Re: [PATCH] perf/record: add num-synthesize-threads option

[Ian Rogers]

On Wed, Apr 22, 2020 at 1:15 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Mon, Apr 20, 2020 at 05:31:41PM -0700, Ian Rogers wrote:
>
> SNIP
>
> > > > +{
> > > > +     static pthread_mutex_t synth_lock = PTHREAD_MUTEX_INITIALIZER;
> > > > +     int ret;
> > > > +
> > > > +     pthread_mutex_lock(&synth_lock);
> > > > +     ret = process_synthesized_event(tool, event, sample, machine);
> > > > +     pthread_mutex_unlock(&synth_lock);
> > >
> > > hum, so how much faster is the synthesizing with threads in record,
> > > given that we serialize it on every event that goes to the file?
> >
> > We see long synthesis times of the order seconds on loaded >100 core
> > servers. I've not been able to create a reproduction on my desktop.
> > You are right that making synthesis multithreaded will suffer from
> > Amdahl's law if the write is a synchronization point. Measuring with
> > the following patch in place:
> > https://lore.kernel.org/lkml/20200415054050.31645-4-irogers@xxxxxxxxxx/
> > without threads the portion that needs a lock is less than 1.5% of
> > execution time and so there's plenty to still run in parallel:
> > ...
> >       - 32.59% __perf_event__synthesize_threads
> >          - 32.54% __event__synthesize_thread
> >             + 22.13% perf_event__synthesize_mmap_events
> >             + 6.68% perf_event__get_comm_ids.constprop.0
> >             + 1.49% process_synthesized_event
> >             + 1.29% __GI___readdir64
> >             + 0.60% __opendir
> > ...
> >
> > The multi-threaded benchmark in this patch (pass -t):
> > https://lore.kernel.org/lkml/20200415054050.31645-2-irogers@xxxxxxxxxx/
> > shows:
> >
> > Computing performance of multi threaded perf event synthesis by
> > synthesizing events on CPU 0:
> >  Number of synthesis threads: 1
> >    Average synthesis took: 127729.000 usec (+- 3372.880 usec)
> >    Average num. events: 21548.600 (+- 0.306)
> >    Average time per event 5.927 usec
> >  Number of synthesis threads: 2
> >    Average synthesis took: 88863.500 usec (+- 385.168 usec)
> >    Average num. events: 21552.800 (+- 0.327)
> >    Average time per event 4.123 usec
> >  Number of synthesis threads: 3
> >    Average synthesis took: 83257.400 usec (+- 348.617 usec)
> >    Average num. events: 21553.200 (+- 0.327)
> >    Average time per event 3.863 usec
> >  Number of synthesis threads: 4
> >    Average synthesis took: 75093.000 usec (+- 422.978 usec)
> >    Average num. events: 21554.200 (+- 0.200)
> >    Average time per event 3.484 usec
> >  Number of synthesis threads: 5
> >    Average synthesis took: 64896.600 usec (+- 353.348 usec)
> >    Average num. events: 21558.000 (+- 0.000)
> >    Average time per event 3.010 usec
> >  Number of synthesis threads: 6
> >    Average synthesis took: 59210.200 usec (+- 342.890 usec)
> >    Average num. events: 21560.000 (+- 0.000)
> >    Average time per event 2.746 usec
> >  Number of synthesis threads: 7
> >    Average synthesis took: 54093.900 usec (+- 306.247 usec)
> >    Average num. events: 21562.000 (+- 0.000)
> >    Average time per event 2.509 usec
> >  Number of synthesis threads: 8
> >    Average synthesis took: 48938.700 usec (+- 341.732 usec)
> >    Average num. events: 21564.000 (+- 0.000)
> >    Average time per event 2.269 usec
> >
> > The event logic there is using an atomic rather than a lock and the
> > scaling isn't linear as not all the logic is threaded. Still with 8
> > threads we see things going about 2.6 times faster. On a large loaded
> > machine that may bring 10 seconds of event synthesis down to less than
> > 4. On a desktop there's no measurable difference and the
> > --num-thread-synthesize is defaulted to 1.
>
> ok, nice ;) sorry I did not get to this before you posted v2,
> but could you plz send v3 with above in the changelog?

Done.
https://lore.kernel.org/lkml/20200422155038.9380-1-irogers@xxxxxxxxxx/T/#u
Not done initially as I wasn't sure the argument was coherent and also
the other two patches haven't landed - it would be nice to avoid
linking to the patches on the mailing list.

Thanks,
Ian

> thanks,
> jirka
>