Avoiding C# JIT overhead

Question

Is there an easy way of JIT-ing C# code up front, rather than waiting for the first time the code is invoked? I have read about NGEN but I don't think that's going to help me.

My application waits and responds to a specific external event that comes from a UDP port, and none of the critical-path code is (a) run before the event arrives, or (b) ever run again, so the cost of JIT is high in this scenario. Checking with ANTS profiler the overhead for JIT is around 40-50%, sometimes it's as high as 90%. My application is highly latency sensitive, and every millisecond counts.

My initial thought is that I could add a bool parameter to every critical path method, and call those methods before the event occurs, in order to initiate the JIT compile. However, is there a prettier and less hacky way?

Many thanks

Answer 1

I'd say use NGEN and if it doesn't work, you likely have deeper problems.

But, to answer your question, this article on how to pre-jit uses System.Runtime.CompilerServices.RuntimeHelpers.PrepareMethod to force a JIT. It includes sample code to use reflection to get the method handles.

Answer 2

What happens the second time the event arrives? Is it faster then or just as slow. If its still slow then JIT is not the problem, because the code gets "JIT"ed only once, the first time it is run.

NGEN would provide the answer for you. My suggestion is to take the bare minimum of the code you need, the critical path if you will, and put it in dummy/sandbox project. Start profiling/NGenning this code and seeing the performance.

If this bare minimum code, even after being NGEN'ed performs poorly on multiple calls, then pre-compiling isn't going to help you. Its something else in the code that is causing performance bottle necks.