After enabling IIS 7.5 autostart, first request is still slow

Question

I've set startMode="AlwaysRunning" attribute on my application pool and serviceAutoStartEnabled="true" attribute on my application in IIS configuration. I've even set up serviceAutoStartProvider and can see that "warm up" code is being executed. I also can see that w3wp process auto-starts after iisreset. Still, the first request to my ASP.NET MVC application is exactly as slow as without auto-start. Is there any point I'm missing or any way to easily debug this without a profiler?

Is this feature expected to affect first request performance at all? What is actually the bulk of work to do on the first request, given that the worker process is ready, .NET appdomain and even all .NET assemblies have been loaded?

Answer 1

I've been looking into this recently.

As far as I can tell, the autoStart feature will cause your IIS worker threads (by default, just the one for the pool) to JIT compile before the first request.

However, what is compiled appears to be just a bulk of the assemblies and dependencies, but not necessarily any methods.

When that first request happens, and your methods you've written get called for the first time, the JITer performs a final compile on those methods that have not yet been compiled.

The benefit of autoStart appears to be it lets .Net do 90% of the work up-front, but the last 10% is still paid for when the first request happens and those methods that were yet to be accessed get run for the first time.