static public function elapsedMemory($start, $end, $real_usage = false, $inMB = true) {
        $start = isset(self::$_marks[$start]) ? self::$_marks[$start]['memory_usage'] : 0;
        $end = isset(self::$_marks[$end]) ? self::$_marks[$end]['memory_usage'] : 0;
        
        $elapsed = !$real_usage ? ($end - $start) : $end;
        
        return $start ? ($inMB ? (($elapsed / 1024 / 1024) . ' MB') : $elapsed) : null;
    }