之前接触过一点这个库，但是并没有深入研究，这次闲着无聊就研究了一下，算是解惑吧。
今天先研究下apply_async与map方法。传闻就是这两个方法分配进程池中的进程给相关函数，我想验证下。
看下官网对这两个的解释：
apply_async(func[, args[, kwds[, callback[, error_callback]]]])
A variant of the apply() method which returns a result object.
If callback is specified then it should be a callable which accepts a single argument. When the result becomes ready callback is applied to it, that is unless the call failed, in which case the error_callback is applied instead.
If error_callback is specified then it should be a callable which accepts a single argument. If the target function fails, then the error_callback is called with the exception instance.
Callbacks should complete immediately since otherwise the thread which handles the results will get blocked.
map(func, iterable[, chunksize])
A parallel equivalent of the map() built-in function (it supports only one iterable argument though). It blocks until the result is ready.
This method chops the iterable into a number of chunks which it submits to the process pool as separate tasks. The (approximate) size of these chunks can be specified by setting chunksize to a positive integer.
Pool可以提供指定数量的进程供用户调用，当有新的请求提交到pool中时，如果池还没有满，那么就会创建一个新的进程用来执行该请求；但如果池中的进程数已经达到规定最大值，那么该请求就会等待，直到池中有进程结束，才会创建新的进程来运行它
下面看下程序吧：
from multiprocessing import Poolimport timeimport osdef func(msg):print('msg: %s %s' % (msg, os.getpid()))
time.sleep(3)print("end")if __name__ == '__main__':
pool = Pool(4)for i in range(4):
msg = 'hello %d' % (i)
pool.apply_async(func, (msg, ))# pool.map(func, range(4))print("Mark~ Mark~ Mark~~~~~~~~~~~~~~~~~~~~~~")
pool.close()
pool.join() # 调用join之前，先调用close函数，否则会出错。执行完close后不会有新的进程加入到pool,join函数等待所有子进程结束print("Sub-process(es) done.")
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