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SubjectRe: [PATCH 6/8] perf top: Implement multithreading for perf_event__synthesize_threads

* Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> From: Kan Liang <kan.liang@intel.com>
>
> The proc files which is sorted with alphabetical order are evenly
> assigned to several synthesize threads to be processed in parallel.
>
> For 'perf top', the threads number hard code to online CPU number. The
> following patch will introduce an option to set it.
>
> For other perf tools, the thread number is 1. Because the process
> function is not ready for multithreading, e.g.
> process_synthesized_event.
>
> This patch series only support event synthesize multithreading for 'perf
> top'. For other tools, it can be done separately later.

Just to give some quick feedback: this is really nice stuff!

Is anyone working on multi-threading 'perf record' (and the recording portion of
'perf top' perhaps)?

Especially with complex, high-frequency profiling there's alot of SMP overhead
coming from a single recording thread. If there was a single thread per CPU, and
it truly only recorded the events from its own CPU, things would become a lot more
scalable.

For example, if we measure the current overhead of perf record of a (limited)
parallel kernel build:

triton:~/tip> perf stat --no-inherit --pre "make clean >/dev/null 2>&1" perf record -F 10000 make -j kernel
...
[ perf record: Captured and wrote 5.124 MB perf.data (108400 samples) ]

Performance counter stats for 'perf record -F 10000 make -j kernel':

183.582587 task-clock (msec) # 0.039 CPUs utilized
2,496 context-switches # 0.014 M/sec
157 cpu-migrations # 0.855 K/sec
6,649 page-faults # 0.036 M/sec
817,478,151 cycles # 4.453 GHz
416,641,913 stalled-cycles-frontend # 50.97% frontend cycles idle
1,018,336,301 instructions # 1.25 insn per cycle
# 0.41 stalled cycles per insn
217,255,137 branches # 1183.419 M/sec
2,970,118 branch-misses # 1.37% of all branches

4.710378510 seconds time elapsed

That's 1018336301 just to record 108400 samples, i.e. every sample takes 9,300
instructions to _record_. That's insanely high overhead from what is in essence a
tracing utility.


Even if I add "-B -N" to disable buildid generation (which is the worst offender),
it's still very high overhead:

[ perf record: Captured and wrote 5.585 MB perf.data ]

Performance counter stats for 'perf record -B -N -F 10000 make -j kernel':

45.625321 task-clock (msec) # 0.009 CPUs utilized
2,950 context-switches # 0.065 M/sec
204 cpu-migrations # 0.004 M/sec
1,992 page-faults # 0.044 M/sec
193,127,853 cycles # 4.233 GHz
117,098,418 stalled-cycles-frontend # 60.63% frontend cycles idle
197,899,633 instructions # 1.02 insn per cycle
# 0.59 stalled cycles per insn
41,221,863 branches # 903.487 M/sec
502,158 branch-misses # 1.22% of all branches

4.858962925 seconds time elapsed

... that's still 1,800+ instructions per event!

As a comparison, ftrace has a tracing overhead of less than 100 instructions per
event.

Thanks,

Ingo

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