Application Pool recycling results in very long response times

Question

I've read somewhere, that application pool recycling shouldn't be very noticeable to the end user, when overlapping is enabled, but in my case that results in at least 10 times longer responses than usually (depending on load, response time from regular 100ms grows up to 5000ms). Also that is not for a single request, but several ones right after pool recycling (I was using ~10 concurrent connections when testing this).

So questions would be:

1. In my opinion I don't do anything, that would take a long time on application start - in general, that is only IoC container and routing initialization, also even I would do something - that is what overlapping should take care, or not?
2. Is sql connection pool destroyed during pool recycling and could that be a reason for long response times?
3. What would be the best method to profile what is taking so long? Also may be there are ideas, what could take so long from IIS/.NET side, and how to avoid that.

Answer 1

1. Overlapping only means that the old worker process will be kept running while the new one is started. As soon as the new one is started, it begins handling all requests. "Started" does not mean that initialization (which might be contained in Application_Start, any static constructors in your application, or any one time, contentious tasks like proxy building) have been completed. This means that new requests will have to wait while these processes are completed, even though the "old" worker process might still be available for a short time. Also, if your application uses any kind of caching, your new caches will be "cold", meaning there will be some additional processing time required until the caches are warmed up.

2. Yes - your new application will have a new sql connection pool.

3. In my experience, in a production environment, with well tested code and an application that requires consistent, high performance, I choose to disable application pool recycling altogether. Application Pool recycling is a "feature" introduced to combat the perception that IIS was unstable, when in fact what was usually really unstable was the applications that it was hosting. In my opinion, it is a crutch that allows people to deploy less than stable code. If it is causing you problems, turn it off and make sure your application doesn't have any memory leaks, etc. that might lead to long term application instability.