Self-Profiling API
性能分析有两种方法：
- Instrumented (or "structured" or "tracing") Profilers, in which an application is invasively instrumented (modified) to add hooks at every function entry and exit, so the exact time spent in each function is known
- Sampled Profilers, which temporarily pause execution of the application at a fixed frequency to note ("sample") what is running on the call stack at that time
Self-Profiling API 使用采样分析，虽然不准确，但是开销小