From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755477Ab1HWOPU (ORCPT ); Tue, 23 Aug 2011 10:15:20 -0400 Received: from mga14.intel.com ([143.182.124.37]:59236 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755331Ab1HWOPM (ORCPT ); Tue, 23 Aug 2011 10:15:12 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.68,269,1312182000"; d="scan'208";a="41504974" Date: Tue, 23 Aug 2011 22:15:04 +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: <20110823141504.GA15949@localhost> References: <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1314093660.8002.24.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 Tue, Aug 23, 2011 at 06:01:00PM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote: > > - not a factor at all for updating balanced_rate (whether or not we do (2)) > > well, in this concept: the balanced_rate formula inherently does not > > derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's > > based on the ratelimit executed for the past 200ms: > > > > balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio > > Ok, this is where it all goes funny.. > > So if you want completely separated feedback loops I would expect If call it feedback loops, then it's a series of independent feedback loops of depth 1. Because each balanced_rate is a fresh estimation dependent solely on - writeout bandwidth - N, the number of dd tasks in the past 200ms. As long as a CONSTANT ratelimit (whatever value it is) is executed in the past 200ms, we can get the same balanced_rate. balanced_rate = CONSTANT_ratelimit * write_bw / dirty_rate The resulted balanced_rate is independent of how large the CONSTANT ratelimit is, because if we start with a doubled CONSTANT ratelimit, we'll see doubled dirty_rate and result in the same balanced_rate. In that manner, balance_rate_(i+1) is not really depending on the value of balance_rate_(i): whatever balance_rate_(i) is, we are going to get the same balance_rate_(i+1) if not considering estimation errors. Note that the estimation errors mainly come from the fluctuations in dirty_rate. That may well be what's already in your mind, just that we disagree about the terms ;) > something like: > > balance_rate_(i+1) = balance_rate_(i) * bw_ratio ; every 200ms > > The former is a complete feedback loop, expressing the new value in the > old value (*) with bw_ratio as feedback parameter; if we throttled too > much, the dirty_rate will have dropped and the bw_ratio will be <1 > causing the balance_rate to drop increasing the dirty_rate, and vice > versa. In principle, the bw_ratio works that way. However since balance_rate_(i) is not the exact _executed_ ratelimit in balance_dirty_pages(). > (*) which is the form I expected and why I thought your primary feedback > loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio Because the executed ratelimit was rate_(i) * pos_ratio. > With the above balance_rate is an independent variable that tracks the > write bandwidth. Now possibly you'd want a low-pass filter on that since > your bw_ratio is a bit funny in the head, but that's another story. Yeah. > Then when you use the balance_rate to actually throttle tasks you apply > your secondary control steering the dirty page count, yielding: > > task_rate = balance_rate * pos_ratio Right. Note the above formula is not a derived one, but an original one that later leads to pos_ratio showing up in the calculation of balanced_rate. > > and task_ratelimit_200ms happen to can be estimated from > > > > task_ratelimit_200ms ~= balanced_rate_i * pos_ratio > > > We may alternatively record every task_ratelimit executed in the > > past 200ms and average them all to get task_ratelimit_200ms. In this > > way we take the "superfluous" pos_ratio out of sight :) > > Right, so I'm not at all sure that makes sense, its not immediately > evident that ~= balance_rate * pos_ratio. Nor is it > clear to me why your primary feedback loop uses task_ratelimit_200ms at > all. task_ratelimit is used and hence defined to be (balance_rate * pos_ratio) by balance_dirty_pages(). So this is an original formula: task_ratelimit = balance_rate * pos_ratio task_ratelimit_200ms is also used as an original data source in balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate Then we try to estimate task_ratelimit_200ms by assuming all tasks have been executing the same CONSTANT ratelimit in balance_dirty_pages(). Hence we get task_ratelimit_200ms ~= prev_balance_rate * pos_ratio > > There is fundamentally no dependency between balanced_rate_(i+1) and > > balanced_rate_i/task_ratelimit_200ms: the balanced_rate estimation > > only asks for _whatever_ CONSTANT task ratelimit to be executed for > > 200ms, then it get the balanced rate from the dirty_rate feedback. > > How can there not be a relation between balance_rate_(i+1) and > balance_rate_(i) ? In this manner: even though balance_rate_(i) is somehow used for calculating balance_rate_(i+1), the latter will evaluate to the same value given whatever balance_rate_(i). That is, there is two dependencies, the seemingly dependency in the