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Message-ID: <20100917075204.GA1998@csn.ul.ie> Date: Fri, 17 Sep 2010 08:52:05 +0100 From: Mel Gorman <mel@....ul.ie> To: Andrew Morton <akpm@...ux-foundation.org> Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, Linux Kernel List <linux-kernel@...r.kernel.org>, Johannes Weiner <hannes@...xchg.org>, Minchan Kim <minchan.kim@...il.com>, Wu Fengguang <fengguang.wu@...el.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> Subject: Re: [PATCH 0/8] Reduce latencies and improve overall reclaim efficiency v2 On Thu, Sep 16, 2010 at 03:28:04PM -0700, Andrew Morton wrote: > On Wed, 15 Sep 2010 13:27:43 +0100 > Mel Gorman <mel@....ul.ie> wrote: > > > This is v2 of a series to reduce some of the latencies seen in page reclaim > > and to improve the efficiency a bit. > > epic changelog! > Thanks > > > > ... > > > > The tests run were as follows > > > > kernbench > > compile-based benchmark. Smoke test performance > > > > sysbench > > OLTP read-only benchmark. Will be re-run in the future as read-write > > > > micro-mapped-file-stream > > This is a micro-benchmark from Johannes Weiner that accesses a > > large sparse-file through mmap(). It was configured to run in only > > single-CPU mode but can be indicative of how well page reclaim > > identifies suitable pages. > > > > stress-highalloc > > Tries to allocate huge pages under heavy load. > > > > kernbench, iozone and sysbench did not report any performance regression > > on any machine. sysbench did pressure the system lightly and there was reclaim > > activity but there were no difference of major interest between the kernels. > > > > X86-64 micro-mapped-file-stream > > > > traceonly-v2r2 lowlumpy-v2r3 waitcongest-v2r3 waitwriteback-v2r4 > > pgalloc_dma 1639.00 ( 0.00%) 667.00 (-145.73%) 1167.00 ( -40.45%) 578.00 (-183.56%) > > pgalloc_dma32 2842410.00 ( 0.00%) 2842626.00 ( 0.01%) 2843043.00 ( 0.02%) 2843014.00 ( 0.02%) > > pgalloc_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgsteal_dma 729.00 ( 0.00%) 85.00 (-757.65%) 609.00 ( -19.70%) 125.00 (-483.20%) > > pgsteal_dma32 2338721.00 ( 0.00%) 2447354.00 ( 4.44%) 2429536.00 ( 3.74%) 2436772.00 ( 4.02%) > > pgsteal_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgscan_kswapd_dma 1469.00 ( 0.00%) 532.00 (-176.13%) 1078.00 ( -36.27%) 220.00 (-567.73%) > > pgscan_kswapd_dma32 4597713.00 ( 0.00%) 4503597.00 ( -2.09%) 4295673.00 ( -7.03%) 3891686.00 ( -18.14%) > > pgscan_kswapd_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgscan_direct_dma 71.00 ( 0.00%) 134.00 ( 47.01%) 243.00 ( 70.78%) 352.00 ( 79.83%) > > pgscan_direct_dma32 305820.00 ( 0.00%) 280204.00 ( -9.14%) 600518.00 ( 49.07%) 957485.00 ( 68.06%) > > pgscan_direct_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pageoutrun 16296.00 ( 0.00%) 21254.00 ( 23.33%) 18447.00 ( 11.66%) 20067.00 ( 18.79%) > > allocstall 443.00 ( 0.00%) 273.00 ( -62.27%) 513.00 ( 13.65%) 1568.00 ( 71.75%) > > > > These are based on the raw figures taken from /proc/vmstat. It's a rough > > measure of reclaim activity. Note that allocstall counts are higher because > > we are entering direct reclaim more often as a result of not sleeping in > > congestion. In itself, it's not necessarily a bad thing. It's easier to > > get a view of what happened from the vmscan tracepoint report. > > > > FTrace Reclaim Statistics: vmscan > > > > traceonly-v2r2 lowlumpy-v2r3 waitcongest-v2r3 waitwriteback-v2r4 > > Direct reclaims 443 273 513 1568 > > Direct reclaim pages scanned 305968 280402 600825 957933 > > Direct reclaim pages reclaimed 43503 19005 30327 117191 > > Direct reclaim write file async I/O 0 0 0 0 > > Direct reclaim write anon async I/O 0 3 4 12 > > Direct reclaim write file sync I/O 0 0 0 0 > > Direct reclaim write anon sync I/O 0 0 0 0 > > Wake kswapd requests 187649 132338 191695 267701 > > Kswapd wakeups 3 1 4 1 > > Kswapd pages scanned 4599269 4454162 4296815 3891906 > > Kswapd pages reclaimed 2295947 2428434 2399818 2319706 > > Kswapd reclaim write file async I/O 1 0 1 1 > > Kswapd reclaim write anon async I/O 59 187 41 222 > > Kswapd reclaim write file sync I/O 0 0 0 0 > > Kswapd reclaim write