public function checkQueue()
    {
        if ($this->backlog !== null) {
            while (!$this->backlog->isEmpty()) {
                if ($this->maxConcurrency !== -1 && ($this->jobsNum - $this->resultsNum > $this->maxConcurrency)) {
                    return;
                }
                list($name, $cb) = $this->backlog->shift();
                $this->addJob($name, $cb);
            }
        }
        if ($this->more !== null) {
            $this->more();
        }
    }