[Rd] external BLAS library triggering erroneous ill-condtionned matrices

[Dirk Eddelbuettel]

On 29 October 2026 at 10:02, bastien_chassagnol--- via R-devel wrote:
| Set-up: R 4.6.1 (Ubuntu 22.04 build), linked to the system OpenBLAS

That's dated / a mismatch.  Why current R but older BLAS/LAPACK?  I recommend
Ubuntu 26.04 or 26.10 if you want to engage with upstream.

It's also ... sort-of well known. Just rotate through the available BLAS /
LAPACK combinations. On a Debian / Ubuntu system is so easy that many (many)
years ago I wrote a package doing just that to compare and time with GPUs,
see https://cran.r-project.org/package=gcbd -- could use some love and an
update to current packages.

There is also flexiblas, already in Fedora and last I checked 'being
packaged' for Debian.

| The reference libraries were loaded with `LD_PRELOAD`; the kernel was set

You don't need to do that. `gcbd` just uninstall / reinstalls. If you check
the manual page, you can see how the `update-alternatives` command (a Debian
/ Ubuntu special) allows you to switch.  The number of available BLAS /
LAPACK varies from year to year but there are way more than just reference
BLAS and OpenBLAS. On my (26.04) machine just now

  $ apt-cache search libblas | wc -l
  33
  $ 
  $ apt-cache search libblas | grep -- -dev | wc -l
  16
  $ 

It's big topic, and if you check source (of e.g. R itself) you will often
find comments about precision varying. "Practical" numerical analysis has
challenges that more theoretical approaches may gloss over.

Dirk

-- 
dirk.eddelbuettel.com | @eddelbuettel | edd using debian.org