[Octave-bug-tracker] [bug #46830] Multiplication about 4x slower than Matlab

[Markus Mützel]

Follow-up Comment #20, bug #46830 (project octave):

That depends how you define "default".
If you use the installer, OpenBLAS is selected as the default. If OpenBLAS is
selected, the installer deletes libblas.dll (the reference implementation by
netlib) and replaces it with libopenblas.dll.
The user can later switch between the reference BLAS implementation and
OpenBLAS with the tool that lostbard created. (That tool appears in the Start
menu, or it can be found at `bin\blas_switch.exe` of the Octave
installation.)

Octave for Windows links to libblas.dll (which is normally the reference
implementation). But it doesn't care if that is actually the reference BLAS or
any other BLAS implementation (like, e.g., OpenBLAS). Renaming libraries
allows switching the BLAS implementation that Octave uses. If you just unzip a
tarball and don't rename anything, libblas.dll is the reference BLAS
implementation. So, in that case the reference BLAS implementation is the
"default".

The reference implementation is usually the one with the "more numerically
stable" algorithms. But it can be orders of magnitudes slower as
implementations of the BLAS library that are optimized for certain hardware.
The OpenBLAS that is included in Octave for Windows is built for dynamic
architecture. I.e., it detects at runtime which CPU features it can use. It is
usually much faster than the reference implementation. But the results might
be "less accurate" in some situations compared to the reference
implementation.

(Having written that, there can still be bugs in either of them that could
lead to the reverse behavior to the one described before. I.e., more accurate
results with OpenBLAS or faster with the reference implementation. But the
"expected" behavior is that OpenBLAS is faster, and that the reference
implementation might be more accurate.)




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