Stack overflow in .NET sends IIS to 100% CPU usage - why no StackOverflowException?

Question

I had some code in an ASP.NET application running on Server 2008 R2 + IIS 7.5. Whenever I loaded a particular page, it would hang forever and send IIS to 100% CPU usage. I eventually tracked down the problem.

public string Comments
{
    get { return this.Comments; }
}

Oops - should have been return this.Photo.Comments. So, my question is, why didn't .NET generate a StackOverflowException, but instead let IIS run at 100% CPU for far longer than it should have taken. In my experience programming with .NET, it takes seconds or less to get a StackOverflowException when doing something like the above. So how could it still be running for almost 30 minutes on IIS?

Answer 1

It's possible the JIT compiler optimised out a method call to YourClass::get_Comments() (which is what the IL would look like) and inlined the code with a jmp (or whatever the x86 assembler would be) loop construct because there weren't any values being passed around. Just a thought.

This old article is worth a look:

Jit Optimizations: Inlining (II)

"A typical example of a really good candidate for inlining is a property getter/setter. These are usually really small methods that usually just do a memory fetch or store, so it's usually a size and speed win to inline them."

As is:

Writing High-Performance Managed Applications : A Primer - Managed Code and the CLR JIT

I also reproduced this with a simple console application:

class Program
{
  static void Main(string[] args)
  {
    MyClass mc = new MyClass();
    string s = mc.Comments;
  }
}

public class MyClass
{
  public string  Comments
  {
    get { return this.Comments; }
  }
}

In debug mode with optimisations turned off I get a stack overflow exception thrown. Upon turning on Jit Optimisations and compiling a release build the app just runs forever. This suggests that inlining to a loop has probably happened.

This also appears to be the case with C#2.0, 3.0 and 4.0.

Answer 2

I tried putting this code into a class library and running it with a unit test.

It crashes the MS test agent with a stack overflow exception.

What may be happening is that you are getting a stackoverflow exception. This is crashing the application pool. IIS then pulls up a new copy of the app pool, and it gets crashed again ....

Check your event logging for application pool recycling / stops.