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Date: Thu, 26 Mar 2015 11:11:19 +0100
From: Daniel Wagner <daniel.wagner@...-carit.de>
To: Jeff Layton <jlayton@...chiereds.net>
CC: <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
"J. Bruce Fields" <bfields@...ldses.org>,
Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH v3 0/2] Use blocked_lock_lock only to protect blocked_hash
Hi Jeff,
Sorry for the long delay. Was I week on holiday and the testing
took a bit longer than I expected.
On 03/14/2015 01:40 PM, Jeff Layton wrote:
> On Tue, 10 Mar 2015 09:20:24 +0100
> Daniel Wagner <daniel.wagner@...-carit.de> wrote:
>> On 03/07/2015 03:00 PM, Jeff Layton wrote:
>>> On Fri, 6 Mar 2015 08:53:30 +0100
>>> Daniel Wagner <daniel.wagner@...-carit.de> wrote:
>>>
>>>> Hi,
>>>>
>>>> Finally, I got a bigger machine and did a quick test round. I expected
>>>> to see some improvements but the resutls do not show any real gain. So
>>>> they are merely refactoring patches.
>>>>
>>>
>>> Ok, in that case is there any point in merging these? I'm all for
>>> breaking up global locks when it makes sense, but if you can't
>>> demonstrate a clear benefit then I'm less inclined to take the churn.
>>>
>>> Perhaps we should wait to see if a benefit emerges when/if you convert
>>> the lglock code to use normal spinlocks (like Andi suggested)? That
>>> seems like a rather simple conversion, and I don't think it's
>>> "cheating" in any sense of the word.
>>>
>>> I do however wonder why Nick used arch_spinlock_t there when he wrote
>>> the lglock code instead of normal spinlocks. Was it simply memory usage
>>> considerations or something else?
>>
>> I did a complete test run with 4.0.0-rc3, the two patches from this
>> thread (fs-locks-v10), the spinlock_t conversion (lglock-v0)
>> and fs-locks-v10 and lglock-v0 combined. Some of the test take rather
>> long on my machine (12 minutes per run) so I tweaked it a bit to get
>> more samples. Each test was run 100 times.
>>
>> The raw data and scripts are here: http://www.monom.org/lglock/data/
>>
>> flock01
>> mean variance sigma max min
>> 4.0.0-rc3 8930.8708 282098.1663 531.1291 9612.7085 4681.8674
>> fs-locks-v10 9081.6789 43493.0287 208.5498 9747.8491 8072.6508
>> lglock-v0 9004.9252 12339.3832 111.0828 9489.5420 8493.0763
>> fs-locks-v10+lglock-v0 9053.6680 17714.5623 133.0961 9588.7413 8555.0727
>>
>>
>> flock02
>> mean variance sigma max min
>> 4.0.0-rc3 553.1720 1057.6026 32.5208 606.2989 479.5528
>> fs-locks-v10 596.0683 1486.8345 38.5595 662.6566 512.4865
>> lglock-v0 595.2150 976.8544 31.2547 646.7506 527.2517
>> fs-locks-v10+lglock-v0 587.5492 989.9467 31.4634 646.2098 509.6020
>>
>>
>> lease01
>> mean variance sigma max min