anon sync I/O 0 0 0 0 > > Time stalled direct reclaim (seconds) 4.34 2.52 6.63 2.96 > > Time kswapd awake (seconds) 11.15 10.25 11.01 10.19 > > > > Total pages scanned 4905237 4734564 4897640 4849839 > > Total pages reclaimed 2339450 2447439 2430145 2436897 > > %age total pages scanned/reclaimed 47.69% 51.69% 49.62% 50.25% > > %age total pages scanned/written 0.00% 0.00% 0.00% 0.00% > > %age file pages scanned/written 0.00% 0.00% 0.00% 0.00% > > Percentage Time Spent Direct Reclaim 29.23% 19.02% 38.48% 20.25% > > Percentage Time kswapd Awake 78.58% 78.85% 76.83% 79.86% > > > > What is interesting here for nocongest in particular is that while direct > > reclaim scans more pages, the overall number of pages scanned remains the same > > and the ratio of pages scanned to pages reclaimed is more or less the same. In > > other words, while we are sleeping less, reclaim is not doing more work and > > as direct reclaim and kswapd is awake for less time, it would appear to be doing less work. > > Yes, I think the reclaimed/scanned ratio (what I call "reclaim > efficiency") is a key metric. > Indeed. > 50% is low! What's the testcase here? micro-mapped-file-stream? > It's a streaming write workload Johannes posted at http://linux--kernel.googlegroups.com/attach/922930ad782c993f/mapped-file-stream.c?gda=C9ZmZUYAAAC7YRbTg15qnVftAVpdAUbEdtSiuVqDFQ7IygxgoOgCJibbrMllVnGRuK4kFCYFogdx40jamwa1UURqDcgHarKEE-Ea7GxYMt0t6nY0uV5FIQ&part=2 He considered it to be a somewhat adverse workload for reclaim. > It's strange that the "total pages reclaimed" increased a little. Just > a measurement glitch? > Probably not a glitch but the measurements are system-wide. Depending on the starting state of the system when the benchmark ran, there will be slightly different scanning numbers. > > FTrace Reclaim Statistics: congestion_wait > > Direct number congest waited 87 196 64 0 > > Direct time congest waited 4604ms 4732ms 5420ms 0ms > > Direct full congest waited 72 145 53 0 > > Direct number conditional waited 0 0 324 1315 > > Direct time conditional waited 0ms 0ms 0ms 0ms > > Direct full conditional waited 0 0 0 0 > > KSwapd number congest waited 20 10 15 7 > > KSwapd time congest waited 1264ms 536ms 884ms 284ms > > KSwapd full congest waited 10 4 6 2 > > KSwapd number conditional waited 0 0 0 0 > > KSwapd time conditional waited 0ms 0ms 0ms 0ms > > KSwapd full conditional waited 0 0 0 0 > > > > The vanilla kernel spent 8 seconds asleep in direct reclaim and no time at > > all asleep with the patches. > > > > MMTests Statistics: duration > > User/Sys Time Running Test (seconds) 10.51 10.73 10.6 11.66 > > Total Elapsed Time (seconds) 14.19 13.00 14.33 12.76 > > Is that user time plus system time? Yes. > If so, why didn't user+sys equal > elapsed in the we-never-slept-in-congestion-wait() case? Because the > test's CPU got stolen by kswapd perhaps? > One possibility. The other is IO wait time. I'll think about it some more. I'm afraid this mail is a bit rushed because I'm about to leave for a wedding. I won't be back online until Monday. > > Overall, the tests completed faster. It is interesting to note that backing off further > > when a zone is congested and not just a BDI was more efficient overall. > > > > PPC64 micro-mapped-file-stream > > pgalloc_dma 3024660.00 ( 0.00%) 3027185.00 ( 0.08%) 3025845.00 ( 0.04%) 3026281.00 ( 0.05%) > > pgalloc_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgsteal_dma 2508073.00 ( 0.00%) 2565351.00 ( 2.23%) 2463577.00 ( -1.81%) 2532263.00 ( 0.96%) > > pgsteal_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgscan_kswapd_dma 4601307.00 ( 0.00%) 4128076.00 ( -11.46%) 3912317.00 ( -17.61%) 3377165.00 ( -36.25%) > > pgscan_kswapd_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pgscan_direct_dma 629825.00 ( 0.00%) 971622.00 ( 35.18%) 1063938.00 ( 40.80%) 1711935.00 ( 63.21%) > > pgscan_direct_normal 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%) > > pageoutrun 27776.00 ( 0.00%) 20458.00 ( -35.77%) 18763.00 ( -48.04%) 18157.00 ( -52.98%) > > allocstall 977.00 ( 0.00%) 2751.00 ( 64.49%) 2098.00 ( 53.43%) 5136.00 ( 80.98%) > > > > ... > > > > > > X86-64 STRESS-HIGHALLOC > > traceonly-v2r2 