Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754012AbZK0F60 (ORCPT ); Fri, 27 Nov 2009 00:58:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753987AbZK0F60 (ORCPT ); Fri, 27 Nov 2009 00:58:26 -0500 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:37298 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752856AbZK0F6X convert rfc822-to-8bit (ORCPT ); Fri, 27 Nov 2009 00:58:23 -0500 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 From: KOSAKI Motohiro To: Mel Gorman Subject: Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 Cc: kosaki.motohiro@jp.fujitsu.com, Corrado Zoccolo , Jens Axboe , Andrew Morton , Linus Torvalds , Frans Pop , Jiri Kosina , Sven Geggus , Karol Lewandowski , Tobias Oetiker , 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: <20091126141738.GE13095@csn.ul.ie> References: <4e5e476b0911260547r33424098v456ed23203a61dd@mail.gmail.com> <20091126141738.GE13095@csn.ul.ie> Message-Id: <20091127143307.A7E1.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: 8BIT X-Mailer: Becky! ver. 2.50.07 [ja] Date: Fri, 27 Nov 2009 14:58:26 +0900 (JST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6331 Lines: 139 > On Thu, Nov 26, 2009 at 02:47:10PM +0100, Corrado Zoccolo wrote: > > 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 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. > > > > 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 = jiffies - cfqd->last_end_sync_rq; > > unsigned int depth; > > > > depth = last_sync / cfqd->cfq_slice[1]; > > if (!depth && !cfqq->dispatched) > > depth = 1; > > if (depth < max_dispatch) > > max_dispatch = 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. > > > > All of it or just the cfq_latency part? > > As it turns out the test machine does report for the disk NCQ (depth 31/32) > and it's the same on the laptop so slowing down dirty page cleaning > could be impacting reclaim. > > > > > > > To show the problem, I used an x86-64 machine booting booted with 512MB of > > > 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-heavy > > > 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. > > > > It may not indeed. > > In case you mean a partial disabling of cfq_latency, I'm try the > following patch. The intention is to disable the low_latency logic if > kswapd is at work and presumably needs clean pages. Alternative > suggestions welcome. I like treat vmscan writeout as special. because - vmscan use various process context. but it doesn't write own process's page. IOW, it doesn't so match cfq's io fairness logic. - plus, the above mean vmscan writeout doesn't need good i/o latency. - vmscan maintain page granularity lru list. It mean vmscan makes awful seekful I/O. it assume block-layer buffered much i/o request. - plus, the above mena vmscan. writeout need good io throughput. otherwise system might cause hangup. However, I don't think kswapd_awake is good choice. because - zone reclaim run before kswapd wakeup. iow, this patch doesn't solve hpc machine. btw, some Core i7 box (at least, Intel's reference box) also use zone reclaim. - On large (many memory node) machine, one of much kswapd always run. Instead, PF_MEMALLOC is good idea? Subject: [PATCH] cfq: Do not limit the async queue depth while memory reclaim Not-Signed-off-by: KOSAKI Motohiro (I haven't test this) --- block/cfq-iosched.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index aa1e953..9546f64 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1308,7 +1308,8 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq) * 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) { + if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_latency && + !(current->flags & PF_MEMALLOC)) { unsigned long last_sync = jiffies - cfqd->last_end_sync_rq; unsigned int depth; -- 1.6.5.2 -- 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/