>> 4.0.0-rc3 505.2654 780.7545 27.9420 564.2530 433.7727
>> fs-locks-v10 523.6855 705.2606 26.5567 570.3401 442.6539
>> lglock-v0 516.7525 1026.7596 32.0431 573.8766 433.4124
>> fs-locks-v10+lglock-v0 513.6507 751.1674 27.4074 567.0972 435.6154
>>
>>
>> lease02
>> mean variance sigma max min
>> 4.0.0-rc3 3588.2069 26736.0422 163.5116 3769.7430 3154.8405
>> fs-locks-v10 3566.0658 34225.6410 185.0017 3747.6039 3188.5478
>> lglock-v0 3585.0648 28720.1679 169.4703 3758.7240 3150.9310
>> fs-locks-v10+lglock-v0 3621.9347 17706.2354 133.0648 3744.0095 3174.0998
>>
>>
>> posix01
>> mean variance sigma max min
>> 4.0.0-rc3 9297.5030 26911.6602 164.0477 9941.8094 8963.3528
>> fs-locks-v10 9462.8665 44762.9316 211.5725 10504.3205 9202.5748
>> lglock-v0 9320.4716 47168.9903 217.1842 10401.6565 9054.1950
>> fs-locks-v10+lglock-v0 9458.1463 58231.8844 241.3128 10564.2086 9193.1177
>>
>>
>> posix02
>> mean variance sigma max min
>> 4.0.0-rc3 920.6533 2648.1293 51.4600 983.4213 790.1792
>> fs-locks-v10 915.3972 4384.6821 66.2169 1004.2339 795.4129
>> lglock-v0 888.1910 4644.0478 68.1473 983.8412 777.4851
>> fs-locks-v10+lglock-v0 926.4184 1834.6481 42.8328 975.8544 794.4582
>>
>>
>> posix03
>> mean variance sigma max min
>> 4.0.0-rc3 7.5202 0.0456 0.2136 7.9697 6.9803
>> fs-locks-v10 7.5203 0.0640 0.2529 7.9619 7.0063
>> lglock-v0 7.4738 0.0349 0.1867 7.8011 7.0973
>> fs-locks-v10+lglock-v0 7.5856 0.0595 0.2439 8.1098 6.8695
>>
>>
>> posix04
>> mean variance sigma max min
>> 4.0.0-rc3 0.6699 0.1091 0.3303 3.3845 0.5247
>> fs-locks-v10 0.6320 0.0026 0.0511 0.9064 0.5195
>> lglock-v0 0.6460 0.0039 0.0623 1.0830 0.5438
>> fs-locks-v10+lglock-v0 0.6589 0.0338 0.1838 2.0346 0.5393
>>
>>
>> Hmm, not really convincing numbers. I hoped to see scaling effects but nope, no fun.
>>
>
> Yeah...
>
> That said, the lglock-v0 numbers do look a little better. Perhaps it
> would make sense to go ahead and consider that change? It's not clear
> to me why the lglock code uses arch_spinlock_t. Was it just the extra
> memory usage or was there some other reason?
If my reading is correct, the main difference between spinlock_t
and arch_spinlock_t is the avoidance of the trylock path:
spin_lock(&lock)
raw_spin_lock(&lock)
_raw_spin_lock(&lock)
__raw_spin_lock(&lock)
static inline void __raw_spin_lock(raw_spinlock_t *lock)
{
preempt_disable();
spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
}
static inline int do_raw_spin_trylock(raw_spinlock_t *lock)
{
return arch_spin_trylock(&(lock)->raw_lock);
}
static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock)
{
__acquire(lock);
arch_spin_lock(&lock->raw_lock);
}
So with using arch_spin_lock(&lock) lglock is short cutting the
path slightly.
