private function addPending()
    {
        while (count($this->inProcess) < $this->concurrency) {

            $item = $this->getUnprocessed();

            if (empty($item)) {

                break;
            }

            $this->inProcess[$item['index']] = $item['process'];

            $item['process']->start();

            if (count($this->inProcess) >= $this->concurrency) {

                break;
            }
        }
    }