formula, and the effective dependency in the data values. Thank, Fengguang From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 2/5] writeback: dirty position control Date: Tue, 23 Aug 2011 22:15:04 +0800 Message-ID: <20110823141504.GA15949@localhost> References: <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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 To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <1314093660.8002.24.camel@twins> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Tue, Aug 23, 2011 at 06:01:00PM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote: > > - not a factor at all for updating balanced_rate (whether or not we do (2)) > > well, in this concept: the balanced_rate formula inherently does not > > derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's > > based on the ratelimit executed for the past 200ms: > > > > balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio > > Ok, this is where it all goes funny.. > > So if you want completely separated feedback loops I would expect If call it feedback loops, then it's a series of independent feedback loops of depth 1. Because each balanced_rate is a fresh estimation dependent solely on - writeout bandwidth - N, the number of dd tasks in the past 200ms. As long as a CONSTANT ratelimit (whatever value it is) is executed in the past 200ms, we can get the same balanced_rate. balanced_rate = CONSTANT_ratelimit * write_bw / dirty_rate The resulted balanced_rate is independent of how large the CONSTANT ratelimit is, because if we start with a doubled CONSTANT ratelimit, we'll see doubled dirty_rate and result in the same balanced_rate. In that manner, balance_rate_(i+1) is not really depending on the value of balance_rate_(i): whatever balance_rate_(i) is, we are going to get the same balance_rate_(i+1) if not considering estimation errors. Note that the estimation errors mainly come from the fluctuations in dirty_rate. That may well be what's already in your mind, just that we disagree about the terms ;) > something like: > > balance_rate_(i+1) = balance_rate_(i) * bw_ratio ; every 200ms > > The former is a complete feedback loop, expressing the new value in the > old value (*) with bw_ratio as feedback parameter; if we throttled too > much, the dirty_rate will have dropped and the bw_ratio will be <1 > causing the balance_rate to drop increasing the dirty_rate, and vice > versa. In principle, the bw_ratio works that way. However since balance_rate_(i) is not the exact _executed_ ratelimit in balance_dirty_pages(). > (*) which is the form I expected and why I thought your primary feedback > loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio Because the executed ratelimit was rate_(i) * pos_ratio. > With the above balance_rate is an independent variable that tracks the > write bandwidth. Now possibly you'd want a low-pass filter on that since > your bw_ratio is a bit funny in the head, but that's another story. Yeah. > Then when you use the balance_rate to actually throttle tasks you apply > your secondary control steering the dirty page count, yielding: > > task_rate = balance_rate * pos_ratio Right. Note the above formula is not a derived one, but an original one that later leads to pos_ratio showing up in the calculation of balanced_rate. > > and task_ratelimit_200ms happen to can be estimated from > > > > task_ratelimit_200ms ~= balanced_rate_i * pos_ratio > > > We may alternatively record every task_ratelimit executed in the > > past 200ms and average them all to get task_ratelimit_200ms. In this > > way we take the "superfluous" pos_ratio out of sight :) > > Right, so I'm not at all sure that makes sense, its not immediately > evident that ~= balance_rate * pos_ratio. Nor is it > clear to me why your primary feedback loop uses task_ratelimit_200ms at > all. task_ratelimit is used and hence defined to be (balance_rate * pos_ratio) by balance_dirty_pages(). So this is an original formula: task_ratelimit = balance_rate * pos_ratio task_ratelimit_200ms is also used as an original data source in balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate Then we try to estimate task_ratelimit_200ms by assuming all tasks have been executing the same CONSTANT ratelimit in balance_dirty_pages(). Hence we get task_ratelimit_200ms ~= prev_balance_rate * pos_ratio > > There is fundamentally no dependency between balanced_rate_(i+1) and > > balanced_rate_i/task_ratelimit_200ms: the balanced_rate estimation > > only asks for _whatever_ CONSTANT task ratelimit to be executed for > > 200ms, then it get the balanced rate from the dirty_rate feedback. > > How can there not be a relation between balance_rate_(i+1) and > balance_rate_(i) ? In this manner: even though balance_rate_(i) is somehow used for calculating balance_rate_(i+1), the latter will evaluate to the same value given whatever balance_rate_(i). That is, there is two dependencies, the seemingly dependency in the formula, and the effective dependency in the data values. Thank, 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