protected function _fillup($max)
    {
        $nb = $max - $this->_running;
        $list = array_splice($this->_queue, 0, $nb);
        foreach ($list as $item) {
            $handle = $this->_initRequest($item['socket'], $item['response']);
            $this->_handles[(integer) $handle] = $item;
            curl_multi_add_handle($this->_curl, $handle);
        }
    }