Note that maxtasksperchild is new in 2.7.

Doc/library/multiprocessing.rst

@@ -1543,6 +1543,7 @@ with the :class:`Pool` class.
    *initializer* is not ``None`` then each worker process will call
    ``initializer(*initargs)`` when it starts.
 
+   .. versionadded:: 2.7
       *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