[plt-scheme] profiling

Hi,

I have almost finished my application and I would like to do some 
profiling. As I understand there is a profiler in errortrace.ss, however
I did not manage to make it work. My app is organised into modules
and I do not know where to require errortrace, inside the module,
outside, in all of the modules (?). So far it always reports that
instrumentation is on, but it always returns empty lists as collected
data. Can somebody tell me how to use the profiler? Is there any other
way to do profiling?

Thanks

Peter Ivanyi