congestion_wait() is a bit stupid in that it goes to sleep even when there
is no congestion. This causes stalls in a number of situations and may be
partially responsible for bug reports about desktop interactivity.
This patch series aims to account for these unnecessary congestion_waits()
and to avoid going to sleep when there is no congestion available. Patches
1 and 2 add instrumentation related to congestion which should be reuable
by alternative solutions to congestion_wait. Patch 3 calls cond_resched()
instead of going to sleep if there is no congestion.
Once again, I shoved this through performance test. Unlike previous tests,
I ran this on a ported version of my usual test-suite that should be suitable
for release soon. It's not quite as good as my old set but it's sufficient
for this and related series. The tests I ran were kernbench vmr-stream
iozone hackbench-sockets hackbench-pipes netperf-udp netperf-tcp sysbench
stress-highalloc. Sysbench was a read/write tests and stress-highalloc is
the usual stress the number of high order allocations that can be made while
the system is under severe stress. The suite contains the necessary analysis
scripts as well and I'd release it now except the documentation blows.
x86: Intel Pentium D 3GHz with 3G RAM (no-brand machine)
x86-64: AMD Phenom 9950 1.3GHz with 3G RAM (no-brand machine)
ppc64: PPC970MP 2.5GHz with 3GB RAM (it's a terrasoft powerstation)
The disks on all of them were single disks and not particularly fast.
Comparison was between a 2.6.36-rc1 with patches 1 and 2 applied for
instrumentation and a second test with patch 3 applied.
In all cases, kernbench, hackbench, STREAM and iozone did not show any
performance difference because none of them were pressuring the system
enough to be calling congestion_wait() so I won't post the results.
About all worth noting for them is that nothing horrible appeared to break.
In the analysis scripts, I record unnecessary sleeps to be a sleep that
had no congestion. The post-processing scripts for cond_resched() will only
count an uncongested call to congestion_wait() as unnecessary if the process
actually gets scheduled. Ordinarily, we'd expect it to continue uninterrupted.
One vague concern I have is when too many pages are isolated, we call
congestion_wait(). This could now actively spin in the loop for its quanta
before calling cond_resched(). If it's calling with no congestion, it's
hard to know what the proper thing to do there is.
X86
Sysbench on this machine was not stressed enough to call congestion_wait
so I'll just discuss the stress-highalloc test. This is the full report
from the testsuite
STRESS-HIGHALLOC
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Pass 1 70.00 ( 0.00%) 72.00 ( 2.00%)
Pass 2 72.00 ( 0.00%) 72.00 ( 0.00%)
At Rest 74.00 ( 0.00%) 73.00 (-1.00%)
FTrace Reclaim Statistics: vmscan
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Direct reclaims 409 755
Direct reclaim pages scanned 185585 212524
Direct reclaim write file async I/O 442 554
Direct reclaim write anon async I/O 31789 27074
Direct reclaim write file sync I/O 17 23
Direct reclaim write anon sync I/O 17825 15013
Wake kswapd requests 895 1274
Kswapd wakeups 387 432
Kswapd pages scanned 16373859 12892992
Kswapd reclaim write file async I/O 29267 18188
Kswapd reclaim write anon async I/O 1243386 1080234
Kswapd reclaim write file sync I/O 0 0
Kswapd reclaim write anon sync I/O 0 0
Time stalled direct reclaim (seconds) 4479.04 3446.81
Time kswapd awake (seconds) 2229.99 1218.52
Total pages scanned 16559444 13105516
%age total pages scanned/written 7.99% 8.71%
%age file pages scanned/written 0.18% 0.14%
Percentage Time Spent Direct Reclaim 74.99% 69.54%
Percentage Time kswapd Awake 41.78% 28.57%
FTrace Reclaim Statistics: congestion_wait
Direct number congest waited 474 38
Direct number schedule waited 0 9478
Direct time congest waited 21564ms 3732ms
Direct time schedule waited 0ms 4ms
Direct unnecessary wait 434 1
KSwapd number congest waited 68 0
KSwapd number schedule waited 0 0
KSwapd time schedule waited 0ms 0ms
KSwapd time congest waited 5424ms 0ms
Kswapd unnecessary wait 44 0
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 1493.97 1509.88
Total Elapsed Time (seconds) 5337.71 4265.07
Allocations under stress were slightly better but by and large there is no
significant difference in success rates. The test completed 1072 seconds
faster which is a pretty decent speedup.
Scanning rates in reclaim were higher but that is somewhat expected because
we weren't going to sleep as much. Time stalled in reclaim for both direct
and kswapd was reduced which is pretty significant.
