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

[Philip Nienhuis]

Follow-up Comment #27, bug #46830 (project octave):

@Markus:
Thanks, I knew that thread, it was about Intel Core CPU and I wasn't sure if
it would apply equally to AMD Ryzen CPUs.
The intruiging thing is that building Octave & crossbuilding on that Ryzen-7
box with "-j16" and (make nsis-installer) JOBS=16 resp., works much faster
than with "-j8" / JOBS=8, so it is just OpenBLAS that is affected by
hyperthreading.

I don't know much about CPU internals, but I suspect that hyperthreading
applies more to "regular" instructions rather than the less basic mathematical
operations.  The latter are usually delegated to dedicated parts of the CPU
that often contain "pipelines" e.g., operations for division, roots and trig,
all of which, unlike read/write from/to memory, cannot be handled in a couple
of clock ticks.



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