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Subject: Re: [RFC PATCH v3 00/15] perf: add workqueue library and use it in
 synthetic-events
From:   Riccardo Mancini <rickyman7@gmail.com>
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Date:   Tue, 31 Aug 2021 18:57:34 +0200
In-Reply-To: <YS5OwQ+MzePxNrRI@krava>
References: <cover.1629454773.git.rickyman7@gmail.com>
         <YSwDTWsihFxn6f1E@krava> <YS5OwQ+MzePxNrRI@krava>
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Hi Jiri,
thanks again for testing it!

On Tue, 2021-08-31 at 17:46 +0200, Jiri Olsa wrote:
> On Sun, Aug 29, 2021 at 11:59:41PM +0200, Jiri Olsa wrote:
> > On Fri, Aug 20, 2021 at 12:53:46PM +0200, Riccardo Mancini wrote:
> > > Changes in v3:
> > >  - improved separation of threadpool and threadpool_entry method
> > >  - replaced shared workqueue with per-thread workqueue. This should
> > >    improve the performance on big machines (Jiri noticed in his
> > >    experiments a significant performance degradation after 15 threads
> > >    with the shared queue).
> > >  - improved error reporting in both threadpool and workqueue
> > >  - added lazy spinup of threads in workqueue [9/15]
> > >  - added global workqueue [10/15]
> > >  - setup global workqueue in perf record, top and synthesize bench
> > >    [12-14/15] and used in in synthetic events
> > 
> > 
> > hi,
> > I ran the test again and there's still the slowdown,
> > adding the stats below

Looking at this experiments and v2 ones, it looks like they are synthesizing a
different number of events, so it's difficult to compare them:

old (v2 tests):                                             new (v2 tests):
Number of synthesis threads: 32                             Number of synthesis
threads: 32
   Average synthesis took: 2295.300 usec (+- 41.538 usec)      Average synthesis
took: 4191.700 usec (+- 149.780 usec)
   Average num. events: 954.000 (+- 0.000)                     Average num.
events: 1002.000 (+- 0.000)
   Average time per event 2.406 usec                           Average time per
event 4.183 usec

old (v3 tests):                                             new (v3 tests):
Number of synthesis threads: 32                              Number of synthesis
threads: 32                          
  Average synthesis took: 3684.900 usec (+- 253.077 usec)      Average synthesis
took: 4400.800 usec (+- 224.134 usec)
  Average num. events: 2319.200 (+- 0.200)                     Average num.
events: 2407.400 (+- 2.544)               
  Average time per event 1.589 usec                            Average time per
event 1.828 usec

Anyways, looking at the overhead of the workqueue, computed as the difference
between old and new time, it looks like v3 is better than v2, since it has a
800usec overhead, compared to 1800usec of v2, even with double number of events.
In any case, still not good enough.

> > 
> > I'm doing the review and I noticed few strange things,
> > but so far nothing that would explain that
> 
> I used trace compass to show the flow and it shows lot of
> extra scheduling in the new code, please check attached
> screenshots
> 
> the current code takes the quickest approach and distribues
> 'equal' load for each thread
> 
> while the lazy thread spin in the new code is nice, I think
> we should have a way to instruct the new code to do the same
> thing as the old one, because it's faster in this case

From the figures, we can see how, in the older code the spinup took ~1usec,
while, with this patchet it takes, around ~2usec, so there's definitely room for
improvement.
Regarding the lazy spawn, I think much of the overhead comes from the
registration mechanism of the workers, which I'm already planning to remove to
get rid of that additional contention on the workqueue lock, which makes it
necessary to wait for the thread to spawn.
By removing it, I think the remaining overhead should not be very big.
In any case, I could add a way to pre-spin the workers, which makes sense in
this scenario, since we'd like to use all of them anyways.

> 
> I think the work_size setup could help with that
> 
> > 
> > like I can see for 40 threads only 35 threads spawned,
> > need to check on that more

It looks like that, by the time 35 works are dispatched, the first worker
becomes ready, so no more than 35 threads are ever spawned.

> > 
> > also I'll try run some tests for parallel_for > 1 to cut
> 
> ugh.. should have been s/parallel_for/work_size/ sorry
> 
> > down some of the workqueue code.. any tests on that?

