参考：http://api.mongodb.com/python/current/faq.html#using-pymongo-with-multiprocessing
如果使用了多进程，则必须在子进程中创建mongo连接。不能创建一个mongo连接传给多个进程！！
# Each process creates its own instance of MongoClient.
def func():
db = pymongo.MongoClient().mydb
# Do something with db. proc = multiprocessing.Process(target=func)
proc.start()