无法释放进程内存

<?php

use PDO;
use Workerman\Worker;
require_once 'Workerman/Autoloader.php';

$w = new Worker();
$w->onWorkerStart = function(){
    echo memory_get_usage()."\n";
    $set = Config::getItem('db');
    $pdo = new PDO(
        $set,
        $set,
        $set,

    );
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $pdo->prepare("SELECT * FROM orders", );
    $stmt->execute(null);
    $stmt->setFetchMode(PDO::FETCH_ASSOC);
    $data = $stmt->fetchAll();
    $stmt = null;
    echo memory_get_usage()."\n";
    unset($data);
    echo memory_get_usage()."\n";
};