio > into account in bdi_update_dirty_ratelimit() and wondering why to take > that into account again in balance_diry_pages(). > > We calculated the pos_rate and balanced_rate and adjusted the > bdi->dirty_ratelimit accordingly in bdi_update_dirty_ratelimit(). Good question. There are some inter-dependencies in the calculation, and the dependency chain is the opposite to the one in your mind: balance_dirty_pages() used pos_ratio in the first place, so that bdi_update_dirty_ratelimit() have to use pos_ratio in the calculation of the balanced dirty rate, too. Let's return to how the balanced dirty rate is estimated. Please pay special attention to the last paragraphs below the "......" line. Start by throttling each dd task at rate task_ratelimit = task_ratelimit_0 (1) (any non-zero initial value is OK) After 200ms, we measured dirty_rate = # of pages dirtied by all dd's / 200ms write_bw = # of pages written to the disk / 200ms For the aggressive dd dirtiers, the equality holds dirty_rate == N * task_rate == N * task_ratelimit == N * task_ratelimit_0 (2) Or task_ratelimit_0 = dirty_rate / N (3) Now we conclude that the balanced task ratelimit can be estimated by balanced_rate = task_ratelimit_0 * (write_bw / dirty_rate) (4) Because with (2) and (3), (4) yields the desired equality (1): balanced_rate == (dirty_rate / N) * (write_bw / dirty_rate) == write_bw / N ............................................................................. Now let's revisit (1). Since balance_dirty_pages() chooses to execute the ratelimit task_ratelimit = task_ratelimit_0 = dirty_ratelimit * pos_ratio (5) Put (5) into (4), we get the final form used in bdi_update_dirty_ratelimit() balanced_rate = (dirty_ratelimit * pos_ratio) * (write_bw / dirty_rate) So you really need to take (dirty_ratelimit * pos_ratio) as a single entity. 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 internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org