多处理池map\u async的意外行为

我有一些代码对python 3应用程序中的多个文件执行相同的操作,因此似乎是 multiprocessing . 我想用 Pool 将工作分配给一些进程。在进行这些计算时,我希望代码继续执行其他操作(主要是为用户显示内容),因此我希望使用 map_async 的功能 multiprocessing.Pool 类。我希望调用此函数后,代码将继续,结果将由我指定的回调处理,但这似乎没有发生。下面的代码显示了我尝试调用的三种方法 map\U异步 我看到的结果是:

import multiprocessing
NUM_PROCS = 4
def func(arg_list):
    arg1 = arg_list[0]
    arg2 = arg_list[1]
    print('start func')
    print ('arg1 = {0}'.format(arg1))
    print ('arg2 = {0}'.format(arg2))
    time.sleep(1)
    result1 = arg1 * arg2
    print('end func')
    return result1

def callback(result):
    print('result is {0}'.format(result))


def error_handler(error1):
    print('error in call\n {0}'.format(error1))


def async1(arg_list1):
    # This is how my understanding of map_async suggests i should
    # call it. When I execute this, the target function func() is not called
    with multiprocessing.Pool(NUM_PROCS) as p1:
        r1 = p1.map_async(func,
                          arg_list1,
                          callback=callback,
                          error_callback=error_handler)


def async2(arg_list1):
    with multiprocessing.Pool(NUM_PROCS) as p1:
        # If I call the wait function on the result for a small
        # amount of time, then the target function func() is called
        # and executes sucessfully in 2 processes, but the callback
        # function is never called so the results are not processed
        r1 = p1.map_async(func,
                          arg_list1,
                          callback=callback,
                          error_callback=error_handler)
        r1.wait(0.1)


def async3(arg_list1):
    # if I explicitly call join on the pool, then the target function func()
    # successfully executes in 2 processes and the callback function is also
    # called, but by calling join the processing is not asynchronous any more
    # as join blocks the main process until the other processes are finished.
    with multiprocessing.Pool(NUM_PROCS) as p1:
        r1 = p1.map_async(func,
                          arg_list1,
                          callback=callback,
                          error_callback=error_handler)
        p1.close()
        p1.join()


def main():
    arg_list1 = [(5, 3), (7, 4), (-8, 10), (4, 12)]
    async3(arg_list1)

    print('pool executed successfully')


if __name__ == '__main__':
    main()

什么时候 async1 , async2 或 async3 在main中调用,结果在每个函数的注释中描述。有人能解释一下为什么不同的电话会这样吗?最后我想打电话 map\U异步 如中所述 异步1 ,这样,当工作进程繁忙时,我可以在主进程中执行其他操作。我已经用python 2.7和3.6在旧的RH6 linux机器和新的ubuntu VM上测试了这段代码,得到了相同的结果。