From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753958AbXJVJuq (ORCPT ); Mon, 22 Oct 2007 05:50:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751175AbXJVJuj (ORCPT ); Mon, 22 Oct 2007 05:50:39 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:51559 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751139AbXJVJuj (ORCPT ); Mon, 22 Oct 2007 05:50:39 -0400 Date: Mon, 22 Oct 2007 02:49:33 -0700 From: Andrew Morton To: Ingo Molnar Cc: Jens Axboe , Arjan van de Ven , linux-kernel@vger.kernel.org Subject: Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority Message-Id: <20071022024933.2381e544.akpm@linux-foundation.org> In-Reply-To: <20071022094014.GB7659@elte.hu> References: <20071015104647.14e60bc5@laptopd505.fenrus.org> <20071015114738.6b5a25c7.akpm@linux-foundation.org> <20071015192822.GA2267@kernel.dk> <20071022091057.GB2781@elte.hu> <20071022022319.a5988afe.akpm@linux-foundation.org> <20071022094014.GB7659@elte.hu> X-Mailer: Sylpheed 2.4.1 (GTK+ 2.8.17; x86_64-unknown-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, 22 Oct 2007 11:40:14 +0200 Ingo Molnar wrote: > > * Andrew Morton wrote: > > > > so lets just goddamn apply this _trivial_ patch. This isnt an > > > intrusive 1000 line rewrite that is hard to revert. If it causes any > > > bandwidth problems, it will be just as trivial to undo. If we do > > > anything else we just stiffle the still young and very much > > > under-represented "lets fix latencies that bothers people" movement. > > > If anything we need _positive_ discrimination for latency related > > > fixes (which treatment this fix does not need at all - all it needs > > > is _equal_ footing with the countless bandwidth patches that go into > > > the kernel all the time), otherwise it will never take off and > > > become as healthy as bandwidth optimizations. Ok? > > > > I think the situation is that we've asked for some additional > > what-can-be-hurt-by-this testing. > > > > Yes, we could sling it out there and wait for the reports. But often > > that's a pretty painful process and regressions can be discovered too > > late for us to do anything about them. > > reverting this oneliner is trivial. Finding bandwidth problems and > tracking them down to this oneliner change is relatively easy too. > Finding latency problems and fixing them is _not_ trivial. > > Boot up a Linux desktop and start OOo or firefox, and measure the time > it takes to start the app up. 10-20 seconds on a top-of-the-line > quad-core 3.2 GHz system - which is a shame. Same box can do in excess > of 1GB/sec block IO. Yes, one could blame the apps but in reality most > of the blame is mostly on the kernel side. We do not make bloat and > latency suckage apparent enough to user-space (due to lack of > intelligent instrumentation), we make latencies hard to fix, we have an > acceptance bias towards bandwidth fixes (because they are easier to > measure and justify) - and that's all what it takes to let such a > situation get out of control. > > and i can bring up the scheduler as an example. CFS broke the bandwidth > performance of one particular app and it took only a few days to get it > back under control. But it was months to get good latency behavior out > of the scheduler. And that is with the help of excellent scheduler > instrumentation. In the IO space the latency situation is much, much > worse. Really. > None of which is an argument for simply not bothering to do a bit more developer testing before merging.