protected function spawnMinimumWorkers()
    {
        $this->logger->debug('verify that the minimum number ['.$this->min_children.'] of workers are running', [
            'category' => get_class($this),
        ]);

        for ($i = $this->count(); $i < $this->min_children; ++$i) {
            $this->spawnWorker();
        }
    }