From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760272AbXJOVNl (ORCPT ); Mon, 15 Oct 2007 17:13:41 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754827AbXJOVNb (ORCPT ); Mon, 15 Oct 2007 17:13:31 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:33804 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751939AbXJOVNa (ORCPT ); Mon, 15 Oct 2007 17:13:30 -0400 Date: Mon, 15 Oct 2007 14:12:36 -0700 From: Andrew Morton To: Rik van Riel Cc: arjan@infradead.org, linux-kernel@vger.kernel.org, jens.axboe@oracle.com, mingo@elte.hu Subject: Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority Message-Id: <20071015141236.62b075c6.akpm@linux-foundation.org> In-Reply-To: <20071015161315.4fe18e07@bree.surriel.com> References: <20071015104647.14e60bc5@laptopd505.fenrus.org> <20071015114738.6b5a25c7.akpm@linux-foundation.org> <20071015161315.4fe18e07@bree.surriel.com> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 15 Oct 2007 16:13:15 -0400 Rik van Riel wrote: > Since you have been involved a lot with ext3 development, > which kinds of workloads do you think will show a performance > degradation with Arjan's patch? What should I test? Gee. Expect the unexpected ;) One problem might be when kjournald is doing its ordered-mode data writeback at the start of commit. That writeout will now be higher-priority and might impact other tasks which are doing synchronous file overwrites (ie: no commits) or O_DIRECT reads or writes or just plain old reads. If the aggregate number of seeks over the long term is the same as before then of course the overall throughput should be the same, in which case the impact might only be upon latency. However if for some reason the total number of seeks is increased then there will be throughput impacts as well. So as a starting point I guess one could set up a copy-a-kernel-tree-in-a-loop running in the background and then see what impact that has upon a large-linear-read, upon a read-a-different-kernel-tree and upon some database-style multithreaded O_DIRECT reader/overwriter.