Allow automatically determining the number of threads

[oscargus]

Pass -p0 to use all cores (determined by multiprocessing.cpu_count().

[LarsAsplund]

Sounds like a good idea but I would like it to take hyper threading into account. I typically run 12 threads on my 6 cores. From what I understand there is also os.cpu_count that might work differently. I'm on my phone so I can't test now. I've also seen (discussion) threads indicating it doesn't always work. Differences between Windows and Linux.

[oscargus]

I am quite sure that it does that. I've tested a few computers to confirm.

On my laptop (i7 1260P, 4P + 8E): cpu_count() = 16
On my workstation (some Xeon, 6 cores, 12 threads): cpu_count() = 12
On my compute server (Two 20-core Xeon, 80 threads): cpu_count() = 80

I was a bit unsure what to write in the note as the concept of threads from a software perspective is a bit different from the hardware perspective... Maybe one should write "logical CPUs"? (os.cpu_count() returns the same btw.)

[LarsAsplund]

Ok, then it should be fine. Regarding the use of "threads". I think we should keep it when explaining the -p option since the verbose option name is --num-threads.

I will look into the PR a bit more when I'm back at my computer but I can't see any major issues with it. I'm not sure how common it is that cpu_count fails to determine the number of threads but I think we should test for that case and raise a descriptive error if it happens.

[LarsAsplund]

I tested this locally and it works as expected.

I take back my comment on how the -p option is described. I'm good with how it is stated and will merge this PR.

[oscargus]

I take back my comment on how the -p option is described. I'm good with how it is stated and will merge this PR.

I reformulated it based on your comment, so glad it improved it. :-)