Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760041AbZKZNhc (ORCPT ); Thu, 26 Nov 2009 08:37:32 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759570AbZKZNhc (ORCPT ); Thu, 26 Nov 2009 08:37:32 -0500 Received: from mail.gmx.net ([213.165.64.20]:35197 "HELO mail.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1753306AbZKZNhb (ORCPT ); Thu, 26 Nov 2009 08:37:31 -0500 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX19P2xw5SrBPoz/qjKSbhVu8jO0hs7WbF4GPC3GsiV cljlRd94X2Q0Am Subject: Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 From: Mike Galbraith To: Bartlomiej Zolnierkiewicz Cc: Mel Gorman , Jens Axboe , Andrew Morton , Linus Torvalds , Frans Pop , Jiri Kosina , Sven Geggus , Karol Lewandowski , Tobias Oetiker , KOSAKI Motohiro , Pekka Enberg , Rik van Riel , Christoph Lameter , Stephan von Krawczynski , "Rafael J. Wysocki" , linux-kernel@vger.kernel.org, linux-mm@kvack.org In-Reply-To: <200911261420.57121.bzolnier@gmail.com> References: <20091126121945.GB13095@csn.ul.ie> <1259240937.7371.15.camel@marge.simson.net> <200911261420.57121.bzolnier@gmail.com> Content-Type: text/plain Date: Thu, 26 Nov 2009 14:37:31 +0100 Message-Id: <1259242651.6622.5.camel@marge.simson.net> Mime-Version: 1.0 X-Mailer: Evolution 2.24.1.1 Content-Transfer-Encoding: 7bit X-Y-GMX-Trusted: 0 X-FuHaFi: 0.6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2347 Lines: 44 On Thu, 2009-11-26 at 14:20 +0100, Bartlomiej Zolnierkiewicz wrote: > On Thursday 26 November 2009 02:08:57 pm Mike Galbraith wrote: > > On Thu, 2009-11-26 at 12:19 +0000, Mel Gorman wrote: > > > (cc'ing the people from the page allocator failure thread as this might be > > > relevant to some of their problems) > > > > > > I know this is very last minute but I believe we should consider disabling > > > the "low_latency" tunable for block devices by default for 2.6.32. There was > > > evidence that low_latency was a problem last week for page allocation failure > > > reports but the reproduction-case was unusual and involved high-order atomic > > > allocations in low-memory conditions. It took another few days to accurately > > > show the problem for more normal workloads and it's a bit more wide-spread > > > than just allocation failures. > > > > > > Basically, low_latency looks great as long as you have plenty of memory > > > but in low memory situations, it appears to cause problems that manifest > > > as reduced performance, desktop stalls and in some cases, page allocation > > > failures. I think most kernel developers are not seeing the problem as they > > > tend to test on beefier machines and without hitting swap or low-memory > > > situations for the most part. When they are hitting low-memory situations, > > > it tends to be for stress tests where stalls and low performance are expected. > > > > Ouch. It was bad desktop stalls under heavy write that kicked the whole > > thing off. > > The problem is that 'desktop' means different things for different people > (for some kernel developers 'desktop' is more like 'a workstation' and for > others it is more like 'an embedded device'). The stalls I'm talking about were reported for garden variety desktop PC. I reproduced them on my supermarket special Q6600 desktop PC. That problem has been with us roughly forever, but I'd hoped it had been cured. Guess not. As an idle speculation, I wonder if the sync vs async slice ratios may not have been knocked out of kilter a bit by giving more to sync. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/