protected function ack()
    {
        $this->queue->ack($this->lastDeliveryTag, Constants::AMQP_MULTIPLE);
        $this->lastDeliveryTag = null;
        $delta = $this->timestampLastMessage - $this->timestampLastAck;

        $this->logger->info(sprintf(
            'Acknowledged %d messages at %.0f msg/s',
            $this->countMessagesUnacked,
            $delta ? $this->countMessagesUnacked / $delta : 0
        ));

        $this->timestampLastAck = microtime(true);
        $this->countMessagesUnacked = 0;
    }