RFC: threadding-aware virtual BLAS/LAPACK?

[Mo Zhou <lumin@debian.org>]

Hi,

Through some previous discussions I realized that the issue of
threadding implementation of the BLAS can be sometimes cumbersome.  For
example, sometimes we can observe severe performance regression from
pthread program + BLAS with gomp, or even observe severe calculation
error from gomp program + BLAS with iomp (aka llvm openmp).

These threadding disasters will be eventually propagated to our end
users, and may possibly harm their scientific computing experience.

------------- Fedora

Recall what Fedora does for the BLAS libraries: they dont do any virtual
package at all. OpenBLAS packages with different threadding support are
given different sonames:

  openblas + pthread: libopenblasp.so.*
  openblas + openmp:  libopenblaso.so.* (IIRC)
  openblas + serial:  libopenblass.so.* (IIRC)

Although this makes implementations not switchable at runtime, but at
least the users won't have to struggle with threads.

------------- Gentoo

I wrote the alternatives mechanism for gentoo's blas/lapack, which
resembles Debian's. However, gentoo's package management system supports
a "USE flag" feature which allows the user to set global threadding
implementation for the whole system.

------------- what can we do

Maybe we can provide some more virtual shared objects such as
libblasp.so, which has candidates such as openblas-pthread and
blis-pthread?

In that case if a Debian maintainer intentionally choose to link against
a pthread BLAS (libblasp) to avoid issues of an openmp BLAS (libblaso),
we can help the users to avoid threadding issues when they don't read
any documentation (actually I think 99% of the users won't read the doc,
or have enough background to understand the threadding issue).

We can dig deeper into this direction if this turns to be a sensible
direction for making improvement.

Acknowledgement
---------------
This is a part of my GSoC2020 project, although this topic is not on the
original plan.