Profiler flamegraphs can be a wonderful tool for understanding actual execution. It turns out that execution time is a useful proxy / reasonable approximation

Profiler flamegraphs can be a wonderful tool for understanding actual execution. It turns out that execution time is a useful proxy / reasonable approximation for code's importance to the overall application. Bonus points for using an intrusive, explicitly annotated tracing API of some sort, which lets you capture enough parameter information to understand not just where, but what, the code is doing.