public function start(string $poolOnly = '') : void
    {
        $this->reloadPids();

        foreach ($this->poolsConfig as $pool => $workerConfig) {
            // don't start workers of different type if filter is set
            if (!empty($poolOnly) && $poolOnly !== $pool) {
                continue;
            }

            // don't start new workers if already running:
            if (!empty($this->pids[$pool])) {
                continue;
            }

            // startup the workers:
            for ($i = 0; $i < $workerConfig['instances']; $i++) {
                $this->startupWorker($pool);
            }
        }
    }