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SubjectRe: [Cocci] [PATCH] scripts: coccicheck: Change default value for parallelism


On Thu, 13 Aug 2020, Sumera Priyadarsini wrote:

>
>
> On Thu, 13 Aug, 2020, 1:45 AM Julia Lawall, <julia.lawall@inria.fr> wrote:
>
>
> 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.
>
>
> How does this sound? 
>  
> However, when all available threads are used, a decrease in performance was noted. The elapsed time was minimum when at most one thread per core was used. 

This looks better, thanks.

julia

>
>
>
>
> > 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.
>
>
> Alright, will add this. 
>
>
> thanks,
> julia
>
> >
> > Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.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@systeme.lip6.fr
> > https://systeme.lip6.fr/mailman/listinfo/cocci
> >
>
>
>
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