From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with SMTP id 85C3C6B009E for ; Thu, 26 Nov 2009 08:37:38 -0500 (EST) Subject: Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 From: Mike Galbraith 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 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org 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 List-ID: 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org