> 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).
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
