function cleanConnection()
    {
        $now = time();
        foreach($this->connections as $client_id => $conn)
        {
            if($conn['time'] < $now - $this->heart_time)
            {
                $this->log("connection[$client_id] timeout.", 'CLOSE');
                $this->close($client_id);
            }
        }
        $this->log('clean connections');
    }