调用Web服务如何导致死锁？

Question

This confusing MSDN article talks about how invoking a web service from an ASP.Net application can lead to a deadlock. I understand the part about each web service call needing a second thread to do IO completion. I understand that there may not always be such a thread available, which means that the thread making the web service call will block and not make progress until such a thread becomes available. But it seems to me that the web service calls that did get an IO thread will advance, and eventually both threads will be released back to the thread pool for use by the previously-blocked calls. So where does the deadlock come from? At the point that IIS kills the process, can you describe what the deadlocked threads are waiting for?

Answer 1

This article is for .NET 1.0 / IIS 6 and doesn't really apply in newer versions of .NET/IIS. The limits on the number of threadpool and IOCP threads are vastly higher in more recent versions of the framework, and the likelihood that you'll run into them is very, very small.

The basic problem that used to happen was that ASP.NET would limit the number of threads that it allowed anything to use and web requests were handled on I/O threads, so incoming web requests would consume all of the available threads and IO completion for outstanding (asynchronous) calls would be queued up behind the very web requests that were waiting for them.

See also http://blogs.msdn.com/b/nicd/archive/2007/04/16/dissection-of-an-asp-net-2-0-request-processing-flow.aspx