Yes, I think we need to increase it.
On my machines differences were not significant, so I opted for the lowest
number. On a faster machine, dispatching one single work item per pid might be
too low.
In fact, all that extra scheduling is happening due to many threads going idle
and being woken up, since they did not have enough work.
We could try increasing it to 10 and see what happens. 
The worst case behaviour should be the "old" one (static partitioning).

Thanks,
Riccardo

> > 
> > jirka
> > 
> > 
> > ---
> > new:                                                                        
> >   
> >           old:
> > ell-r440-01 perf]# ./perf bench internals synthesize -
> > t                                      [root@dell-r440-01 perf]# ./perf
> > bench
> > internals synthesize -t
> > # Running 'internals/synthesize'
> > benchmark:                                                  # Running
> > 'internals/synthesize' benchmark:
> > Computing performance of multi threaded perf event synthesis
> > by                              Computing performance of multi threaded perf
> > event synthesis by
> > synthesizing events on CPU
> > 0:                                                               
> > synthesizing
> > events on CPU 0:
> >   Number of synthesis threads:
> > 1                                                               Number of
> > synthesis threads: 1
> >     Average synthesis took: 13970.400 usec (+- 339.216
> > usec)                                     Average synthesis took: 13563.700
> > usec (+- 348.354 usec)
> >     Average num. events: 2349.000 (+-
> > 0.000)                                                     Average num.
> > events: 2317.000 (+- 0.000)
> >     Average time per event 5.947
> > usec                                                            Average time
> > per event 5.854 usec
> >   Number of synthesis threads:
> > 2                                                               Number of
> > synthesis threads: 2
> >     Average synthesis took: 15651.800 usec (+- 1612.798
> > usec)                                    Average synthesis took: 8433.600
> > usec
> > (+- 83.725 usec)
> >     Average num. events: 2353.000 (+-
> > 0.000)                                                     Average num.
> > events: 2321.600 (+- 0.306)
> >     Average time per event 6.652
> > usec                                                            Average time
> > per event 3.633 usec
> >   Number of synthesis threads:
> > 3                                                               Number of
> > synthesis threads: 3
> >     Average synthesis took: 12114.100 usec (+- 1208.208
> > usec)                                    Average synthesis took: 6716.200
> > usec
> > (+- 16.889 usec)
> >     Average num. events: 2355.000 (+-
> > 0.000)                                                     Average num.
> > events: 2325.000 (+- 0.000)
> >     Average time per event 5.144
> > usec                                                            Average time
> > per event 2.889 usec
> >   Number of synthesis threads:
> > 4                                                               Number of
> > synthesis threads: 4
> >     Average synthesis took: 9812.500 usec (+- 951.284
> > usec)                                      Average synthesis took: 5981.400
> > usec (+- 11.102 usec)
> >     Average num. events: 2357.000 (+-
> > 0.000)                                                     Average num.
> > events: 2323.000 (+- 0.000)
> >     Average time per event 4.163
> > usec                                                            Average time
> > per event 2.575 usec
> >   Number of synthesis threads:
> > 5                                                               Number of
> > synthesis threads: 5
> >     Average synthesis took: 7338.300 usec (+- 661.620
> > usec)                                      Average synthesis took: 5538.800
> > usec (+- 12.990 usec)
> >     Average num. events: 2359.000 (+-
> > 0.000)                                                     Average num.
> > events: 2329.000 (+- 0.000)
> >     Average time per event 3.111
> > usec                                                            Average time
> > per event 2.378 usec
> >   Number of synthesis threads:
> > 6                                                               Number of
> > synthesis threads: 6
> >     Average synthesis took: 7256.800 usec (+- 680.312
> > usec)                                      Average synthesis took: 5255.700
> > usec (+- 7.454 usec)
> >     Average num. events: 2361.000 (+-
> > 0.000)                                                     Average num.
> > events: 2331.000 (+- 0.000)
> >     Average time per event 3.074
> > usec                                                            Average time
> > per event 2.255 usec
> >   Number of synthesis threads:
> > 7                                                               Number of
> > synthesis threads: 7
> >     Average synthesis took: 6119.600 usec (+- 479.409
