ThreadPool exceptions in IIS Express

When debugging a WebAPI application recently which used an asynchronous library, I came accross a situation where the application kept on throwing the following InvalidOperationException:

There were not enough free threads in the ThreadPool to complete the operation.

Running the following code from a WebAPI controller:

Results in the following:

workers = 2

completions = 2

This is too low to run any other async tasks. For example, when run in a console application, we are returned significantly higher numbers.

workers = 1023

completions = 1000

After posting a question on stack overflow, and doing some digging, I came accross the following github issue:

https://github.com/aspnet/Home/issues/94

The default values for the thread pool seem really low under Helios. I personally get workerThreads = completionPortThreads = 16 on a Core i7 machine with 4 cores and hyper-threading enabled (= 8 cores).

An ASP.NET Web Forms app running IIS Express on the same machine comes with far higher default values: workerThreads = 4096 and completionPortThreads = 1000.

You should consider increasing these values to get rid of these weird exceptions:

InvalidOperationException: There were not enough free threads in the ThreadPool to complete the operation. In the meantime, manually calling ThreadPool.SetMaxThreads with higher values will do the trick.

Specifically, the following comment explains the issue:

This is an issue with Helios specifically. WebEngine4 sets the limits to 2 * cpuCount and normally System.Web sets the limits to 1000 and 4096 (basically the autoConfig settings in web.config).

The fix is to change the calculation method for how Helios calculates the max threadpool count. To do this yourself, add the following code to your Startup class:

This will be the fix implemented in Helios by the ASP.Net team in the future.

For now, I have set the Startup class in my WebAPI application to call this code on construction. The code is wrapped in conditional compilation symbols to ensure that the code is only run when the build configuration is DEBUG.

Happy coding!