From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 8486E6B0169 for ; Wed, 24 Aug 2011 12:13:19 -0400 (EDT) Subject: Re: [PATCH 2/5] writeback: dirty position control From: Peter Zijlstra Date: Wed, 24 Aug 2011 18:12:58 +0200 In-Reply-To: <20110824001257.GA6349@localhost> References: <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> <20110823141504.GA15949@localhost> <20110823174757.GC15820@redhat.com> <20110824001257.GA6349@localhost> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Message-ID: <1314202378.6925.48.camel@twins> Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Wu Fengguang Cc: Vivek Goyal , "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML On Wed, 2011-08-24 at 08:12 +0800, Wu Fengguang wrote: > > You somehow directly jump to =20 > >=20 > > balanced_rate =3D task_ratelimit_200ms * write_bw / dirty_rate > >=20 > > without explaining why following will not work. > >=20 > > balanced_rate_(i+1) =3D balance_rate(i) * write_bw / dirty_rate >=20 > Thanks for asking that, it's probably the root of confusions, so let > me answer it standalone. >=20 > It's actually pretty simple to explain this equation: >=20 > write_bw > balanced_rate =3D task_ratelimit_200ms * ---------- (1) > dirty_rate >=20 > If there are N dd tasks, each task is throttled at task_ratelimit_200ms > for the past 200ms, we are going to measure the overall bdi dirty rate >=20 > dirty_rate =3D N * task_ratelimit_200ms (2) >=20 > put (2) into (1) we get >=20 > balanced_rate =3D write_bw / N (3) >=20 > So equation (1) is the right estimation to get the desired target (3). >=20 >=20 > As for >=20 > write_bw > balanced_rate_(i+1) =3D balanced_rate_(i) * ---------- (4) > dirty_rate >=20 > Let's compare it with the "expanded" form of (1): >=20 > write_bw > balanced_rate_(i+1) =3D balanced_rate_(i) * pos_ratio * ---------= - (5) > dirty_rate >=20 > So the difference lies in pos_ratio. >=20 > Believe it or not, it's exactly the seemingly use of pos_ratio that > makes (5) independent(*) of the position control. >=20 > Why? Look at (4), assume the system is in a state >=20 > - dirty rate is already balanced, ie. balanced_rate_(i) =3D write_bw / N > - dirty position is not balanced, for example pos_ratio =3D 0.5 >=20 > balance_dirty_pages() will be rate limiting each tasks at half the > balanced dirty rate, yielding a measured >=20 > dirty_rate =3D write_bw / 2 (6) >=20 > Put (6) into (4), we get >=20 > balanced_rate_(i+1) =3D balanced_rate_(i) * 2 > =3D (write_bw / N) * 2 >=20 > That means, any position imbalance will lead to balanced_rate > estimation errors if we follow (4). Whereas if (1)/(5) is used, we > always get the right balanced dirty ratelimit value whether or not > (pos_ratio =3D=3D 1.0), hence make the rate estimation independent(*) of > dirty position control. >=20 > (*) independent as in real values, not the seemingly relations in equatio= n The assumption here is that N is a constant.. in the above case pos_ratio would eventually end up at 1 and things would be good again. I see your argument about oscillations, but I think you can introduce similar effects by varying N. -- 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