On 5/12/20 10:23 AM, Andrii Nakryiko wrote:
On Tue, May 12, 2020 at 8:11 AM Yonghong Song <yhs@xxxxxx> wrote:
On 5/11/20 9:22 PM, Andrii Nakryiko wrote:
On Sat, May 9, 2020 at 10:24 AM Yonghong Song <yhs@xxxxxx> wrote:
On 5/8/20 4:20 PM, Andrii Nakryiko wrote:
Add fmod_ret BPF program to existing test_overhead selftest. Also re-implement
user-space benchmarking part into benchmark runner to compare results. Results
with ./bench are consistently somewhat lower than test_overhead's, but relative
performance of various types of BPF programs stay consisten (e.g., kretprobe is
noticeably slower).
run_bench_rename.sh script (in benchs/ directory) was used to produce the
following numbers:
base : 3.975 ± 0.065M/s
kprobe : 3.268 ± 0.095M/s
kretprobe : 2.496 ± 0.040M/s
rawtp : 3.899 ± 0.078M/s
fentry : 3.836 ± 0.049M/s
fexit : 3.660 ± 0.082M/s
fmodret : 3.776 ± 0.033M/s
While running test_overhead gives:
task_rename base 4457K events per sec
task_rename kprobe 3849K events per sec
task_rename kretprobe 2729K events per sec
task_rename raw_tp 4506K events per sec
task_rename fentry 4381K events per sec
task_rename fexit 4349K events per sec
task_rename fmod_ret 4130K events per sec
Do you where the overhead is and how we could provide options in
bench to reduce the overhead so we can achieve similar numbers?
For benchmarking, sometimes you really want to see "true"
potential of a particular implementation.
Alright, let's make it an official bench-off... :) And the reason for
this discrepancy, turns out to be... not atomics at all! But rather a
single-threaded vs multi-threaded process (well, at least task_rename
happening from non-main thread, I didn't narrow it down further).
It would be good to find out why and have a scheme (e.g. some kind
of affinity binding) to close the gap.
I don't think affinity has anything to do with this. test_overhead
sets affinity for entire process, and that doesn't change results at
all. Same for bench, both with and without setting affinity, results
are pretty much the same. Affinity helps a bit to get a bit more
stable and consistent results, but doesn't hurt or help performance
for this benchmark.
I don't think we need to spend that much time trying to understand
behavior of task renaming for such a particular setup. Benchmarking
has to be multi-threaded in most cases anyways, there is no way around
that.
Okay. This might be related to kernel scheduling of main thread vs.
secondary threads? This then indeed beyond this patch.
I am fine with the current mechanism as is. Maybe put the above
experimental data in commit message? If later other people
want to do further investigation, they have some data to
start with.
Atomics actually make very little difference, which gives me a good
peace of mind :)
So, I've built and ran test_overhead (selftest) and bench both as
multi-threaded and single-threaded apps. Corresponding results match
almost perfectly. And that's while test_overhead doesn't use atomics
at all, while bench still does. Then I also ran test_overhead with
added generics to match bench implementation. There are barely any
differences, see two last sets of results.
BTW, selftest results seems bit lower from the ones in original
commit, probably because I made it run more iterations (like 40 times
more) to have more stable results.
So here are the results:
Single-threaded implementations
===============================
/* bench: single-threaded, atomics */
base : 4.622 ± 0.049M/s
kprobe : 3.673 ± 0.052M/s
kretprobe : 2.625 ± 0.052M/s
rawtp : 4.369 ± 0.089M/s
fentry : 4.201 ± 0.558M/s
fexit : 4.309 ± 0.148M/s
fmodret : 4.314 ± 0.203M/s
/* selftest: single-threaded, no atomics */
task_rename base 4555K events per sec
task_rename kprobe 3643K events per sec
task_rename kretprobe 2506K events per sec
task_rename raw_tp 4303K events per sec
task_rename fentry 4307K events per sec
task_rename fexit 4010K events per sec
task_rename fmod_ret 3984K events per sec
Multi-threaded implementations
==============================
/* bench: multi-threaded w/ atomics */
base : 3.910 ± 0.023M/s
kprobe : 3.048 ± 0.037M/s
kretprobe : 2.300 ± 0.015M/s
rawtp : 3.687 ± 0.034M/s
fentry : 3.740 ± 0.087M/s
fexit : 3.510 ± 0.009M/s
fmodret : 3.485 ± 0.050M/s
/* selftest: multi-threaded w/ atomics */
task_rename base 3872K events per sec
task_rename kprobe 3068K events per sec
task_rename kretprobe 2350K events per sec
task_rename raw_tp 3731K events per sec
task_rename fentry 3639K events per sec
task_rename fexit 3558K events per sec
task_rename fmod_ret 3511K events per sec
/* selftest: multi-threaded, no atomics */
task_rename base 3945K events per sec
task_rename kprobe 3298K events per sec
task_rename kretprobe 2451K events per sec
task_rename raw_tp 3718K events per sec
task_rename fentry 3782K events per sec
task_rename fexit 3543K events per sec
task_rename fmod_ret 3526K events per sec
[...]
