On Thu, Mar 18, 2021 at 12:52 PM Song Liu <songliubrav...@fb.com> wrote: > > > > > On Mar 17, 2021, at 6:11 AM, Arnaldo Carvalho de Melo <a...@kernel.org> > > wrote: > > > > Em Wed, Mar 17, 2021 at 02:29:28PM +0900, Namhyung Kim escreveu: > >> Hi Song, > >> > >> On Wed, Mar 17, 2021 at 6:18 AM Song Liu <songliubrav...@fb.com> wrote: > >>> > >>> perf uses performance monitoring counters (PMCs) to monitor system > >>> performance. The PMCs are limited hardware resources. For example, > >>> Intel CPUs have 3x fixed PMCs and 4x programmable PMCs per cpu. > >>> > >>> Modern data center systems use these PMCs in many different ways: > >>> system level monitoring, (maybe nested) container level monitoring, per > >>> process monitoring, profiling (in sample mode), etc. In some cases, > >>> there are more active perf_events than available hardware PMCs. To allow > >>> all perf_events to have a chance to run, it is necessary to do expensive > >>> time multiplexing of events. > >>> > >>> On the other hand, many monitoring tools count the common metrics (cycles, > >>> instructions). It is a waste to have multiple tools create multiple > >>> perf_events of "cycles" and occupy multiple PMCs. > >> > >> Right, it'd be really helpful when the PMCs are frequently or mostly > >> shared. > >> But it'd also increase the overhead for uncontended cases as BPF programs > >> need to run on every context switch. Depending on the workload, it may > >> cause a non-negligible performance impact. So users should be aware of it. > > > > Would be interesting to, humm, measure both cases to have a firm number > > of the impact, how many instructions are added when sharing using > > --bpf-counters? > > > > I.e. compare the "expensive time multiplexing of events" with its > > avoidance by using --bpf-counters. > > > > Song, have you perfmormed such measurements? > > I have got some measurements with perf-bench-sched-messaging: > > The system: x86_64 with 23 cores (46 HT) > > The perf-stat command: > perf stat -e cycles,cycles,instructions,instructions,ref-cycles,ref-cycles > <target, etc.> > > The benchmark command and output: > ./perf bench sched messaging -g 40 -l 50000 -t > # Running 'sched/messaging' benchmark: > # 20 sender and receiver threads per group > # 40 groups == 1600 threads run > Total time: 10X.XXX [sec] > > > I use the "Total time" as measurement, so smaller number is better. > > For each condition, I run the command 5 times, and took the median of > "Total time". > > Baseline (no perf-stat) 104.873 [sec] > # global > perf stat -a 107.887 [sec] > perf stat -a --bpf-counters 106.071 [sec] > # per task > perf stat 106.314 [sec] > perf stat --bpf-counters 105.965 [sec] > # per cpu > perf stat -C 1,3,5 107.063 [sec] > perf stat -C 1,3,5 --bpf-counters 106.406 [sec] > > From the data, --bpf-counters is slightly better than the regular event > for all targets. I noticed that the results are not very stable. There > are a couple 108.xx runs in some of the conditions (w/ and w/o > --bpf-counters).
Hmm.. so this result is when multiplexing happened, right? I wondered how/why the regular perf stat is slower.. Thanks, Namhyung > > > I also measured the average runtime of the BPF programs, with > > sysctl kernel.bpf_stats_enabled=1 > > For each event, if we have one leader and two followers, the total run > time is about 340ns. IOW, 340ns for two perf-stat reading instructions, > 340ns for two perf-stat reading cycles, etc. > > Thanks, > Song