為什么開了多進程之后，一直停不下來

不知道是不是我格式錯誤，我用了多進程之后，程序一直停不下來

def main():
    start = time.clock()
    ulist = []
    pool = multiprocessing.Pool(processes=4)
    for i in range(1,10):
        url = "http://www.xicidaili.com/nn/{}".format(i)
        html = getHTML(url)
        pool.apply_async(paraHTML, (ulist,html))
        print('Process start!')
    pool.close()
    pool.join()
    end = time.clock()
    print(end-start)
    export(ulist)

main()

望大神指教