[R-sig-Fedora] Changing the BLAS from openblas on a F32 box

[Iñaki Ucar]

Hi Gavin,

On Wed, 27 May 2020 at 01:15, Gavin Simpson <ucfagls using gmail.com> wrote:
>
> Dear list,
>
> What is the recommended incantation on Fedora 32 to swap out the
> openblas BLAS that the packaged (rpm) version of R-core installs for
> ATLAS?

I'm afraid there is no official mechanism in place to do that yet.
There was a proposal [1], but it was never pushed forward due to some
issues and lack of time. There's a simple hack you can do though.

Since recently, R in Fedora no longer uses openblas-Rblas and links
against system openblas, as you can see here:

$ ldd /usr/lib64/R/bin/exec/R | grep blas
        libopenblas.so.0 => /lib64/libopenblas.so.0 (0x00007f8fff849000)

So one simple trick to override that is something along these lines:

$ mkdir ~/blas
$ ln -s /lib64/atlas/libsatlas.so.3 ~/blas/libopenblas.so.0
$ LD_LIBRARY_PATH=~/blas ldd /usr/lib64/R/bin/exec/R | grep blas
        libopenblas.so.0 => /home/****/blas/libopenblas.so.0
(0x00007f78b3ea2000)

As you can see, now R points to the link in my home, which in turn
points to system's atlas. Now you can define an alias in your .bashrc
to always prepend that override to R and Rscript. If you remove the
link in your home, it will default to system's openblas. If you point
it to another BLAS implementation, it will pick it up.

Hope it helps.

[1] https://fedoraproject.org/w/index.php?title=PackagingDrafts:BLAS_LAPACK

> If anyone is interested, the issue Simon Wood reports with openblas
> and *mgcv* is:
>
> Issues:
>
> ** openblas 0.3.x x<7 is not thread safe if itself compiled for single thread
>    use and then called from multiple threads (unlike the reference BLAS, say).
>    0.2.20 appears to be OK. For 0.3.x x>6 make USE_THREAD=0 USE_LOCKING=1
>    to make openblas ensures thread safety (currently unclear if USE_LOCKING
>    will be default from 0.3.7).

Does this mean that single-threaded openblas should be built with
USE_LOCKING=1 by default? Is there any upstream recommendation about
this?

-- 
Iñaki Úcar