python调用进程.start()报错 ...The "freeze_support()" line can be omitted if the program         is not going to be frozen to produce an executable.

我的代码

import time
import os
import multiprocessing


def work_a():
    for i in range(1, 11):
        print('a', i)
        time.sleep(1)


def work_b():
    for i in range(1, 11):
        print('b', i)
        time.sleep(1)


start = time.time()
a_p = multiprocessing.Process(target=work_a)
a_p.start()
work_b()
print("消耗的时间为{}".format(time.time() - start))
print(os.getpid())

：修改后

import time
import os
import multiprocessing


def work_a():
    for i in range(1, 11):
        print('a', i)
        time.sleep(1)


def work_b():
    for i in range(1, 11):
        print('b', i)
        time.sleep(1)

if __name__ == '__main__':

    start = time.time()

    a_p = multiprocessing.Process(target=work_a)
    a_p.start()
    work_b()
    print("消耗的时间为{}".format(time.time() - start))
    print(os.getpid())

修改后就可以正常运行了，希望有懂的大神解释下。