Multiprocessing(1)---Pool

Example 1 The use of multiprocessing.Pool

Program

from multiprocessing import Pool # with first alphebet capital
import os, time, random
# define the task function for processes
def run_task(name):
    print('task {} (pid={}) is running '.format(name, os.getpid()))
    #set the delay time for the process
    time.sleep(random.random()*3) # annotation 1
    print('task {} ends'.format(name))
# mian process
if __name__ == '__main__':
    print('current process',os.getpid())# annotation 2
    # construct the process pool of three processes
    p=Pool(processes=3)# pay attention to 'processes'
    # apply the 5 task funtions to the pool 
    for i in range(5):
        p.apply_async(run_task,args=(str(i),))# annotation 3
    print('waiting for all processes end')
    # close the pool
    p.close() # p.close() to excute the pool
    # block the main process
    p.join() 
    print('all processes end')

Output

current process 6780
waiting for all processes end
task 0 (pid=7576) is running
task 1 (pid=10112) is running
task 2 (pid=14572) is running
task 0 ends
task 3 (pid=7576) is running
task 2 ends
task 4 (pid=14572) is running
task 4 ends
task 1 ends
task 3 ends
all processes end

Annotation

1：(1)time.sleep(secs) delay the process by secs. (2)random.random() return x in the interval [0,1) .

2:(1) os.getpid() return the process id of the current process. (2) os.getppid() return the process id of the father process.

3: apply_async(func,args=(,)) apply funtion to the pool( there is no ‘target’ ).

Attention

1. Before join() there must a close(), when using Pool.