> > usec)                                      Average synthesis took: 4836.200
> > usec (+- 8.132 usec)
> >     Average num. events: 2363.000 (+-
> > 0.000)                                                     Average num.
> > events: 2323.000 (+- 0.000)
> >     Average time per event 2.590
> > usec                                                            Average time
> > per event 2.082 usec
> >   Number of synthesis threads:
> > 8                                                               Number of
> > synthesis threads: 8
> >     Average synthesis took: 5899.600 usec (+- 506.285
> > usec)                                      Average synthesis took: 4643.000
> > usec (+- 4.913 usec)
> >     Average num. events: 2365.000 (+-
> > 0.000)                                                     Average num.
> > events: 2335.000 (+- 0.000)
> >     Average time per event 2.495
> > usec                                                            Average time
> > per event 1.988 usec
> >   Number of synthesis threads:
> > 9                                                               Number of
> > synthesis threads: 9
> >     Average synthesis took: 5459.100 usec (+- 431.725
> > usec)                                      Average synthesis took: 4526.600
> > usec (+- 5.207 usec)
> >     Average num. events: 2367.000 (+-
> > 0.000)                                                     Average num.
> > events: 2337.000 (+- 0.000)
> >     Average time per event 2.306
> > usec                                                            Average time
> > per event 1.937 usec
> >   Number of synthesis threads:
> > 10                                                              Number of
> > synthesis threads: 10
> >     Average synthesis took: 4977.100 usec (+- 251.378
> > usec)                                      Average synthesis took: 4128.700
> > usec (+- 5.911 usec)
> >     Average num. events: 2369.000 (+-
> > 0.000)                                                     Average num.
> > events: 2327.800 (+- 0.533)
> >     Average time per event 2.101
> > usec                                                            Average time
> > per event 1.774 usec
> >   Number of synthesis threads:
> > 11                                                              Number of
> > synthesis threads: 11
> >     Average synthesis took: 5428.700 usec (+- 513.409
> > usec)                                      Average synthesis took: 3890.800
> > usec (+- 15.051 usec)
> >     Average num. events: 2371.000 (+-
> > 0.000)                                                     Average num.
> > events: 2323.000 (+- 0.000)
> >     Average time per event 2.290
> > usec                                                            Average time
> > per event 1.675 usec
> >   Number of synthesis threads:
> > 12                                                              Number of
> > synthesis threads: 12
> >     Average synthesis took: 5517.800 usec (+- 508.171
> > usec)                                      Average synthesis took: 3367.800
> > usec (+- 14.261 usec)
> >     Average num. events: 2373.000 (+-
> > 0.000)                                                     Average num.
> > events: 2343.000 (+- 0.000)
> >     Average time per event 2.325
> > usec                                                            Average time
> > per event 1.437 usec
> >   Number of synthesis threads:
> > 13                                                              Number of
> > synthesis threads: 13
> >     Average synthesis took: 5279.500 usec (+- 432.819
> > usec)                                      Average synthesis took: 3974.300
> > usec (+- 12.437 usec)
> >     Average num. events: 2375.000 (+-
> > 0.000)                                                     Average num.
> > events: 2328.200 (+- 1.405)
> >     Average time per event 2.223
> > usec                                                            Average time
> > per event 1.707 usec
> >   Number of synthesis threads:
> > 14                                                              Number of
> > synthesis threads: 14
> >     Average synthesis took: 4993.100 usec (+- 392.485
> > usec)                                      Average synthesis took: 4157.100
> > usec (+- 163.268 usec)
> >     Average num. events: 2377.000 (+-
> > 0.000)                                                     Average num.
> > events: 2319.800 (+- 0.533)
> >     Average time per event 2.101
> > usec                                                            Average time
> > per event 1.792 usec
> >   Number of synthesis threads:
> > 15                                                              Number of
> > synthesis threads: 15
> >     Average synthesis took: 5584.700 usec (+- 379.862
> > usec)                                      Average synthesis took: 4065.700
> > usec (+- 25.656 usec)
> >     Average num. events: 2379.000 (+-
> > 0.000)                                                     Average num.
> > events: 2322.800 (+- 0.467)
