From d99c22eabee45f40ca44b877a1adde028f14b6b4 Mon Sep 17 00:00:00 2001 From: Stephane Eranian Date: Wed, 22 Apr 2020 08:50:38 -0700 Subject: perf record: Add num-synthesize-threads option To control degree of parallelism of the synthesize_mmap() code which is scanning /proc/PID/task/PID/maps and can be time consuming. Mimic perf top way of handling the option. If not specified will default to 1 thread, i.e. default behavior before this option. On a desktop computer the processing of /proc/PID/task/PID/maps isn't slow enough to warrant parallel processing and the thread creation has some cost - hence the default of 1. On a loaded server with >100 cores it is possible to see synthesis times in the order of seconds and in this case having the option is desirable. As the processing is a synchronization point, it is legitimate to worry if Amdahl's law will apply to this patch. Profiling with this patch in place: https://lore.kernel.org/lkml/20200415054050.31645-4-irogers@google.com/ shows: ... - 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 ... That is the processing is 1.49% of execution time and there is plenty to make parallel. This is shown in the benchmark in this patch: https://lore.kernel.org/lkml/20200415054050.31645-2-irogers@google.com/ 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 Where average time per synthesized event goes from 5.927 usec with 1 thread to 2.269 usec with 8. This isn't a linear speed up as not all of synthesize code has been made parallel. If the synthesis time was about 10 seconds then using 8 threads may bring this down to less than 4. Signed-off-by: Stephane Eranian Reviewed-by: Ian Rogers Acked-by: Jiri Olsa Cc: Adrian Hunter Cc: Alexander Shishkin Cc: Alexey Budankov Cc: Kan Liang Cc: Mark Rutland Cc: Namhyung Kim Cc: Peter Zijlstra Cc: Tony Jones Cc: yuzhoujian Link: http://lore.kernel.org/lkml/20200422155038.9380-1-irogers@google.com Signed-off-by: Arnaldo Carvalho de Melo --- tools/perf/Documentation/perf-record.txt | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'tools/perf/Documentation') diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt index b3f3b3f1c161..6e8b4649307c 100644 --- a/tools/perf/Documentation/perf-record.txt +++ b/tools/perf/Documentation/perf-record.txt @@ -596,6 +596,10 @@ Make a copy of /proc/kcore and place it into a directory with the perf data file Limit the sample data max size, is expected to be a number with appended unit character - B/K/M/G +--num-thread-synthesize:: + The number of threads to run when synthesizing events for existing processes. + By default, the number of threads equals 1. + SEE ALSO -------- linkperf:perf-stat[1], linkperf:perf-list[1], linkperf:perf-intel-pt[1] -- cgit