Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760384AbZKZORh (ORCPT ); Thu, 26 Nov 2009 09:17:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754458AbZKZORg (ORCPT ); Thu, 26 Nov 2009 09:17:36 -0500 Received: from gir.skynet.ie ([193.1.99.77]:40318 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753899AbZKZORf (ORCPT ); Thu, 26 Nov 2009 09:17:35 -0500 Date: Thu, 26 Nov 2009 14:17:38 +0000 From: Mel Gorman To: Corrado Zoccolo 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 Subject: Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 Message-ID: <20091126141738.GE13095@csn.ul.ie> References: <20091126121945.GB13095@csn.ul.ie> <4e5e476b0911260547r33424098v456ed23203a61dd@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4e5e476b0911260547r33424098v456ed23203a61dd@mail.gmail.com> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6011 Lines: 140 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. ====== cfq: Do not limit the async queue depth while kswapd is awake diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index aa1e953..dcab74e 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1308,7 +1308,7 @@ 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 && !kswapd_awake()) { unsigned long last_sync = jiffies - cfqd->last_end_sync_rq; unsigned int depth; diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 6f75617..b593aff 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -655,6 +655,7 @@ typedef struct pglist_data { void get_zone_counts(unsigned long *active, unsigned long *inactive, unsigned long *free); void build_all_zonelists(void); +int kswapd_awake(void); void wakeup_kswapd(struct zone *zone, int order); int zone_watermark_ok(struct zone *z, int order, unsigned long mark, int classzone_idx, int alloc_flags); diff --git a/mm/vmscan.c b/mm/vmscan.c index 777af57..75cdd9a 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2201,6 +2201,15 @@ static int kswapd(void *p) return 0; } +int kswapd_awake(void) +{ + pg_data_t *pgdat; + for_each_online_pgdat(pgdat) + if (!waitqueue_active(&pgdat->kswapd_wait)) + return 1; + return 0; +} + /* * A zone is low on free memory, so wake its kswapd task to service it. */ -- 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/