高并发下Apache+mongodb的php驱动不稳定

我的环境 : Apache/2.2.21 (Win32) PHP/5.4.3

<?php
$db=new Mongo("mongodb://localhost:27017");
$c=$db->selectDB("npm")->selectCollection("user");
$user = $c->findOne(array('userName' => 'admin','passWord'=>'admin'));
if($user){
echo var_dump($user);
}
echo var_dump($db);
?>
使用apache并发测试工具 ab.exe 测试，测试命令如下

D:\Apache2.2\bin>ab.exe -n 100  http://192.168.15.21/a.php
测试结果:

Concurrency Level:      1
Time taken for tests:   0.266 seconds
Complete requests:      100
Failed requests:        58
   (Connect: 0, Receive: 0, Length: 58, Exceptions: 0)
Write errors:           0
Total transferred:      888192 bytes
HTML transferred:       870668 bytes
Requests per second:    376.47 [#/sec](mean)
Time per request:       2.656 [ms](mean)
Time per request:       2.656 [ms](mean, across all concurrent requests)
Transfer rate:          3265.41 [Kbytes/sec] received
php错误信息:

[13-Jun-2012 07:40:26 UTC] PHP Notice:  Unknown: localhost:27017: pool done (0x3877a10) in Unknown on