Bring the number of pool processes up to the specified number,
        for use after reaping workers which have exited.

def _repopulate_pool(self):
        """Bring the number of pool processes up to the specified number,
        for use after reaping workers which have exited.
        """
        for i in range(self._processes - len(self._pool)):
            w = self.Process(target=self._worker_f,
                             args=(self._inqueue, self._outqueue,
                                   self._initializer,
                                   self._initargs, self._maxtasksperchild))
            self._pool.append(w)
            w.name = w.name.replace('Process', 'PoolWorker')
            w.daemon = True
            w.start()
            debug('added worker')