ruby-prof is a module that runs inside the ruby process, and hooks into the interpreter to track what's happening. You have options for how to run it. You can

ruby-prof is a module that runs inside the ruby process, and hooks into the interpreter to track what's happening. You have options for how to run it. You can run the entire program within the profiler, but if you do that, you won't get data out until the process ends (you kill the webserver, in this case). Or, you can do what's more typical for profiling Rails, which is to explicitly activate the profiler for the course of a single request, and report on that single request.