In terms of congestion_wait, the time spent asleep was massively reduced
by 17 seconds for direct reclaim and 5 seconds for kswapd. cond_reched
is called a number of times instead of course but the time it spent
being scheduled was a mere 4ms. Overall, this looked positive.
X86-64
Sysbench again wasn't under enough pressure so here is the high alloc test.
STRESS-HIGHALLOC
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Pass 1 69.00 ( 0.00%) 73.00 ( 4.00%)
Pass 2 71.00 ( 0.00%) 74.00 ( 3.00%)
At Rest 72.00 ( 0.00%) 75.00 ( 3.00%)
FTrace Reclaim Statistics: vmscan
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Direct reclaims 646 1091
Direct reclaim pages scanned 94779 102392
Direct reclaim write file async I/O 164 216
Direct reclaim write anon async I/O 12162 15413
Direct reclaim write file sync I/O 64 45
Direct reclaim write anon sync I/O 5366 6987
Wake kswapd requests 3950 3912
Kswapd wakeups 613 579
Kswapd pages scanned 7544412 7267203
Kswapd reclaim write file async I/O 14660 16256
Kswapd reclaim write anon async I/O 964824 1065445
Kswapd reclaim write file sync I/O 0 0
Kswapd reclaim write anon sync I/O 0 0
Time stalled direct reclaim (seconds) 3279.00 3564.59
Time kswapd awake (seconds) 1445.70 1870.70
Total pages scanned 7639191 7369595
%age total pages scanned/written 13.05% 14.99%
%age file pages scanned/written 0.19% 0.22%
Percentage Time Spent Direct Reclaim 70.48% 72.04%
Percentage Time kswapd Awake 35.62% 42.94%
FTrace Reclaim Statistics: congestion_wait
Direct number congest waited 801 97
Direct number schedule waited 0 16079
Direct time congest waited 37448ms 9004ms
Direct time schedule waited 0ms 0ms
Direct unnecessary wait 696 0
KSwapd number congest waited 10 1
KSwapd number schedule waited 0 0
KSwapd time schedule waited 0ms 0ms
KSwapd time congest waited 900ms 100ms
Kswapd unnecessary wait 6 0
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 1373.11 1383.7
Total Elapsed Time (seconds) 4058.33 4356.47
Success rates were slightly higher again, not by a massive amount but some.
Time to complete the test was unfortunately increased slightly though and
I'm not sure where that is coming from. The increased number of successful
allocations would account for some of that because the system is under
greater memory pressure as a result of the allocations.
Scanning rates are comparable. Writing back files from reclaim was slighly
increased which I believe it due to less time being spent asleep so there
was a smaller window for the flusher threads to do their work. Reducing
that is the responsibility of another series.
Again, the time spent asleep in congestion_wait() is reduced by a large
amount - 28 seconds for direct reclaim and none of the cond_resched()
resulted in sleep times.
Overally, seems reasonable.
PPC64
Unlike the other two machines, sysbench called congestion_wait a few times so here are the full
results for sysbench
SYSBENCH
sysbench-traceonly-v1r1-sysbenchsysbench-nocongest-v1r1-sysbench
traceonly-v1r1 nocongest-v1r1
1 5307.36 ( 0.00%) 5349.58 ( 0.79%)
2 9886.45 ( 0.00%) 10274.78 ( 3.78%)
3 14165.01 ( 0.00%) 14210.64 ( 0.32%)
4 16239.12 ( 0.00%) 16201.46 (-0.23%)
5 15337.09 ( 0.00%) 15541.56 ( 1.32%)
6 14763.64 ( 0.00%) 15805.80 ( 6.59%)
7 14216.69 ( 0.00%) 15023.57 ( 5.37%)
8 13749.62 ( 0.00%) 14492.34 ( 5.12%)
9 13647.75 ( 0.00%) 13969.77 ( 2.31%)
10 13275.70 ( 0.00%) 13495.08 ( 1.63%)
11 13324.91 ( 0.00%) 12879.81 (-3.46%)
12 13169.23 ( 0.00%) 12967.36 (-1.56%)
13 12896.20 ( 0.00%) 12981.43 ( 0.66%)
14 12793.44 ( 0.00%) 12768.26 (-0.20%)
15 12627.98 ( 0.00%) 12522.86 (-0.84%)
16 12228.54 ( 0.00%) 12352.07 ( 1.00%)
FTrace Reclaim Statistics: vmscan
sysbench-traceonly-v1r1-sysbenchsysbench-nocongest-v1r1-sysbench
traceonly-v1r1 nocongest-v1r1
Direct reclaims 0 0
Direct reclaim pages scanned 0 0
Direct reclaim write file async I/O 0 0
Direct reclaim write anon async I/O 0 0
Direct reclaim write file sync I/O 0 0
Direct reclaim write anon sync I/O 0 0
Wake kswapd requests 0 0
Kswapd wakeups 202 194
Kswapd pages scanned 5990987 5618709
Kswapd reclaim write file async I/O 24 16
Kswapd reclaim write anon async I/O 1509 1564
Kswapd reclaim write file sync I/O 0 0
Kswapd reclaim write anon sync I/O 0 0
Time stalled direct reclaim (seconds) 0.00 0.00
Time kswapd awake (seconds) 174.23 152.17
Total pages scanned 5990987 5618709
%age total pages scanned/written 0.03% 0.03%
%age file pages scanned/written 0.00% 0.00%
Percentage Time Spent Direct Reclaim 0.00% 0.00%
Percentage Time kswapd Awake 2.80% 2.60%
FTrace Reclaim Statistics: congestion_wait
Direct number congest waited 0 0
Direct number schedule waited 0 0
Direct time congest waited 0ms 0ms
Direct time schedule waited 0ms 0ms
Direct unnecessary wait 0 0
KSwapd number congest waited 10 3
KSwapd number schedule waited 0 0
KSwapd time schedule waited 0ms 0ms
KSwapd time congest waited 800ms 300ms
Kswapd unnecessary wait 6 0
Performance is improved by a decent marging although I didn't check if
it was statistically significant or not. The time kswapd spent asleep was
slightly reduced.
