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Is this really asynchronous?

Posted June 16, 2008

Topics: Programming, Windows Programming

I've been working on a relatively simple program whose purpose is to see just how fast I can issue Web requests. The idea is to get one machine hooked directly to an Internet connection and see how many concurrent connections it can maintain and how much bandwidth it can consume. A straight bandwidth test is easy: just start three or four Linux distribution downloads from different sites. That'll usually max out a cable modem connection.

But determining the sustained concurrent connection rate is a bit more difficult. It requires that you issue a lot of requests, very quickly, for an extended period of time. By slowly increasing the number of concurrent connections and monitoring the bandwidth used, I should be able to find an optimum range of request rates: one that makes maximum use of bandwidth, but doesn't cause requests to timeout.

I got the program up and limping the other day, and was somewhat surprised to find that it couldn't maintain more than 15 concurrent connections for any length of time. Considering that my crawler can maintain 200 or more connections without a problem, I found that quite curious. It had to be something about the different way I was issuing requests.

Because this is a simple tool, I figured I'd use the .NET Framework's WebClient component to issue the requests. In order to avoid the overhead of constructing a new WebClient for every request, I initialized 100 WebClient instances to be served from a queue, and then issued the requests in a loop, kind of like this:
while (!shutdown)
{
    if (currentConnections < MaxConnections)
    {
        WebClient cli = GetClientFromQueue();
        ++currentConnections;
        cli.DownloadStringAsync(GetNextUrlFromQueue());
    }
}

The actual code is a bit more involved, of course, but that's the gist of it. The currentConnections counter gets decremented in the download completed event handler. The important thing to note here is that I'm issuing asynchronous requests. The call to DownloadStringAsync executes on a thread pool thread. This code should issue requests at a blindingly fast rate, and keep the number of concurrent connections right near the maximum. Even with MaxConnections set to 50, the best I could do was 20 concurrent--and that for only a very short time. Most often I had somewhere between 10 and 15 concurrent connections.

After eliminating everything else, I finally got around to timing just how long it takes to issue that asynchronous request. The result was pretty surprising: in my brief tests, it took anywhere from 0 to 300 milliseconds to issue those requests. The average seemed to be around 100 or 150 ms. That would explain why I could only keep 10 or 15 connections open. If it takes 100 ms to issue a request, then I can only make 10 requests per second. Since it takes about 2 seconds (on average) to complete a request, the absolute best I'll be able to do is 20 concurrent requests.

So I got to thinking, why would it take 100 milliseconds or more to issue an asynchronous Web request? And the only reasonable answer I could come up with was DNS: resolving the domain name. And it turns out I was right. I flushed the DNS cache and ran my test by requesting a small number of URLs from different domains. Sure enough, it averaged about 150 ms per request. I then ran the program again and it took almost no time at all to issue the requests. Why? Because the DNS cache already had those domain names resolved. Just to make sure, I flushed the DNS cache again and re-ran the test.

By the way, the HttpWebRequest.BeginGetResponse method (the low-level counterpart to WebClient.DownloadStringAsync) exhibits the same behavior. That's not surprising, considering that WebClient uses HttpWebRequest to do its thing.

This is a fatal flaw in the design of the .NET Framework's support for asynchronous Web requests. The whole idea of supplying asynchronous methods for I/O requests is to push the waiting off on to background threads so that the main thread can continue processing. What's the use of providing an asynchronous method if you have to wait for a high latency task like DNS resolution to complete before the asynchronous request is issued? Why can't the DNS resolution be done on the thread pool thread, just like the actual Web request is?

There is a way around the problem: queue a background thread to issue the asynchronous request. Yes, I know it sounds crazy, but it works. And it's incredibly easy to do with anonymous delegates:

ThreadPool.QueueUserWorkItem(delegate(object state)
    {
        cli.DownloadStringAsync((Uri)state);
    }, GetNextUrlFromQueue());
That spawns a thread, which then issues the asynchronous Web request. The time waiting for DNS lookup is spent in the background thread rather than on the main processing thread. It looks pretty goofy, and unless it's commented well somebody six months from now will wonder what I was smoking when I wrote it.

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