Question

How much performance gain (if any) can a windows service gain between a debug build and release build and why?

Answer 1

For managed code, unless you have a lot of stuff conditionally compiled in for DEBUG builds there should be little difference – the IL should be pretty much the same. The Jitter generates differently when run under the debugger or not – the compilation to IL isn’t affected much.

There are some things the /optimize does when compiling to IL, but they aren’t particularly aggressive. And some of those IL optimizations will probably be handled by the jitter optimizations, even if they aren’t optimized in the IL (like the removal of nops).

See Eric Lippert’s article http://blogs.msdn.com/ericlippert/archive/2009/06/11/what-does-the-optimize-switch-do.aspx for details:

The /optimize flag does not change a huge amount of our emitting and generation logic. We try to always generate straightforward, verifiable code and then rely upon the jitter to do the heavy lifting of optimizations when it generates the real machine code. But we will do some simple optimizations with that flag set.

Read Eric’s article for information about /optimize does do differently in IL generation.