1 import multiprocessing
2
3 def do_calculation(data):
4 return data*2
5 def start_process():
6 print 'Starting',multiprocessing.current_process().name
7
8 if __name__=='__main__':
9 inputs=list(range(10))
10 print 'Inputs :',inputs
11
12 builtin_output=map(do_calculation,inputs)
13 print 'Build-In :', builtin_output
14
15 pool_size=multiprocessing.cpu_count()*2
16 pool=multiprocessing.Pool(processes=pool_size,
17 initializer=start_process,)
18
19 pool_outputs=pool.map(do_calculation,inputs)
20 pool.close()
21 pool.join()
22
23 print 'Pool :',pool_outputs
运行结果：
1 Inputs : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
2 Build-In : [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
3 Starting PoolWorker-2
4 Starting PoolWorker-1
5 Starting PoolWorker-3
6 Starting PoolWorker-4
7 Pool : [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
默认情况下，Pool会创建固定数目的工作进程，并向这些工作进程传递作业，直到再没有更多作业为止。maxtasksperchild参数为每个进程执行task的最大数目，设置maxtasksperchild参数可以告诉池在完成一定数量任务之后重新启动一个工作进程，来避免运行时间很长的工作进程消耗太多的系统资源。
maxtasksperchild is the number of tasks a worker process can complete before it will exit and be replaced with a fresh worker process, to enable unused resources to be freed. The default maxtasksperchild is None, which means worker processes will live as long as the pool.
Worker processes within a Pool typically live for the complete duration of the Pool’s work queue. A frequent pattern found in other systems (such as Apache, mod_wsgi, etc) to free resources held by workers is to allow a worker within a pool to complete only a set amount of work before being exiting, being cleaned up and a new process spawned to replace the old one. The maxtasksperchild argument to the Pool exposes this ability to the end user.
