What is profiling

Profiling should be an indispensable function for any performance analysis tool. But wait, what is profiling on earth? The following definition is from Wikipedia):

In software engineering, profiling (“program profiling”, “software profiling”) is a form of dynamic program analysis that measures, for example, the space (memory) or time complexity of a program, the usage of particular instructions, or the frequency and duration of function calls. Most commonly, profiling information serves to aid program optimization.

Profiling is achieved by instrumenting either the program source code or its binary executable form using a tool called a profiler (or code profiler). Profilers may use a number of different techniques, such as event-based, statistical, instrumented, and simulation methods.

As a profiler, perf provides two working mode: sampling (perf record) and counting (perf stat). Sampling is checking running system periodically to show the hotspot of the application, while counting is just a statistic of some events (mostly need support from underlying hardware).