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Message-ID: <CAEf4BzY71QEmq74B8y-AmW1LFhFZ35TwO5vLn4AOiJPOSVqtjw@mail.gmail.com>
Date: Tue, 12 May 2020 10:23:26 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>,
John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH v2 bpf-next 2/3] selftest/bpf: fmod_ret prog and implement
test_overhead as part of bench
On Tue, May 12, 2020 at 8:11 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 5/11/20 9:22 PM, Andrii Nakryiko wrote:
> > On Sat, May 9, 2020 at 10:24 AM Yonghong Song <yhs@...com> 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.
>
> > 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
> >
> >
> [...]
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