Profiling

Profiling is a method for analyzing the performance characteristics of a program. There are a variety of types of profilers out there (instrumentation, statistical, emulation, etc.).

Statistical Profilers

Statistical profilers work by using operating system provided hooks to stop the program being profiled at a fixed sampling interval (typically 1ms or shorter). Every time the program is stopped the profiler takes a backtrace of every thread. This gives the profiler a snapshot of what the program was doing at that instant. The profiler tabulates the number of times a given piece of code appears in a sample to give you an estimate of how "expensive" a function is in relation to the rest of the tasks the program is performing.

What can statistical profiling tell you?

The number of times the program counter was on a given line of code during a sample.
The relative CPU burden of different tasks a program performs by comparing the number of samples that fall within two execution paths.
The various code paths taken by a particular execution tree in your program. (i.e. -- "Hey I didn't realize this code somehow called QVector::expensiveCopy!").
Usually a profiler can connect the source code with the assembly

What can't statistical profiling tell you?

How expensive a function is in absolute terms (i.e. how long a function takes to complete, statistics about methods that Mixxx's performance timers might provide, etc.).
The full set of functions that are called by another function (if they don't appear in a sample then they will appear to not exist to the profiler).