From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758589Ab1KVVEl (ORCPT ); Tue, 22 Nov 2011 16:04:41 -0500 Received: from cantor2.suse.de ([195.135.220.15]:55648 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758545Ab1KVVEj (ORCPT ); Tue, 22 Nov 2011 16:04:39 -0500 Date: Tue, 22 Nov 2011 22:04:37 +0100 From: Jan Kara To: Wu Fengguang Cc: Jan Kara , "linux-fsdevel@vger.kernel.org" , Peter Zijlstra , Christoph Hellwig , Andrew Morton , LKML Subject: Re: [PATCH 1/5] writeback: balanced_rate cannot exceed write bandwidth Message-ID: <20111122210437.GH8058@quack.suse.cz> References: <20111121130342.211953629@intel.com> <20111121131215.645601798@intel.com> <20111121225049.GE4017@quack.suse.cz> <20111122064149.GA12864@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111122064149.GA12864@localhost> 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 22-11-11 14:41:49, Wu Fengguang wrote: > On Tue, Nov 22, 2011 at 06:50:49AM +0800, Jan Kara wrote: > > On Mon 21-11-11 21:03:43, Wu Fengguang wrote: > > > Add an upper limit to balanced_rate according to the below inequality. > > > This filters out some rare but huge singular points, which at least > > > enables more readable gnuplot figures. > > > > > > When there are N dd dirtiers, > > > > > > balanced_dirty_ratelimit = write_bw / N > > > > > > So it holds that > > > > > > balanced_dirty_ratelimit <= write_bw > > The change makes sense, but do we understand why there are such huge > > singular points? Are they due to errors in estimation of bandwidth or due > > to errors in dirtying rate computations (e.g. due to truncates), or > > something else? > > Good point. I'll add this to the changelog: > > The singular points originate from dirty_rate in the below formular: > > balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate > where > dirty_rate = (number of page dirties in the past 200ms) / 200ms > > In the extreme case, if all dd tasks suddenly get blocked on something > else and hence no pages are dirtied at all, dirty_rate will be 0 and > balanced_dirty_ratelimit will be inf. This could happen in reality. > > There won't be tiny singular points though, as long as the dirty pages > lie inside the dirty control area (above the freerun region). > Because there the dd tasks will be throttled by balanced_dirty_pages() > and won't be able to suddenly dirty much more pages than average. OK, I see. Thanks for explanation. Honza -- Jan Kara SUSE Labs, CR