Is there a good reason to worry about overeager worker spawning? ProcessPoolExecutor spawns all workers when the first work item is submitted ( https://hg.python.org/cpython/file/3.4/Lib/concurrent/futures/process.py#l361 ), only ThreadPoolExecutor even makes an effort to limit the number of threads spawned. Threads are typically more lightweight than processes, and with the recent GIL improvements, the CPython specific costs associated with threads (particularly threads that are just sitting around waiting on a lock) are fairly minimal.

It just seems like if eager process spawning isn't a problem, neither is (cheaper) eager thread spawning.