PushStreamContent 流在负载下不会刷新

我正在使用 PushStreamContent 来保持与每个客户端的持久连接。每 20 秒向每个客户端流推送短心跳消息对于 100 个客户端来说效果很好，但在大约 200 个客户端时，客户端首先开始延迟几秒钟接收，然后根本不显示。

我的控制器代码是

    // Based loosely on https://aspnetwebstack.codeplex.com/discussions/359056
    // and http://blogs.msdn.com/b/henrikn/archive/2012/04/23/using-cookies-with-asp-net-web-api.aspx
    public class LiveController : ApiController
    {
        public HttpResponseMessage Get(HttpRequestMessage request)
        {
            if (_timer == null)
            {
                // 20 second timer
                _timer = new Timer(TimerCallback, this, 20000, 20000);    
            }

            // Get '?clientid=xxx'
            HttpResponseMessage response = request.CreateResponse();
            var kvp = request.GetQueryNameValuePairs().Where(q => q.Key.ToLower() == "clientid").FirstOrDefault();
            string clientId = kvp.Value; 

            HttpContext.Current.Response.ClientDisconnectedToken.Register(
                                                                    delegate(object obj)
                                                                    {
                                                                        // Client has cleanly disconnected
                                                                        var disconnectedClientId = (string)obj;
                                                                        CloseStreamFor(disconnectedClientId);
                                                                    }
                                                                    , clientId);

            response.Content = new PushStreamContent(
                                    delegate(Stream stream, HttpContent content, TransportContext context)
                                    {
                                        SaveStreamFor(clientId, stream);
                                    }
                                    , "text/event-stream");


            return response;
        }

        private static void CloseStreamFor(string clientId)
        {
            Stream oldStream;
            _streams.TryRemove(clientId, out oldStream);
            if (oldStream != null)
                oldStream.Close();
        }

        private static void SaveStreamFor(string clientId, Stream stream)
        {
            _streams.TryAdd(clientId, stream);
        }


        private static void TimerCallback(object obj)
        {
            DateTime start = DateTime.Now;

            // Disable timer
            _timer.Change(Timeout.Infinite, Timeout.Infinite);      

            // Every 20 seconds, send a heartbeat to each client
            var recipients = _streams.ToArray();
            foreach (var kvp in recipients)
            {
                string clientId = kvp.Key;
                var stream = kvp.Value;
                try
                {
                    // ***
                    // Adding this Trace statement and running in debugger caused
                    // heartbeats to be reliably flushed!
                    // ***
                    Trace.WriteLine(string.Format("** {0}: Timercallback: {1}", DateTime.Now.ToString("G"), clientId));

                    WriteHeartBeat(stream);
                }
                catch (Exception ex)
                {
                    CloseStreamFor(clientId);
                }
            }

           // Trace... (this trace statement had no effect) 
            _timer.Change(20000, 20000);      // re-enable timer
        }

        private static void WriteHeartBeat(Stream stream)
        {
            WriteStream(stream, "event:heartbeat\ndata:-\n\n");
        }


        private static void WriteStream(Stream stream, string data)
        {
            byte[] arr = Encoding.ASCII.GetBytes(data);
            stream.Write(arr, 0, arr.Length);
            stream.Flush();
        }

        private static readonly ConcurrentDictionary<string, Stream> _streams = new ConcurrentDictionary<string, Stream>();
        private static Timer _timer;
    }

是否有某些 ASP.NET 或 IIS 设置会影响此问题？我在 Windows Server 2008 R2 上运行。

更新：

如果 1) 添加了 Trace.WriteLine 语句，2) 连接了 Visual Studio 2013 调试器并调试和捕获 Trace.WriteLines，则可以可靠地发送心跳。

这两点都是必要的；如果删除 Trace.WriteLine，则在调试器下运行没有任何效果。如果 Trace.WriteLine 存在，但程序未在调试器下运行（而是 SysInternals 的 DbgView 显示跟踪消息），则心跳不可靠。

更新2：

后来与微软发生了两次支持事件，结论如下：

1) 通过使用企业级互联网连接而不是家庭连接解决了 200 个客户端的延迟问题

2）是否附加调试器实际上没有任何区别；

3) 需要在 web.config 中添加以下两个内容，以确保及时发送心跳，并且由于客户端“不干净”地断开连接（例如，通过拔掉计算机而不是正常关闭干净地发出 TCP RST 的程序）而导致心跳失败，会触发及时的 ClientDisconnected回调以及：

<httpRuntime executionTimeout="5" />

<serverRuntime appConcurrentRequestLimit="50000" uploadReadAheadSize="1" frequentHitThreshold="2147483647" />