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

[Olivier Benz]

> On 29 Oct 2026, at 13:41, Dirk Eddelbuettel <edd using debian.org> wrote:
> 
> 
> 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.

Cross reference: Incorrect results on Intel Xeon Gold 6448H due to OpenBLAS <0.3.21

https://github.com/rocker-org/rocker-versioned2/issues/836

> 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
> 
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