The memory consumption is even less using spinlock_t:
4.0.0-rc5
text data bss dec hex filename
19941 2409 1088 23438 5b8e fs/locks.o
822 0 0 822 336 kernel/locking/lglock.o
[ 0.000000] setup_percpu: NR_CPUS:64 nr_cpumask_bits:64 nr_cpu_ids:64 nr_node_ids:4
[ 0.000000] PERCPU: Embedded 32 pages/cpu @ffff881fbfc00000 s92888 r8192 d29992 u131072
[ 0.000000] pcpu-alloc: s92888 r8192 d29992 u131072 alloc=1*2097152
[ 0.000000] pcpu-alloc: [0] 00 04 08 12 16 20 24 28 32 36 40 44 48 52 56 60
[ 0.000000] pcpu-alloc: [1] 01 05 09 13 17 21 25 29 33 37 41 45 49 53 57 61
[ 0.000000] pcpu-alloc: [2] 02 06 10 14 18 22 26 30 34 38 42 46 50 54 58 62
[ 0.000000] pcpu-alloc: [3] 03 07 11 15 19 23 27 31 35 39 43 47 51 55 59 63
[ 0.000000] Built 4 zonelists in Node order, mobility grouping on. Total pages: 132109066
4.0.0-rc5-lglock-v0
text data bss dec hex filename
19941 2409 1088 23438 5b8e fs/locks.o
620 0 0 620 26c kernel/locking/lglock.o
[ +0.000000] setup_percpu: NR_CPUS:64 nr_cpumask_bits:64 nr_cpu_ids:64 nr_node_ids:4
[ +0.000000] PERCPU: Embedded 32 pages/cpu @ffff881fbfc00000 s92888 r8192 d29992 u131072
[ +0.000000] pcpu-alloc: s92888 r8192 d29992 u131072 alloc=1*2097152
[ +0.000000] pcpu-alloc: [0] 00 04 08 12 16 20 24 28 32 36 40 44 48 52 56 60
[ +0.000000] pcpu-alloc: [1] 01 05 09 13 17 21 25 29 33 37 41 45 49 53 57 61
[ +0.000000] pcpu-alloc: [2] 02 06 10 14 18 22 26 30 34 38 42 46 50 54 58 62
[ +0.000000] pcpu-alloc: [3] 03 07 11 15 19 23 27 31 35 39 43 47 51 55 59 63
[ +0.000000] Built 4 zonelists in Node order, mobility grouping on. Total pages: 132109066
Legend: s: static size, r: reserved size, d: dynamic size, u: unit size
I did another round of measurements with different parameters and saw
some unexpected things:
- flock01: The number of child processes is close to the number
of cores (eg. 128 processes on 64 cores) and the number of test
iterations is relatively low (32 iterations). In this configuration
the results are not stable:
while true; do
rm -rf /tmp/a; flock01 -n 128 -l 32 /tmp/a;
done
38.392769508
1.054781151
113.731122000
66.362571593
97.581588309
0.015311589
117.311633231
0.015412247
0.014909320
0.015469361
0.015481439
38.779573512
101.239880635
0.822888216
...
I see this on 4.0.0-rc5 with or without the lglock-v0 patch.
This looks like if we are lucky the children of flock01 get
along without interfering and that results in low numbers.
If they system is not idle (kernel build in background
'make -j200') then the numbers get more consistent:
0.034442009
0.035964874
0.026305154
0.030738657
0.024400840
0.028685513
0.025869458
0.027475024
0.023971313
0.026113323
0.033676295
....
- I also played with lockdep detection. With lglock-v0 applied
some tests like flock02 and posix02 get considerable worse
results. The difference between flock01 and flock02 is that
the children of flock01 fight over one file lock versus
the children of flock02 lock and unlock their own lock.
My best guess is that the lockdep tracing is adding
far more to the per child lock configuration. I didn't find
any other explanation than this. Although I have to admit
I can't find a good argument why this makes a difference
between arch_spinlock_t and spinlock_t.