> >     Average time per event 2.347
> > usec                                                            Average time
> > per event 1.750 usec
> >   Number of synthesis threads:
> > 16                                                              Number of
> > synthesis threads: 16
> >     Average synthesis took: 5009.800 usec (+- 381.018
> > usec)                                      Average synthesis took: 4580.600
> > usec (+- 129.218 usec)
> >     Average num. events: 2381.000 (+-
> > 0.000)                                                     Average num.
> > events: 2324.800 (+- 0.200)
> >     Average time per event 2.104
> > usec                                                            Average time
> > per event 1.970 usec
> >   Number of synthesis threads:
> > 17                                                              Number of
> > synthesis threads: 17
> >     Average synthesis took: 5543.300 usec (+- 376.064
> > usec)                                      Average synthesis took: 4089.700
> > usec (+- 54.096 usec)
> >     Average num. events: 2383.000 (+-
> > 0.000)                                                     Average num.
> > events: 2320.200 (+- 0.611)
> >     Average time per event 2.326
> > usec                                                            Average time
> > per event 1.763 usec
> >   Number of synthesis threads:
> > 18                                                              Number of
> > synthesis threads: 18
> >     Average synthesis took: 5191.800 usec (+- 342.317
> > usec)                                      Average synthesis took: 4219.000
> > usec (+- 61.395 usec)
> >     Average num. events: 2385.000 (+-
> > 0.000)                                                     Average num.
> > events: 2323.000 (+- 0.516)
> >     Average time per event 2.177
> > usec                                                            Average time
> > per event 1.816 usec
> >   Number of synthesis threads:
> > 19                                                              Number of
> > synthesis threads: 19
> >     Average synthesis took: 4647.000 usec (+- 273.303
> > usec)                                      Average synthesis took: 3998.800
> > usec (+- 49.221 usec)
> >     Average num. events: 2387.000 (+-
> > 0.000)                                                     Average num.
> > events: 2325.200 (+- 0.200)
> >     Average time per event 1.947
> > usec                                                            Average time
> > per event 1.720 usec
> >   Number of synthesis threads:
> > 20                                                              Number of
> > synthesis threads: 20
> >     Average synthesis took: 4710.600 usec (+- 179.874
> > usec)                                      Average synthesis took: 3930.300
> > usec (+- 67.725 usec)
> >     Average num. events: 2389.000 (+-
> > 0.000)                                                     Average num.
> > events: 2319.000 (+- 0.000)
> >     Average time per event 1.972
> > usec                                                            Average time
> > per event 1.695 usec
> >   Number of synthesis threads:
> > 21                                                              Number of
> > synthesis threads: 21
> >     Average synthesis took: 4959.100 usec (+- 318.519
> > usec)                                      Average synthesis took: 3696.400
> > usec (+- 30.953 usec)
> >     Average num. events: 2390.800 (+-
> > 0.200)                                                     Average num.
> > events: 2319.800 (+- 0.533)
> >     Average time per event 2.074
> > usec                                                            Average time
> > per event 1.593 usec
> >   Number of synthesis threads:
> > 22                                                              Number of
> > synthesis threads: 22
> >     Average synthesis took: 4422.300 usec (+- 236.998
> > usec)                                      Average synthesis took: 3394.000
> > usec (+- 63.254 usec)
> >     Average num. events: 2392.800 (+-
> > 0.200)                                                     Average num.
> > events: 2319.000 (+- 0.000)
> >     Average time per event 1.848
> > usec                                                            Average time
> > per event 1.464 usec
> >   Number of synthesis threads:
> > 23                                                              Number of
> > synthesis threads: 23
> >     Average synthesis took: 4640.800 usec (+- 245.604
> > usec)                                      Average synthesis took: 4091.100
> > usec (+- 134.320 usec)
> >     Average num. events: 2394.400 (+-
> > 0.600)                                                     Average num.
> > events: 2323.400 (+- 0.267)
> >     Average time per event 1.938
> > usec                                                            Average time
> > per event 1.761 usec
> >   Number of synthesis threads:
> > 24                                                              Number of
