XHProf is a powerful tool for profiling PHP code. We have used it many times in the past.

When profiling it is often desired to profile the full stack rather than just current request.

One limitation of the way we use XHProf is that it will only generate the profiling information only for the current active request.

When the current request triggers many other requests (in archive.php we call the API, in tests we call tracker + API, etc.) then we'd like to also profile these requests.