linux-kernel - Re: [Cocci] [PATCH] scripts: coccicheck: Change default value for parallelism

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Date:   Wed, 12 Aug 2020 22:15:04 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Sumera Priyadarsini <sylphrenadin@...il.com>
cc:     michal.lkml@...kovi.net, Gilles.Muller@...6.fr,
        gregkh@...uxfoundation.org, nicolas.palix@...g.fr,
        linux-kernel@...r.kernel.org, cocci@...teme.lip6.fr
Subject: Re: [Cocci] [PATCH] scripts: coccicheck: Change default value for
 parallelism

On Wed, 12 Aug 2020, Sumera Priyadarsini wrote:

> By default, coccicheck utilizes all available threads to implement
> parallelisation. However, when hyperthreading is enabled, this leads
> to all threads per core being occupied resulting in longer wall-clock
> times and higher power consumption.

I have the feeling that the above sentence is not quite optimal.
Actually, using all of the available hardware threads would not be a bad
thing, if it was giving a benefit.  The point is that it doesn't.  It
makes the performance worse instead.

> Hence, to improve performance,
> modify coccicheck to use only one thread per core atmost.

"atmost" is not a word.  It would be clearer to say "to use at most one
thread per core".

> In the cases where the total number of threads is more than 8 and
> hyperthreading is enabled, it was observed that optimum performance
> is achieved around one-fourth of the total number of cores.
> Modify the script further to accommodate this use case.

It would be nice to give some performance numbers and some information
about the machine used.

thanks,
julia

>
> Signed-off-by: Sumera Priyadarsini <sylphrenadin@...il.com>
> ---
>  scripts/coccicheck | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/scripts/coccicheck b/scripts/coccicheck
> index e04d328210ac..dd228dcc915e 100755
> --- a/scripts/coccicheck
> +++ b/scripts/coccicheck
> @@ -75,8 +75,17 @@ 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
> +		if [ $NPROC -gt 8 ] ; then
> +			NPROC=$((NPROC/4))
> +		else
> +			NPROC=$((NPROC/2))
> +		fi
> +	fi
>      else
>          NPROC="$J"
>      fi
> --
> 2.17.1
>
> _______________________________________________
> Cocci mailing list
> Cocci@...teme.lip6.fr
> https://systeme.lip6.fr/mailman/listinfo/cocci
>

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