Messages in this thread | | | Date | Wed, 4 Oct 2023 09:26:46 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] perf/core: Introduce cpuctx->cgrp_ctx_list |
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* Namhyung Kim <namhyung@kernel.org> wrote:
> AFAIK we don't have a tool to measure the context switch overhead > directly. (I think I should add one to perf ftrace latency). But I can > see it with a simple perf bench command like this. > > $ perf bench sched pipe -l 100000 > # Running 'sched/pipe' benchmark: > # Executed 100000 pipe operations between two processes > > Total time: 0.650 [sec] > > 6.505740 usecs/op > 153710 ops/sec > > It runs two tasks communicate each other using a pipe so it should > stress the context switch code. This is the normal numbers on my > system. But after I run these two perf stat commands in background, > the numbers vary a lot. > > $ sudo perf stat -a -e cycles -G user.slice -- sleep 100000 & > $ sudo perf stat -a -e uncore_imc/cas_count_read/ -- sleep 10000 & > > I will show the last two lines of perf bench sched pipe output for > three runs. > > 58.597060 usecs/op # run 1 > 17065 ops/sec > > 11.329240 usecs/op # run 2 > 88267 ops/sec > > 88.481920 usecs/op # run 3 > 11301 ops/sec > > I think the deviation comes from the fact that uncore events are managed > a certain number of cpus only. If the target process runs on a cpu that > manages uncore pmu, it'd take longer. Otherwise it won't affect the > performance much.
The numbers of pipe-message context switching will vary a lot depending on CPU migration patterns as well.
The best way to measure context-switch overhead is to pin that task to a single CPU with something like:
$ taskset 1 perf stat --null --repeat 10 perf bench sched pipe -l 10000 >/dev/null
Performance counter stats for 'perf bench sched pipe -l 10000' (10 runs):
0.049798 +- 0.000102 seconds time elapsed ( +- 0.21% )
as you can see the 0.21% stddev is pretty low.
If we allow 2 CPUs, both runtime and stddev is much higher:
$ taskset 3 perf stat --null --repeat 10 perf bench sched pipe -l 10000 >/dev/null
Performance counter stats for 'perf bench sched pipe -l 10000' (10 runs):
1.4835 +- 0.0383 seconds time elapsed ( +- 2.58% )
Thanks,
Ingo
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