From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935098AbYDQLOL (ORCPT ); Thu, 17 Apr 2008 07:14:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933739AbYDQLJT (ORCPT ); Thu, 17 Apr 2008 07:09:19 -0400 Received: from smtp2.sssup.it ([193.205.80.100]:50636 "EHLO smtp2.sssup.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934383AbYDQLJP (ORCPT ); Thu, 17 Apr 2008 07:09:15 -0400 Date: Thu, 17 Apr 2008 13:14:07 +0200 From: Fabio Checconi To: Aaron Carroll Cc: Paolo Valente , Jens Axboe , Pavel Machek , linux-kernel@vger.kernel.org Subject: Re: [RESEND][RFC] BFQ I/O Scheduler Message-ID: <20080417111407.GS62286@gandalf.sssup.it> References: <20080401152903.GB34860@gandalf.sssup.it> <20080416184441.GA3923@ucw.cz> <4806EACB.7040408@unimore.it> <20080417071012.GP12774@kernel.dk> <480709A2.7040606@unimore.it> <48072549.7040104@cse.unsw.edu.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <48072549.7040104@cse.unsw.edu.au> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Aaron Carroll > Date: Thu, Apr 17, 2008 08:24:09PM +1000 > > Paolo Valente wrote: > >In my opinion, the time-slice approach of cfq is definitely better > >suited than the (sector) budget approach for this type of workloads. On > >the opposite end, the price of time-slices is unfairness towards, e.g., > >threads doing sequential accesses. In bfq we were mainly thinking about > > How do you figure that? This is a situation where time-slices work nicely, > because they implicitly account for the performance penalty of poor access > patterns. The sequential-accessing processes (and the system overall) ends > up with higher throughput. > The unfairness is not WRT tasks generating poor access patterns. If you have two tasks doing sequential accesses on two different regions of the disk the exact amount of service they receive in the same amount of time depends on the transfer rate of the disk on that regions, and, depending on the media, it is not always the same. We showed some example of that in the original post and it is quite easy to try it out if you put two partitions at the ends of a disk and you try to read from them concurrently.