Multiprocessing Queue need Manager?

问题

Why program A deadlock but program B runs fine?

Both uses a queue to pass result between main process and sub-processes. Both wait for all sub-processes to end before retrieving the results from the queue.

In the actual program, I need a queue to pass streams of results between steps. This is for working on results when the results are generated and before one of the step completes.

Changing the mp.queue in program A to a queue of a mp.Manager would fix the deadlock. But using manager seems to have performance penalty, because the manager is managed by the main process.

Program A (deadlocks without manager):

import multiprocessing as mp
def worker(q, i): 
    q.put(i)

if __name__ == "__main__":
    q = mp.Queue()

    # Start sub-processes
    p = mp.Pool()
    for i in range(4):
        p.apply_async(worker, args=(q, i)) 

    # wait till all workers complete their task.
    p.close()
    p.join()

    # Get results from queue
    for i in range(4):
        print(q.get())

Program B is my simplification of a program at (https://docs.python.org/3/howto/logging-cookbook.html) below the line "basis for code meeting your own specific requirements", and it doesn't use a manager ...

Program B:

import multiprocessing as mp
def worker(q, i):
    q.put(i)

if __name__  == "__main__":
    q = mp.Queue()

    # Start sub-processes
    ws = []
    for i in range(4):
        w = mp.Process(target=worker, args=(q, i))
        ws.append(w)
        w.start()

    # wait till all workers complete their task.
    for w in ws:
        w.join()

    # Get results from queue
    for i in range(4):
        print(q.get())

来源：https://stackoverflow.com/questions/45346814/multiprocessing-queue-need-manager