public function retrieveTotalMemoryUsage()
    {
        $base = $this->retrieve(0);
        $total = $last = 0;

        foreach ($this->retrieve() as $item) {
            $total += max(0, (($item[5]-$base[5]) - $last));
            $last = $item[5]-$base[5];
        }

        return $total;
    }