linux-kernel - Re: [PATCH] kunit: tool: Default --jobs to number of CPUs

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Message-ID: <CAGS_qxqAqnEo5iojk85uhuD9dcRNxTw030nfCMCsMADm9RsmbA@mail.gmail.com>
Date:   Sat, 11 Dec 2021 12:37:22 -0800
From:   Daniel Latypov <dlatypov@...gle.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Brendan Higgins <brendanhiggins@...gle.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        kunit-dev@...glegroups.com, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kunit: tool: Default --jobs to number of CPUs

On Sat, Dec 11, 2021 at 12:49 AM David Gow <davidgow@...gle.com> wrote:
>
> The --jobs parameter for kunit_tool currently defaults to 8 CPUs,
> regardless of the number available. For systems with significantly more
> (or less), this is not as efficient. Instead, default --jobs to the
> number of CPUs present in the system: while there are as many
> superstitions as to exactly what the ideal jobs:CPU ratio is, this seems
> sufficiently sensible to me.
>
> Signed-off-by: David Gow <davidgow@...gle.com>

Reminder: the unit tests depend on this hard-coded value.
$ ag '\b8\b' tools/testing/kunit/kunit_tool_test.py
422:
self.linux_source_mock.build_kernel.assert_called_once_with(False, 8,
'.kunit', None)
529:
self.linux_source_mock.build_kernel.assert_called_once_with(False, 8,
build_dir, None)

> ---
>  tools/testing/kunit/kunit.py | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/kunit/kunit.py b/tools/testing/kunit/kunit.py
> index 68e6f461c758..2cb6c7db5683 100755
> --- a/tools/testing/kunit/kunit.py
> +++ b/tools/testing/kunit/kunit.py
> @@ -310,7 +310,7 @@ def add_build_opts(parser) -> None:
>         parser.add_argument('--jobs',
>                             help='As in the make command, "Specifies  the number of '
>                             'jobs (commands) to run simultaneously."',
> -                           type=int, default=8, metavar='jobs')
> +                           type=int, default=os.cpu_count(), metavar='jobs')

Just looking for edge cases:
https://docs.python.org/3/library/os.html#os.cpu_count says
> Returns None if undetermined
and
> This number is not equivalent to the number of CPUs the current process can use. The number of usable CPUs can be obtained with len(os.sched_getaffinity(0))

I assume the None caveat is mainly for other operating systems and
doubt it'll impact any users.
The second point is a bit more interesting, but still niche.
Up to you if you want to use that instead.

Super unscientific comparison (n=1) running all on CPU #0

$ taskset 0x1 ./tools/testing/kunit/kunit.py run --jobs=1
Elapsed time: ... 155.978s building ...

--jobs=2 (some people swear by the 2x ratio)
Elapsed time: ... 158.891s building ...

--jobs=8 (Old behavior)
...
Elapsed time: ... 171.448s building

--jobs=32
Elapsed time: ...  170.765s building ...

So the overhead of j being "too high" isn't that bad and it doesn't
seem to matter much either way.


>
>  def add_exec_opts(parser) -> None:
>         parser.add_argument('--timeout',
> --
> 2.34.1.173.g76aa8bc2d0-goog
>