# Timing

When measuring the performance of parallel programs, it is usually wall clock time, not CPU time, that matters. The reason is that better parallelization typically increases aggregate CPU time by employing more CPUs. The goal of parallelizing a program is usually to make it run faster in real time.

The class `tick_count` in oneAPI Threading Building Blocks (oneTBB) provides a simple interface for measuring wall clock time. A `tick_count` value obtained from the static method tick_count::now() represents the current absolute time. Subtracting two `tick_count` values yields a relative time in `tick_count::interval_t`, which you can convert to seconds, as in the following example:

    tick_count t0 = tick_count::now();
    ... do some work ...
    tick_count t1 = tick_count::now();
    printf("work took %g seconds\n",(t1-t0).seconds());

Unlike some timing interfaces, `tick_count` is guaranteed to be safe to use across threads. It is valid to subtract `tick_count` values that were created by different threads. A `tick_count` difference can be converted to seconds.

The resolution of `tick_count` corresponds to the highest resolution timing service on the platform that is valid across threads in the same process. Since the CPU timer registers are not valid across threads on some platforms, this means that the resolution of tick_count can not be guaranteed to be consistent across platforms.

Note

On Linux* OS, you may need to add -lrt to the linker command when you use oneapi::tbb::tick_count class. For more information, see http://fedoraproject.org/wiki/Features/ChangeInImplicitDSOLinking.