> > synthesis threads: 24
> >     Average synthesis took: 4554.900 usec (+- 201.121
> > usec)                                      Average synthesis took: 3346.600
> > usec (+- 78.846 usec)
> >     Average num. events: 2395.800 (+-
> > 0.854)                                                     Average num.
> > events: 2321.000 (+- 0.667)
> >     Average time per event 1.901
> > usec                                                            Average time
> > per event 1.442 usec
> >   Number of synthesis threads:
> > 25                                                              Number of
> > synthesis threads: 25
> >     Average synthesis took: 4668.300 usec (+- 248.254
> > usec)                                      Average synthesis took: 3794.300
> > usec (+- 191.158 usec)
> >     Average num. events: 2398.000 (+-
> > 0.803)                                                     Average num.
> > events: 2317.900 (+- 6.248)
> >     Average time per event 1.947
> > usec                                                            Average time
> > per event 1.637 usec
> >   Number of synthesis threads:
> > 26                                                              Number of
> > synthesis threads: 26
> >     Average synthesis took: 4683.300 usec (+- 226.836
> > usec)                                      Average synthesis took: 3285.700
> > usec (+- 18.785 usec)
> >     Average num. events: 2399.000 (+-
> > 1.265)                                                     Average num.
> > events: 2317.100 (+- 6.198)
> >     Average time per event 1.952
> > usec                                                            Average time
> > per event 1.418 usec
> >   Number of synthesis threads:
> > 27                                                              Number of
> > synthesis threads: 27
> >     Average synthesis took: 4590.300 usec (+- 158.000
> > usec)                                      Average synthesis took: 3604.600
> > usec (+- 35.487 usec)
> >     Average num. events: 2400.200 (+-
> > 1.497)                                                     Average num.
> > events: 2319.800 (+- 0.533)
> >     Average time per event 1.912
> > usec                                                            Average time
> > per event 1.554 usec
> >   Number of synthesis threads:
> > 28                                                              Number of
> > synthesis threads: 28
> >     Average synthesis took: 4683.500 usec (+- 233.543
> > usec)                                      Average synthesis took: 3594.700
> > usec (+- 21.267 usec)
> >     Average num. events: 2402.400 (+-
> > 1.688)                                                     Average num.
> > events: 2319.200 (+- 0.200)
> >     Average time per event 1.950
> > usec                                                            Average time
> > per event 1.550 usec
> >   Number of synthesis threads:
> > 29                                                              Number of
> > synthesis threads: 29
> >     Average synthesis took: 4830.700 usec (+- 235.730
> > usec)                                      Average synthesis took: 3531.700
> > usec (+- 15.935 usec)
> >     Average num. events: 2405.000 (+-
> > 2.530)                                                     Average num.
> > events: 2322.200 (+- 0.800)
> >     Average time per event 2.009
> > usec                                                            Average time
> > per event 1.521 usec
> >   Number of synthesis threads:
> > 30                                                              Number of
> > synthesis threads: 30
> >     Average synthesis took: 4684.500 usec (+- 210.137
> > usec)                                      Average synthesis took: 3505.700
> > usec (+- 58.332 usec)
> >     Average num. events: 2407.600 (+-
> > 2.495)                                                     Average num.
> > events: 2315.100 (+- 5.900)
> >     Average time per event 1.946
> > usec                                                            Average time
> > per event 1.514 usec
> >   Number of synthesis threads:
> > 31                                                              Number of
> > synthesis threads: 31
> >     Average synthesis took: 4823.300 usec (+- 213.480
> > usec)                                      Average synthesis took: 3431.100
> > usec (+- 42.022 usec)
> >     Average num. events: 2407.400 (+-
> > 2.647)                                                     Average num.
> > events: 2319.000 (+- 0.000)
> >     Average time per event 2.004
> > usec                                                            Average time
> > per event 1.480 usec
> >   Number of synthesis threads:
> > 32                                                              Number of
> > synthesis threads: 32
> >     Average synthesis took: 4400.800 usec (+- 224.134
> > usec)                                      Average synthesis took: 3684.900
> > usec (+- 253.077 usec)
> >     Average num. events: 2407.400 (+-
