From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id 9B66A6B00A1 for ; Thu, 26 Nov 2009 08:47:13 -0500 (EST) Received: by gxk24 with SMTP id 24so675441gxk.6 for ; Thu, 26 Nov 2009 05:47:12 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20091126121945.GB13095@csn.ul.ie> References: <20091126121945.GB13095@csn.ul.ie> Date: Thu, 26 Nov 2009 14:47:10 +0100 Message-ID: <4e5e476b0911260547r33424098v456ed23203a61dd@mail.gmail.com> Subject: Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 From: Corrado Zoccolo Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: 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, Nov 26, 2009 at 1:19 PM, Mel Gorman wrote: > (cc'ing the people from the page allocator failure thread as this might b= e > relevant to some of their problems) > > I know this is very last minute but I believe we should consider disablin= g > the "low_latency" tunable for block devices by default for 2.6.32. =C2=A0= There was > evidence that low_latency was a problem last week for page allocation fai= lure > reports but the reproduction-case was unusual and involved high-order ato= mic > allocations in low-memory conditions. It took another few days to accurat= ely > show the problem for more normal workloads and it's a bit more wide-sprea= d > 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 th= ey > 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 expe= cted. The low latency tunable controls various policies inside cfq. The one that could affect memory reclaim is: /* * Async queues must wait a bit before being allowed dispatch. * We also ramp up the dispatch depth gradually for async IO, * based on the last sync IO we serviced */ if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_latency) { unsigned long last_sync =3D jiffies - cfqd->last_end_sync_r= q; unsigned int depth; depth =3D last_sync / cfqd->cfq_slice[1]; if (!depth && !cfqq->dispatched) depth =3D 1; if (depth < max_dispatch) max_dispatch =3D depth; } here the async queues max depth is limited to 1 for up to 200 ms after a sync I/O is completed. Note: dirty page writeback goes through an async queue, so it is penalized by this. This can affect both low and high end hardware. My non-NCQ sata disk can handle a depth of 2 when writing. NCQ sata disks can handle a depth up to 31, so limiting depth to 1 can cause write performance drop, and this in turn will slow down dirty page reclaim, and cause allocation failures. It would be good to re-test the OOM conditions with that code commented out= . > > To show the problem, I used an x86-64 machine booting booted with 512MB o= f > memory. This is a small amount of RAM but the bug reports related to page > allocation failures were on smallish machines and the disks in the system > are not very high-performance. > > I used three tests. The first was sysbench on postgres running an IO-heav= y > test against a large database with 10,000,000 rows. The second was IOZone > running most of the automatic tests with a record length of 4KB and the > last was a simulated launching of gitk with a music player running in the > background to act as a desktop-like scenario. The final test was similar > to the test described here http://lwn.net/Articles/362184/ except that > dm-crypt was not used as it has its own problems. low_latency was tested on other scenarios: http://lkml.indiana.edu/hypermail/linux/kernel/0910.0/01410.html http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-11/msg04855.html where it improved actual and perceived performance, so disabling it completely may not be good. Thanks, Corrado -- 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