Add an API to allow getting the number of "performance" cores-only

Feature or enhancement

Proposal:

Currently Python provides os.cpu_count() which returns the total number of physical CPU cores.

On macOS M-class machines (and probably other ARM hardware), there are actually two types of CPU cores: performance cores, and efficiency cores.

For many tasks where a user wants to do "1 task per core", it really only makes sense to do 1 task per performance core.

Therefore it'd be sensible to expose this number. On macOS it can be found with the hw.perflevel0.logicalcpu sysctl.

This could be done in one of two ways, a new function, e.g. os.performance_cpu_count(), or as a keyword-argument to os.cpu_count(performance_cores_only=True).

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Start by reading the existing os.cpu_count() API and its platform-specific implementations, then investigate the macOS hw.perflevel0.logicalcpu sysctl described in the proposal. Compare the new-function and keyword-argument options across supported platforms, and identify the relevant tests. Done means the chosen API reliably exposes performance-core counts with documented behavior and coverage for supported systems.