From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752398Ab1HLNUC (ORCPT ); Fri, 12 Aug 2011 09:20:02 -0400 Received: from mga14.intel.com ([143.182.124.37]:32207 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751793Ab1HLNUA (ORCPT ); Fri, 12 Aug 2011 09:20:00 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.67,362,1309762800"; d="scan'208";a="37833697" Date: Fri, 12 Aug 2011 21:19:54 +0800 From: Wu Fengguang To: Peter Zijlstra Cc: "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML Subject: Re: [PATCH 2/5] writeback: dirty position control Message-ID: <20110812131954.GA17781@localhost> References: <20110806084447.388624428@intel.com> <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> <1312910427.1083.68.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1312910427.1083.68.camel@twins> 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 On Wed, Aug 10, 2011 at 01:20:27AM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote: > > > origin - dirty > > > pos_ratio = -------------- > > > origin - goal > > > > > which comes from the below [*] control line, so that when (dirty == goal), > > > pos_ratio == 1.0: > > > > OK, so basically you want a linear function for which: > > > > f(goal) = 1 and has a root somewhere > goal. > > > > (that one line is much more informative than all your graphs put > > together, one can start from there and derive your function) > > > > That does indeed get you the above function, now what does it mean? > > So going by: > > write_bw > ref_bw = dirty_ratelimit * pos_ratio * -------- > dirty_bw > > pos_ratio seems to be the feedback on the deviation of the dirty pages > around its setpoint. Yes. > So we adjust the reference bw (or rather ratelimit) > to take account of the shift in output vs input capacity as well as the > shift in dirty pages around its setpoint. However the above function should better be interpreted as write_bw ref_bw = task_ratelimit_in_past_200ms * -------- dirty_bw where task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratio It would be highly confusing if trying to find the direct "logical" relationships between ref_bw and pos_ratio in the above equation. > From that we derive the condition that: > > pos_ratio(setpoint) := 1 Right. > Now in order to create a linear function we need one more condition. We > get one from the fact that once we hit the limit we should hard throttle > our writers. We get that by setting the ratelimit to 0, because, after > all, pause = nr_dirtied / ratelimit would yield inf. in that case. Thus: > > pos_ratio(limit) := 0 > > Using these two conditions we can solve the equations and get your: > > limit - dirty > pos_ratio(dirty) = ---------------- > limit - setpoint > > Now, for some reason you chose not to use limit, but something like > min(limit, 4*thresh) something to do with the slope affecting the rate > of adjustment. This wants a comment someplace. Thanks to your reasoning that lead to the more elegant setpoint - dirty 3 pos_ratio(dirty) := 1 + (----------------) limit - setpoint Thanks, Fengguang From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta12.messagelabs.com (mail6.bemta12.messagelabs.com [216.82.250.247]) by kanga.kvack.org (Postfix) with ESMTP id C5DC6900138 for ; Fri, 12 Aug 2011 09:20:00 -0400 (EDT) Date: Fri, 12 Aug 2011 21:19:54 +0800 From: Wu Fengguang Subject: Re: [PATCH 2/5] writeback: dirty position control Message-ID: <20110812131954.GA17781@localhost> References: <20110806084447.388624428@intel.com> <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> <1312910427.1083.68.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1312910427.1083.68.camel@twins> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML On Wed, Aug 10, 2011 at 01:20:27AM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote: > > > origin - dirty > > > pos_ratio = -------------- > > > origin - goal > > > > > which comes from the below [*] control line, so that when (dirty == goal), > > > pos_ratio == 1.0: > > > > OK, so basically you want a linear function for which: > > > > f(goal) = 1 and has a root somewhere > goal. > > > > (that one line is much more informative than all your graphs put > > together, one can start from there and derive your function) > > > > That does indeed get you the above function, now what does it mean? > > So going by: > > write_bw > ref_bw = dirty_ratelimit * pos_ratio * -------- > dirty_bw > > pos_ratio seems to be the feedback on the deviation of the dirty pages > around its setpoint. Yes. > So we adjust the reference bw (or rather ratelimit) > to take account of the shift in output vs input capacity as well as the > shift in dirty pages around its setpoint. However the above function should better be interpreted as write_bw ref_bw = task_ratelimit_in_past_200ms * -------- dirty_bw where task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratio It would be highly confusing if trying to find the direct "logical" relationships between ref_bw and pos_ratio in the above equation. > From that we derive the condition that: > > pos_ratio(setpoint) := 1 Right. > Now in order to create a linear function we need one more condition. We > get one from the fact that once we hit the limit we should hard throttle > our writers. We get that by setting the ratelimit to 0, because, after > all, pause = nr_dirtied / ratelimit would yield inf. in that case. Thus: > > pos_ratio(limit) := 0 > > Using these two conditions we can solve the equations and get your: > > limit - dirty > pos_ratio(dirty) = ---------------- > limit - setpoint > > Now, for some reason you chose not to use limit, but something like > min(limit, 4*thresh) something to do with the slope affecting the rate > of adjustment. This wants a comment someplace. Thanks to your reasoning that lead to the more elegant setpoint - dirty 3 pos_ratio(dirty) := 1 + (----------------) limit - setpoint 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