[Twisted-Python] profiling twisted

Hi, first of all, thank you very much for this great peace of software, I'm using twisted for many different projects and really enjoy it's clean and simple design. Now, I'm wondering if there's a profiling tool, which can quickly tell me what events are blocking my server. I have even deferred the file I/O to separate threads, as I've suspected them to make the server behave poorly when under load. But there still seems to be something blocking... What do you use to find the callbacks which block (for too long)? Regards Markus