lowlumpy-v2r3 waitcongest-v2r3waitwriteback-v2r4 > > Pass 1 82.00 ( 0.00%) 84.00 ( 2.00%) 85.00 ( 3.00%) 85.00 ( 3.00%) > > Pass 2 90.00 ( 0.00%) 87.00 (-3.00%) 88.00 (-2.00%) 89.00 (-1.00%) > > At Rest 92.00 ( 0.00%) 90.00 (-2.00%) 90.00 (-2.00%) 91.00 (-1.00%) > > > > Success figures across the board are broadly similar. > > > > traceonly-v2r2 lowlumpy-v2r3 waitcongest-v2r3waitwriteback-v2r4 > > Direct reclaims 1045 944 886 887 > > Direct reclaim pages scanned 135091 119604 109382 101019 > > Direct reclaim pages reclaimed 88599 47535 47863 46671 > > Direct reclaim write file async I/O 494 283 465 280 > > Direct reclaim write anon async I/O 29357 13710 16656 13462 > > Direct reclaim write file sync I/O 154 2 2 3 > > Direct reclaim write anon sync I/O 14594 571 509 561 > > Wake kswapd requests 7491 933 872 892 > > Kswapd wakeups 814 778 731 780 > > Kswapd pages scanned 7290822 15341158 11916436 13703442 > > Kswapd pages reclaimed 3587336 3142496 3094392 3187151 > > Kswapd reclaim write file async I/O 91975 32317 28022 29628 > > Kswapd reclaim write anon async I/O 1992022 789307 829745 849769 > > Kswapd reclaim write file sync I/O 0 0 0 0 > > Kswapd reclaim write anon sync I/O 0 0 0 0 > > Time stalled direct reclaim (seconds) 4588.93 2467.16 2495.41 2547.07 > > Time kswapd awake (seconds) 2497.66 1020.16 1098.06 1176.82 > > > > Total pages scanned 7425913 15460762 12025818 13804461 > > Total pages reclaimed 3675935 3190031 3142255 3233822 > > %age total pages scanned/reclaimed 49.50% 20.63% 26.13% 23.43% > > %age total pages scanned/written 28.66% 5.41% 7.28% 6.47% > > %age file pages scanned/written 1.25% 0.21% 0.24% 0.22% > > Percentage Time Spent Direct Reclaim 57.33% 42.15% 42.41% 42.99% > > Percentage Time kswapd Awake 43.56% 27.87% 29.76% 31.25% > > > > Scanned/reclaimed ratios again look good with big improvements in > > efficiency. The Scanned/written ratios also look much improved. With a > > better scanned/written ration, there is an expectation that IO would be more > > efficient and indeed, the time spent in direct reclaim is much reduced by > > the full series and kswapd spends a little less time awake. > > Wait. The reclaim efficiency got *worse*, didn't it? To reclaim > 3,xxx,xxx pages, the number of pages we had to scan went from 7,xxx,xxx > up to 13,xxx,xxx? > Arguably, yes. The biggest change here is due to lumpy reclaim giving up a range of pages when one fails to reclaim. An impact of this is that it will end up scanning more for a suitable contiguous range of pages because it aborted trying to reclaim the same page stupidly. So, it looks worse from a scanning/reclaim perspective but it's more sensible behaviour (and finishes faster) Similarly, when reclaimers are no longer unnecessarily sleeping, they have more time to be scanning pushing up the rates slightly. The allocation success rates are slightly higher which might be a reflection of the higher scanning. The reclaim efficiency is improved by the later two patches again and while not as good as the "vanilla" kernel, that only has good efficiency figures because it's grinding on the same useless pages chewing up CPU time. Overall, it's still better behaviour. > > > > ... > > > > I think this series is ready for much wider testing. The lowlumpy patches in > > particular should be relatively uncontroversial. While their largest impact > > can be seen in the high order stress tests, they would also have an impact > > if SLUB was configured (these tests are based on slab) and stalls in lumpy > > reclaim could be partially responsible for some desktop stalling reports. > > slub sucks :( > > Is this patchset likely to have any impact on the "hey my net driver > couldn't do an order 3 allocation" reports? I guess not. > Some actually. direct reclaimers and kswapd are not going to waste as much time trying to reclaim those order-3 pages so there will be less stalling and kswapd might keep ahead of the rush of allocators. Sorry I won't get the chance to respond to other mails for the next few days. Have to hit the road. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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