From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753936Ab1HZJEx (ORCPT ); Fri, 26 Aug 2011 05:04:53 -0400 Received: from casper.infradead.org ([85.118.1.10]:41835 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752693Ab1HZJEt convert rfc822-to-8bit (ORCPT ); Fri, 26 Aug 2011 05:04:49 -0400 Subject: Re: [PATCH 2/5] writeback: dirty position control From: Peter Zijlstra 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 Date: Fri, 26 Aug 2011 11:04:29 +0200 In-Reply-To: <20110826001846.GA6118@localhost> References: <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> <1314202378.6925.48.camel@twins> <20110826001846.GA6118@localhost> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1314349469.26922.24.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2011-08-26 at 08:18 +0800, Wu Fengguang wrote: > On Thu, Aug 25, 2011 at 12:12:58AM +0800, Peter Zijlstra wrote: > > On Wed, 2011-08-24 at 08:12 +0800, Wu Fengguang wrote: > > > Put (6) into (4), we get > > > > > > balanced_rate_(i+1) = balanced_rate_(i) * 2 > > > = (write_bw / N) * 2 > > > > > > 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 == 1.0), hence make the rate estimation independent(*) of > > > dirty position control. > > > > > > (*) independent as in real values, not the seemingly relations in equation > > > > > > 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. > > Yeah, it's very possible for N to change over time, in which case > balanced_rate will adapt to new N in similar way. Gah.. but but but, that gives the same stuff as your (6)+(4). Why won't you accept that for pos_ratio but you don't mind for N ? From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 2/5] writeback: dirty position control Date: Fri, 26 Aug 2011 11:04:29 +0200 Message-ID: <1314349469.26922.24.camel@twins> References: <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> <1314202378.6925.48.camel@twins> <20110826001846.GA6118@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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 To: Wu Fengguang Return-path: In-Reply-To: <20110826001846.GA6118@localhost> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, 2011-08-26 at 08:18 +0800, Wu Fengguang wrote: > On Thu, Aug 25, 2011 at 12:12:58AM +0800, Peter Zijlstra wrote: > > On Wed, 2011-08-24 at 08:12 +0800, Wu Fengguang wrote: > > > 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 equ= ation > >=20 > >=20 > > 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. >=20 > Yeah, it's very possible for N to change over time, in which case > balanced_rate will adapt to new N in similar way. Gah.. but but but, that gives the same stuff as your (6)+(4). Why won't you accept that for pos_ratio but you don't mind for 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/ . 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