STRESS-HIGHALLOC
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Pass 1 40.00 ( 0.00%) 35.00 (-5.00%)
Pass 2 50.00 ( 0.00%) 45.00 (-5.00%)
At Rest 61.00 ( 0.00%) 64.00 ( 3.00%)
FTrace Reclaim Statistics: vmscan
stress-highalloc stress-highalloc
traceonly-v1r1 nocongest-v1r1
Direct reclaims 166 926
Direct reclaim pages scanned 167920 183644
Direct reclaim write file async I/O 391 412
Direct reclaim write anon async I/O 31563 31986
Direct reclaim write file sync I/O 54 52
Direct reclaim write anon sync I/O 21696 17087
Wake kswapd requests 123 128
Kswapd wakeups 143 143
Kswapd pages scanned 3899414 4229450
Kswapd reclaim write file async I/O 12392 13098
Kswapd reclaim write anon async I/O 673260 709817
Kswapd reclaim write file sync I/O 0 0
Kswapd reclaim write anon sync I/O 0 0
Time stalled direct reclaim (seconds) 1595.13 1692.18
Time kswapd awake (seconds) 1114.00 1210.48
Total pages scanned 4067334 4413094
%age total pages scanned/written 18.18% 17.50%
%age file pages scanned/written 0.32% 0.31%
Percentage Time Spent Direct Reclaim 45.89% 47.50%
Percentage Time kswapd Awake 46.09% 48.04%
FTrace Reclaim Statistics: congestion_wait
Direct number congest waited 233 16
Direct number schedule waited 0 1323
Direct time congest waited 10164ms 1600ms
Direct time schedule waited 0ms 0ms
Direct unnecessary wait 218 0
KSwapd number congest waited 11 13
KSwapd number schedule waited 0 3
KSwapd time schedule waited 0ms 0ms
KSwapd time congest waited 1100ms 1244ms
Kswapd unnecessary wait 0 0
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 1880.56 1870.36
Total Elapsed Time (seconds) 2417.17 2519.51
Allocation success rates are slightly down but on PPC64, they are always
very difficult and I have other ideas on how allocation success rates could
be improved.
What is more important is that again the time spent asleep due to
congestion_wait() was reduced for direct reclaimers.
The results here aren't as positive as the other two machines but they
still seem acceptable.
Broadly speaking, I think sleeping in congestion_wait() has been responsible
for some bugs related to stalls under large IO, particularly the read IO,
so we need to do something about it. These tests seem overall positive but
it'd be interesting if someone with a workload that stalls in congestion_wait
unnecessarily are helped by this patch. Desktop interactivity would be harder
to test because I think it has multiple root causes of which congestion_wait
is just one of them. I've included Christian Ehrhardt in the cc because he
had a bug back in April that was rooted in congestion_wait() that I think
this might help and hopefully he can provide hard data for a workload
with lots of IO but constrained memory. I cc'd Johannes because we were
discussion congestion_wait() at LSF/MM and he might have some thoughts and
I think I was talking to Jan briefly about congestion_wait() as well. As
this affects writeback, Wu and fsdevel might have some opinions.
include/trace/events/writeback.h | 22 ++++++++++++++++++++++
mm/backing-dev.c | 31 ++++++++++++++++++++++++++-----
2 files changed, 48 insertions(+), 5 deletions(-)
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