请教关于阿里云 RDS 数据库的问题

并发 500 个连接左右，就会报错。如下：

[09-Aug-2017 12:39:07 Asia/Shanghai]

PHP Fatal error:

Uncaught PDOException: PDO::__construct(): php_network_getaddresses: getaddrinfo failed: Name or service not known in /www/connect/public_html/function.php:14

Stack trace:

#0 /www/web/connect/function.php(14): PDO->__construct('mysql:host=rm-b...', '数据库用户名', '数据库密码')

#1 /www/web/connect/function.php(10): mysql->connect()

#2 /www/web/connect/function.php(2): mysql->__construct()

#3 /www/web/connect/direct.php(8): require_once('/www/web/function.php')

数据库连接代码如下：

$mysql = new mysql();

class mysql {

protected $pdo;

protected $res;

protected $config;

/*构造函数*/

function __construct() {

    //$this->Config = $config;
    
    $this->connect();
    
}

/*数据库连接*/

public function connect() {

    $this->pdo = new PDO(DB_TYPE.':host='.DB_HOSTNAME.';dbname='.DB_NAME,DB_USERNAME,DB_PASSWORD);
    
    $this->pdo->query('set names utf8;');
    
    $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    
}

/*数据库关闭*/

public function close() {

    $this->pdo = null;
    
}