Add the optional ``buffersize`` parameter to
:meth:`multiprocessing.pool.Pool.imap` and
:meth:`multiprocessing.pool.Pool.imap_unordered` to limit the number of
submitted tasks whose results have not yet been yielded. If the buffer is
full, iteration over the *iterables* pauses until a result is yielded from
the buffer. To fully utilize pool's capacity when using this feature, set
*buffersize* at least to the number of processes in pool (to consume
*iterable* as you go), or even higher (to prefetch the next
``N=buffersize-processes`` arguments).
