buildstream/_project.py:

Multiprocessing with n_jobs given by multiprocessing.cpu_count() is not optimal for
systems where not all CPU can be used (in particular, Docker, CI etc).

As suggested in the multiprocessing docs
(http://docs.python.org/3/library/multiprocessing.html#multiprocessing.cpu_count)
using len(os.sched_getaffinity(0)) might be better.