With lockdep enabled:
flock01
mean variance sigma max min
4.0.0-rc5 339.6094 2174.4070 46.6305 446.6720 219.1196
4.0.0-rc5-lglock-v0 499.1753 8123.7092 90.1316 666.3474 315.5089
flock02
mean variance sigma max min
4.0.0-rc5 201.2253 64.5758 8.0359 219.6059 179.1278
4.0.0-rc5-lglock-v0 785.1701 1049.7690 32.4001 844.4298 715.6951
lease01
mean variance sigma max min
4.0.0-rc5 8.6606 4.2222 2.0548 13.3409 4.2273
4.0.0-rc5-lglock-v0 12.1898 3.5871 1.8940 16.5744 9.2004
lease02
mean variance sigma max min
4.0.0-rc5 42.0945 1.2928 1.1370 44.8503 37.0932
4.0.0-rc5-lglock-v0 38.5887 0.4077 0.6385 39.8308 36.3703
posix01
mean variance sigma max min
4.0.0-rc5 407.8706 3005.7657 54.8249 581.1921 293.4723
4.0.0-rc5-lglock-v0 613.6238 5604.3537 74.8622 781.7903 487.7466
posix02
mean variance sigma max min
4.0.0-rc5 340.7774 186.4059 13.6531 365.8146 315.1692
4.0.0-rc5-lglock-v0 1319.7676 726.9997 26.9629 1377.5981 1242.2350
posix03
mean variance sigma max min
4.0.0-rc5 0.9615 0.0040 0.0629 1.1086 0.8280
4.0.0-rc5-lglock-v0 1.2682 0.0009 0.0299 1.3415 1.1902
posix04
mean variance sigma max min
4.0.0-rc5 0.0527 0.0003 0.0172 0.1156 0.0237
4.0.0-rc5-lglock-v0 0.0365 0.0001 0.0101 0.0887 0.0249
Without lockdep:
mean variance sigma max min
4.0.0-rc5 448.0287 15417.8359 124.1686 527.8083 0.0081
4.0.0-rc5-lglock-v0 395.1646 28713.4347 169.4504 520.7507 0.0075
flock02
mean variance sigma max min
4.0.0-rc5 6.9185 0.8830 0.9397 10.6138 4.9707
4.0.0-rc5-lglock-v0 6.2474 0.9234 0.9610 9.5478 4.3703
lease01
mean variance sigma max min
4.0.0-rc5 7.7040 0.3930 0.6269 9.1874 5.4179
4.0.0-rc5-lglock-v0 7.6862 0.7794 0.8828 9.0623 1.3639
lease02
mean variance sigma max min
4.0.0-rc5 16.3074 0.1418 0.3766 17.1600 15.0240
4.0.0-rc5-lglock-v0 16.2698 0.2772 0.5265 17.2221 13.4127
posix01
mean variance sigma max min
4.0.0-rc5 531.5151 181.3078 13.4651 596.5883 501.2940
4.0.0-rc5-lglock-v0 531.3600 209.0023 14.4569 600.7317 507.1767
posix02
mean variance sigma max min
4.0.0-rc5 13.8395 2.9768 1.7253 22.0783 9.9136
4.0.0-rc5-lglock-v0 12.6822 3.1645 1.7789 18.1258 9.0030
posix03
mean variance sigma max min
4.0.0-rc5 0.8939 0.0006 0.0242 0.9392 0.8360
4.0.0-rc5-lglock-v0 0.9050 0.0006 0.0254 0.9617 0.8454
posix04
mean variance sigma max min
4.0.0-rc5 0.0122 0.0000 0.0023 0.0227 0.0083
4.0.0-rc5-lglock-v0 0.0115 0.0000 0.0016 0.0165 0.0091
> You had mentioned at one point that lglocks didn't play well with the
> -rt kernels. What's the actual problem there?
-rt kernels like to preempt everything possible. One mean to achieve
this is by exchanging normal spinlock_t with rt_mutex. arch_spinlock_t
does not get this treatment automatically via the lock framework.
For this following patch is carried around:
https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit/?h=v3.14-rt-rebase&id=da1cbed0dcf6ab22a4b50b0c5606271067749aef
struct lglock {
+#ifndef CONFIG_PREEMPT_RT_FULL
arch_spinlock_t __percpu *lock;
+#else
+ struct rt_mutex __percpu *lock;
+#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lock_class_key lock_key;
struct lockdep_map lock_dep_map;
#endif
};
[...]
I have modified version of the above patch ontop of of the lglock-v0
which drops all the ifdefing around arch_spinlock_t and rt_mutex. The
results are identical.
If there aren't any objection I send the lglock-v0 patch with a proper
commit message.
cheers,
daniel
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