* 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/secIt 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/secI 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