我有一个类似的代码：

import numpy as np
import multiprocessing as mp

a = np.zeros((4, 4))  # 4x4 array containing zeros

def f(x, y):
    # uses scipy functions
    # takes long to compute
    #result = <someValue after calculation>
    global a
    a[x][y] = x+y  # simple example function

# since f takes long to compute, I want to run it in parallel
jobs = []
for x in range(4):
    for y in range(4):
        p = mp.Process(target=f, args=(x, y))
        p.start()
        jobs.append(p)

# wait for a to be filled for the next step
for j in jobs:
    j.join()

print a  # prints an array of zeros! 

为什么没有将值分配给 a ，我该如何解决这个问题？谢谢！