Hello all,
I have been experimenting with the idea of leaving ftrace enabled, with sched
events,
on production systems.
The main concern that I am having at the moment is about the impact on the
system.
Enabling the sched events that I currently need for the tracing application
seems to slow down context-switches considerably, and make the system less
responsive.
I have tested with cyclictest on the mainline kernel, and noticed an increase
of min, avg latencies of around 25%.
Is this expected?
Some initial investigation into ftrace seems to point at the reservation and
commit of the events into the ring buffer
as the highest sources of overhead, while event parameters copying, including
COMM, does not seem to have any noticeable effect
relative to those costs.
I have been running 20 times the following test, and thrown away the first
results:
$ sudo ./cyclictest --smp -p95 -m -s -N -l 100000 -q
$ uname -a
Linux claudio-HP-ProBook-470-G5 4.18.0-rc3+ #3 SMP Tue Jul 3 15:50:30 CEST 2018
x86_64 x86_64 x86_64 GNU/Linux
For brevity, this is a comparison of one test's results. All other test results
show the same ~25% increase.
On the left side, the run without ftrace sched events, on the right side with
ftrace sched events enabled.
CPU Count Min Act Avg Max Count
Min-ftrace Act-ftrace Avg-ftrace Max-ftrace
0 100000 2339 2936 2841 139478 100000 2900
3182 3566 93056
1 66742 2365 3386 2874 93639 66750 2959
3786 3646 154074
2 50080 2376 3058 2910 196221 50097 2997
4209 3655 18707
3 40076 2394 3461 2931 17914 40091 3006
4417 3750 17159
4 33404 2371 3612 2834 15336 33419 2997
3836 3594 23172
5 28635 2387 3313 2885 25863 28649 2995
3795 3647 9956
6 25058 2384 3428 2968 12162 25071 3051
4366 3719 18151
7 22275 2381 2859 2982 10706 22287 3046
5078 3825 10781
I would be thankful for any advice or comments on this,
especially with the goal in mind to lower as much as possible the runtime
impact on the system.
Thank you,
Claudio Fontana
