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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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