C# SmtpClient可伸缩性问题（只有两个正在运行的请求）

我正在尝试使用Windows Server 2008 64位企业版上32个CPU的.NET 4.5中的方法发送大量电子邮件

在调试期间，我发现只有两个请求同时运行。考虑到我使用的是服务器操作系统，这很奇怪

下面是一个简单的测试用例：

static void Main(string[] args)
{
    var sw = Stopwatch.StartNew();
    var t1 = SendEmail();
    var t2 = SendEmail();
    var t3 = SendEmail();
    var t4 = SendEmail();
    var t5 = SendEmail();
    var t6 = SendEmail();
    var t7 = SendEmail();
    var t8 = SendEmail();
    Task.WaitAll(t1, t2, t3, t4, t5, t6, t7, t8);
    Console.WriteLine(sw.Elapsed);
}

static async Task SendEmail()
{
    using (var smtp = new SmtpClient())
    {
        Console.WriteLine("Starting [{0}]", DateTime.UtcNow.ToString("o"));
        await smtp.SendMailAsync("...", "...", "Test", "Testing...");
        Console.WriteLine("Done [{0}]", DateTime.UtcNow.ToString("o"));
    }
}

结果呢。它启动很快，但分两批完成（参见《泰晤士报》）。在Process Explorer中检查打开的连接也会显示相同的行为

Starting [2013-03-07T14:27:17.4643296Z]
Starting [2013-03-07T14:27:17.4783382Z]
Starting [2013-03-07T14:27:17.4783382Z]
Starting [2013-03-07T14:27:17.4793387Z]
Starting [2013-03-07T14:27:17.4793387Z]
Starting [2013-03-07T14:27:17.4793387Z]
Starting [2013-03-07T14:27:17.4793387Z]
Starting [2013-03-07T14:27:17.4793387Z]
Done [2013-03-07T14:27:19.2070974Z]
Done [2013-03-07T14:27:19.2070974Z]
Done [2013-03-07T14:27:19.7541596Z]
Done [2013-03-07T14:27:19.7602234Z]
Done [2013-03-07T14:27:20.3880420Z]
Done [2013-03-07T14:27:20.3906050Z]
Done [2013-03-07T14:27:20.9254624Z]
Done [2013-03-07T14:27:20.9324577Z]
00:00:03.7476343

---

有没有办法真正加速更多请求？

每个ServicePoint对象的默认并发连接数限制为2个。您可以通过设置ServicePointManager.DefaultConnectionLimit为将来的ServicePoint实例修改此设置。您还可以修改现有实例的ServicePoint.ConnectionLimit-SmtpClient会公开一个ServicePoint属性，您可以使用该属性获取此属性。

在创建SmtpClient对象之前，您是否尝试过修改System.Net.ServicePointManager.DefaultConnectionLimit？或上述代码中的smtp.ServicePoint.ConnectionLimit？是！这就是原因。如果你加上那个作为答案，我会记下来的。谢谢