What is Sampling profiler, hardware performance counter?

A sampling profiler interrupts a program at regular intervals (of time or of hardware events) and records where it was, estimating the share of time spent in each function. A hardware performance counter is a register of the processor that counts events (cycles, instructions retired, cache misses, branch mispredictions) for the code running on a core; on Linux they are read through perf_event_open and the perf tool.