we need to evaluate the result across many random seeds, so each task needs to log the result independently.

Ohh that kind of makes sense to me 🙂
Yes I'm also getting:

/usr/local/lib/python3.6/multiprocessing/semaphore_tracker.py:143: UserWarning: semaphore_tracker: There appear to be 74 leaked semaphores to clean up at shutdown
  len(cache))

Not sure about that ...