Add --parallel-threads="auto" or similar

[ngoldbaum]

This would choose a number of CPUs equal to the number of logical cores. It's surprisingly tricky and platform-dependent to get the correct answer so it would be best for pytest-run-parallel to depend on e.g. os.process_cpu_count on 3.13 and newer and use platform-specific fallbacks on older Python versions.

[ngoldbaum]

This might require a dependency on psutil to get this information in a cross-platform way on Python < 3.12.

[rgommers]

Seems like a good idea to me.

This might require a dependency on psutil to get this information in a cross-platform way on Python < 3.12.

I think so too. I don't remember all of the details, but I needed this for SciPy's dev.py before and ended up extracting this from threadpool_ctl:

https://github.com/scipy/scipy/blob/564da7926ac0900259bf62898f847b27252b6a49/dev.py#L1526-L1736

[lysnikolaou]

Sounds like a good idea to me too. FWIW here's what pytest-xdist does.

[ngoldbaum]

This ended up not being needed by bcrypt so I probably won't implement it quickly. If someone else gets a chance or has a need, please feel free.

[lysnikolaou]

This ended up not being needed by bcrypt so I probably won't implement it quickly. If someone else gets a chance or has a need, please feel free.

Will work on this tomorrow.