Detect number of CPUs with `--num-workers 0`

[hugovk]

Feature

Mypy 2.0 added an experimental flag:

  -n, --num-workers NUM_WORKERS
                            Number of separate mypy worker processes (experimental)

Thanks for this! Running mypy --num-workers 8 on this 8-core machine for Pillow takes 3s instead of 10s (both with no cache).

I suggest --num-workers 0 detects the number of CPUs available.

Pitch

This means the value doesn't need changing for other machines. We can put --num-workers 0 in our tox config and it will use the correct number for each machine and also for the CI.

Alternatively, --num-workers auto.

Other tools commonly use 0 or auto to use the number of available CPUs (or sometimes unlimited). Some default to this.

Further

Right now, it looks like --num-workers 0 disables the parallel machinery, and --num-workers 1 sets it up with a single worker. It could be more efficient for 1 to run without this overhead.

mypy/mypy/build.py

Line 391 in d2b0234

if options.num_workers > 0:

[ilevkivskyi]

FWIW I think 0 should mean "disable parallel checking" (i.e. use in-process checking, same as now). While auto would mean "auto". We can then make auto the default.

Also 0 and 1 are intentionally different to have precise control over execution (mostly for debugging).