def send_task(self, task, force=True):
        '''
        dispatch task to fetcher

        out queue may have size limit to prevent block, a send_buffer is used
        '''
        try:
            self.out_queue.put_nowait(task)
        except Queue.Full:
            if force:
                self._send_buffer.appendleft(task)
            else:
                raise
dispatch task to fetcher

        out queue may have size limit to prevent block, a send_buffer is used