jupyter notebook中multiprocessing无法执行的问题

 output = mp.Queue()

def rand_string(length, output):
    output.put(length)

if __name__ == "__main__":
    # Setup a list of processes that we want to run
    processes = [mp.Process(target=rand_string, args=(x, output)) for x in range(4)]

    # Run processes
    for p in processes:
        p.start()

    # Exit the completed processes
    for p in processes:
        p.join()

    # Get process results from the output queue
    results = [output.get() for p in processes]

    print(results)

这是一段官方文档中代码，只是稍加改造而已，我在pycharm中运行正常，但是到了jupyter中，却没有反应，点了运行，然后代码框前面的序号变成星号，就没有然后了，也不报错，一只就这样。请问各位大神，有知道的吗？不仅如此，很多涉及到多进程的例子，我在pycharm下跑的通，到了jupyter下，就没反应。