Max number of concurrent HttpWebRequests

Question

Hello

I'm stress testing a web app and have set up a windows test program that spins up a number of threads and sends out a web request on each one.

Problem is I get the following output:

01/09/09 11:34:04 Starting new HTTP request on 10
01/09/09 11:34:04 Starting new HTTP request on 11
01/09/09 11:34:04 Starting new HTTP request on 13
01/09/09 11:34:05 Starting new HTTP request on 14
01/09/09 11:34:05 Starting new HTTP request on 11
01/09/09 11:34:05 11 has finished!
01/09/09 11:34:05 Starting new HTTP request on 13
01/09/09 11:34:05 13 has finished!
01/09/09 11:34:05 Starting new HTTP request on 14
01/09/09 11:34:05 14 has finished!
01/09/09 11:34:05 Starting new HTTP request on 11
01/09/09 11:34:05 11 has finished!
01/09/09 11:34:05 Starting new HTTP request on 14
01/09/09 11:34:05 14 has finished!
01/09/09 11:34:05 Starting new HTTP request on 13
01/09/09 11:34:05 13 has finished!
01/09/09 11:34:05 Starting new HTTP request on 15
01/09/09 11:34:06 Starting new HTTP request on 11
01/09/09 11:34:06 11 has finished!
01/09/09 11:34:06 Starting new HTTP request on 14
01/09/09 11:34:06 14 has finished!

which sort of looks like there's a maximum of 5 threads, even if I create 100 as so:

int numberOfThreads = Convert.ToInt32(txtConcurrentThreads.Text);

    List<BackgroundWorker> workers = new List<BackgroundWorker>();

    for (int N = 0; N < numberOfThreads; N++)
    {

        BackgroundWorker worker = new BackgroundWorker();
        worker.DoWork += new DoWorkEventHandler(worker_DoWork);
        worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);
        workers.Add(worker);
    }


    foreach(BackgroundWorker worker in workers)
    {
        worker.RunWorkerAsync();
    }

Can anyone enlighten me as to what is going on?

Thanks

EDIT: If as suggested I sleep for 5 seconds, instead of httpwebrequest, then I do get more threads firing but not as many as I would have expected:

01/09/09 11:56:14 Starting new HTTP request on 7
01/09/09 11:56:14 Starting new HTTP request on 11
01/09/09 11:56:15 Starting new HTTP request on 12
01/09/09 11:56:15 Starting new HTTP request on 13
01/09/09 11:56:16 Starting new HTTP request on 14
01/09/09 11:56:16 Starting new HTTP request on 15
01/09/09 11:56:17 Starting new HTTP request on 16
01/09/09 11:56:17 Starting new HTTP request on 17
01/09/09 11:56:18 Starting new HTTP request on 18
01/09/09 11:56:19 Starting new HTTP request on 7
01/09/09 11:56:19 7 has finished!
01/09/09 11:56:19 Starting new HTTP request on 11
01/09/09 11:56:19 11 has finished!
01/09/09 11:56:19 Starting new HTTP request on 19
01/09/09 11:56:20 Starting new HTTP request on 20
01/09/09 11:56:20 Starting new HTTP request on 12
01/09/09 11:56:20 12 has finished!

It still looks like I'm only getting 2 threads starting every second, which seems mighty slow to me. I suppose the Console.WriteLine could be a problem?

EDIT: I set

ThreadPool.SetMinThreads(100, 4);

and

System.Net.ServicePointManager.DefaultConnectionLimit = 100;

and got the following results:

01/09/09 14:00:07 Starting new HTTP request on 11
01/09/09 14:00:07 Starting new HTTP request on 81
01/09/09 14:00:07 Starting new HTTP request on 82
01/09/09 14:00:07 Starting new HTTP request on 79
01/09/09 14:00:07 Starting new HTTP request on 83
01/09/09 14:00:07 Starting new HTTP request on 84
01/09/09 14:00:07 Starting new HTTP request on 85
01/09/09 14:00:07 Starting new HTTP request on 87
01/09/09 14:00:07 Starting new HTTP request on 88
...
01/09/09 14:00:07 84 has finished! Took 323.0323 milliseconds
01/09/09 14:00:08 88 has finished! Took 808.0808 milliseconds
01/09/09 14:00:08 96 has finished! Took 806.0806 milliseconds
01/09/09 14:00:08 94 has finished! Took 806.0806 milliseconds
01/09/09 14:00:08 98 has finished! Took 801.0801 milliseconds
01/09/09 14:00:08 80 has finished! Took 799.0799 milliseconds
01/09/09 14:00:08 86 has finished! Took 799.0799 milliseconds
01/09/09 14:00:08 92 has finished! Took 799.0799 milliseconds
01/09/09 14:00:08 100 has finished! Took 812.0812 milliseconds
01/09/09 14:00:08 82 has finished! Took 1010.101 milliseconds

so was able to push out a whole lot of web requests concurrently. Which seemed to queue (calling out to an STA COM+ server) so that's what I expected.

Thanks for your help

Answer 1

Are all (or most) of your requests going to the same host by any chance? There's a built-in limit on a per-host basis. You can change this in app.config in the system.Net connectionManagement element.

The other thing is that the thread pool only ramps up its number of threads gradually - it starts a new thread every half second, IIRC. Could that be what you're seeing? Try getting rid of HttpWebRequest from the equation - just sleep for a couple of seconds instead...

I suspect the latter problem is the one you're initially running into, but the first one is going to cause you problems as well.

Answer 2

There is a limit in the number of simultaneous outgoing HTTP connections. I think you can control this by using the System.Net.ServicePointManager.DefaultConnectionLimit static property before creating the HttpWebRequest objects.