> > 2.544)                                                     Average num.
> > events: 2319.200 (+- 0.200)
> >     Average time per event 1.828
> > usec                                                            Average time
> > per event 1.589 usec
> >   Number of synthesis threads:
> > 33                                                              Number of
> > synthesis threads: 33
> >     Average synthesis took: 4452.600 usec (+- 231.034
> > usec)                                      Average synthesis took: 3233.000
> > usec (+- 24.035 usec)
> >     Average num. events: 2409.300 (+-
> > 3.190)                                                     Average num.
> > events: 2316.500 (+- 6.069)
> >     Average time per event 1.848
> > usec                                                            Average time
> > per event 1.396 usec
> >   Number of synthesis threads:
> > 34                                                              Number of
> > synthesis threads: 34
> >     Average synthesis took: 4770.900 usec (+- 182.325
> > usec)                                      Average synthesis took: 3016.300
> > usec (+- 13.343 usec)
> >     Average num. events: 2411.200 (+-
> > 3.032)                                                     Average num.
> > events: 2322.800 (+- 0.200)
> >     Average time per event 1.979
> > usec                                                            Average time
> > per event 1.299 usec
> >   Number of synthesis threads:
> > 35                                                              Number of
> > synthesis threads: 35
> >     Average synthesis took: 4442.800 usec (+- 248.017
> > usec)                                      Average synthesis took: 3246.700
> > usec (+- 71.765 usec)
> >     Average num. events: 2412.000 (+-
> > 3.296)                                                     Average num.
> > events: 2321.800 (+- 0.611)
> >     Average time per event 1.842
> > usec                                                            Average time
> > per event 1.398 usec
> >   Number of synthesis threads:
> > 36                                                              Number of
> > synthesis threads: 36
> >     Average synthesis took: 5005.200 usec (+- 235.823
> > usec)                                      Average synthesis took: 3329.000
> > usec (+- 122.028 usec)
> >     Average num. events: 2410.400 (+-
> > 2.750)                                                     Average num.
> > events: 2310.800 (+- 8.133)
> >     Average time per event 2.077
> > usec                                                            Average time
> > per event 1.441 usec
> >   Number of synthesis threads:
> > 37                                                              Number of
> > synthesis threads: 37
> >     Average synthesis took: 4654.000 usec (+- 208.838
> > usec)                                      Average synthesis took: 3011.600
> > usec (+- 46.026 usec)
> >     Average num. events: 2409.400 (+-
> > 2.473)                                                     Average num.
> > events: 2322.200 (+- 0.533)
> >     Average time per event 1.932
> > usec                                                            Average time
> > per event 1.297 usec
> >   Number of synthesis threads:
> > 38                                                              Number of
> > synthesis threads: 38
> >     Average synthesis took: 4763.700 usec (+- 197.409
> > usec)                                      Average synthesis took: 3163.500
> > usec (+- 36.589 usec)
> >     Average num. events: 2406.200 (+-
> > 2.462)                                                     Average num.
> > events: 2319.000 (+- 0.000)
> >     Average time per event 1.980
> > usec                                                            Average time
> > per event 1.364 usec
> >   Number of synthesis threads:
> > 39                                                              Number of
> > synthesis threads: 39
> >     Average synthesis took: 4333.100 usec (+- 194.456
> > usec)                                      Average synthesis took: 3170.900
> > usec (+- 30.538 usec)
> >     Average num. events: 2408.600 (+-
> > 3.124)                                                     Average num.
> > events: 2319.000 (+- 0.000)
> >     Average time per event 1.799
> > usec                                                            Average time
> > per event 1.367 usec
> >   Number of synthesis threads:
> > 40                                                              Number of
> > synthesis threads: 40
> >     Average synthesis took: 4520.200 usec (+- 188.901
> > usec)                                      Average synthesis took: 3111.900
> > usec (+- 24.287 usec)
> >     Average num. events: 2409.600 (+-
> > 3.184)                                                     Average num.
> > events: 2307.600 (+- 7.600)
> >     Average time per event 1.876
> > usec                                                            Average time
> > per event 1.349 usec