SIGPROF
The profilers' metronome: a steady tick in CPU time, user plus system.
The family's third clock adds the time in user mode to the time the kernel spends on the process's behalf. Armed with a short interval, it becomes a metronome: at every tick, the handler notes where the program was.
It is the foundation of the statistical profilers of the gprof era: thousands of samples of the program counter paint the portrait of where the time went. Modern tools prefer perf and the hardware, but the bell is still in the tower.
$ ./servidor-perfilado
$ gprof ./servidor-perfilado gmon.out | head -4
Flat profile:
% cumulative self
time seconds seconds name
62.1 1.24 1.24 parse_request