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Date:   Sun, 27 Sep 2020 22:55:52 +0200 (CEST)
From:   Julia Lawall <julia.lawall@inria.fr>
To:     Sumera Priyadarsini <sylphrenadin@gmail.com>
cc:     corbet@lwn.net, michal.lkml@markovi.net, Gilles.Muller@lip6.fr,
        linux-doc@vger.kernel.org, nicolas.palix@imag.fr,
        linux-kernel@vger.kernel.org, cocci@systeme.lip6.fr
Subject: Re: [Cocci] [PATCH 1/2] scripts: coccicheck: Change default value
 for    parallelism
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References: <cover.1600945451.git.sylphrenadin@gmail.com> <3beb97122995eafe3f0b831e36167b1edadb47c5.1600945451.git.sylphrenadin@gmail.com>
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On Thu, 24 Sep 2020, Sumera Priyadarsini wrote:

> By default, coccicheck utilizes all available threads to implement
> parallelisation. However, when all available threads are used,
> a decrease in performance is noted. The elapsed time is  minimum
> when at most one thread per core is used.
>
> For example, on benchmarking the semantic patch kfree.cocci for
> usb/serial using hyperfine, the outputs obtained for J=5 and J=2
> are 1.32 and 1.90 times faster than those for J=10 and J=9
> respectively for two separate runs. For the larger drivers/staging
> directory, minimium elapsed time is obtained for J=3 which is 1.86
> times faster than that for J=12. The optimal J value does not
> exceed 6 in any of the test runs. The benchmarks are run on a machine
> with 6 cores, with 2 threads per core, i.e, 12 hyperthreads in all.
>
> To improve performance, modify coccicheck to use at most only
> one thread per core by default.
>
> Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>

I have applied this one, so just the patch on the docuemtnation needs to
be improved.

julia

>
> ---
> Changes in V2:
> 	- Change commit message as suggested by Julia Lawall
> Changes in V3:
> 	- Use J/2 as optimal value for machines with more
> than 8 hyperthreads as well.
> ---
>  scripts/coccicheck | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/scripts/coccicheck b/scripts/coccicheck
> index e04d328210ac..a72aa6c037ff 100755
> --- a/scripts/coccicheck
> +++ b/scripts/coccicheck
> @@ -75,8 +75,13 @@ else
>          OPTIONS="--dir $KBUILD_EXTMOD $COCCIINCLUDE"
>      fi
>
> +    # Use only one thread per core by default if hyperthreading is enabled
> +    THREADS_PER_CORE=$(lscpu | grep "Thread(s) per core: " | tr -cd [:digit:])
>      if [ -z "$J" ]; then
>          NPROC=$(getconf _NPROCESSORS_ONLN)
> +	if [ $THREADS_PER_CORE -gt 1 -a $NPROC -gt 2 ] ; then
> +		NPROC=$((NPROC/2))
> +	fi
>      else
>          NPROC="$J"
>      fi
> --
> 2.25.1
>
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