Re: FlexiBLAS as BLAS/LAPACK manager - Fedora 33 System-Wide Change proposal

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On Wed, 1 Jul 2020 at 20:13, Susi Lehtola
<jussilehtola@xxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 1 Jul 2020 10:54:16 -0600
> Jerry James <loganjerry@xxxxxxxxx> wrote:
>
> > On Wed, Jul 1, 2020 at 10:26 AM Iñaki Ucar <iucar@xxxxxxxxxxxxxxxxx>
> > wrote:
> > > BTW, I would also like to discuss here, as part of this proposal,
> > > which backend should be the system-wide default. I believe we all
> > > would agree that OpenBLAS nowadays is the best choice. But then, the
> > > serial or the openmp version?
> >
> > First, I want to make sure I understand the current openblas
> > packaging.  Is this correct?
> >
> > openblas-openmp: use if the application uses OpenMP
>
> Yes; then OpenBLAS calls get run in parallel in single-threaded regions
> of the code, and if sequential mode in regions that are already
> running parallel.
>
> > openblas-serial: use if the application is multithreaded
> > openblas-threads: use if the application is single-threaded
>
> No, this is exactly the wrong way around. You should use the serial
> library for code that you want to be running in serial (this way you
> can get several instances of the program running efficiently), and the
> pthreads version if you want to run the BLAS/LAPACK regions in parallel
> (but are somehow opposed to OpenMP!)..

I think I'm more confused than before. :D

> > The question of the default is a hard one.  What happens if a
> > multithreaded application that does not use OpenMP is linked with the
> > OpenMP build of OpenBLAS?
>
> Then you get too much parallellism, i.e. every call to OpenBLAS will
> launch several threads, and this will naturally ruin your scaling.

So would you say that openblas-serial is the most sensible system-wide default?

-- 
Iñaki Úcar
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