
The Parallelism Blues: when faster code is slower - itamarst
https://pythonspeed.com/articles/parallelism-slower/
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navjack27
Is what you think you're testing actually what you're testing here? You could
be threaded into hyperthreads/smt. If on Zen 1 or Zen refresh you could be
hitting cross CCX communication issues. There are a number of things here that
I'm just like shrugging my shoulders at, like, neat, your computer shows a
behavior.

Edit: what was the CPU frequency during the single core load vs the multi core
load?

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itamarst
Threads per core is 1, according lscpu.

This is a Xeon desktop.

Here's a documented example of an OpenBLAS threading issue from someone rather
more knowledgeable than I am:
[https://github.com/xianyi/OpenBLAS/issues/731](https://github.com/xianyi/OpenBLAS/issues/731)

