Measuring Clock Precision

I’ve been reading Operating Systems: Three Easy Pieces, which is a great book BTW, and one of the questions in the TLB section is

For timing, you’ll need to use a timer (e.g., gettimeofday()). How precise is such a timer? How long does an operation have to take in order for you to time it precisely? (this will help determine how many times, in a loop, you’ll have to repeat a page access in order to time it successfully)

gettimeofday is not a good fit for this because it has only microsecond resolution so I was considering using std::chrono::steady_clock instead. Unfortunately the standard doesn’t give a way to determine the actual clock resolution. We only know that it is at least clock::time_point::period seconds. For steady_clock the period is normally std::ratio<1, 1000000000> which gives us 1ns but the real resolution is a multiple of that.

On POSIX steady_clock is normally implemented using clock_gettime and the resolution can be as high as 1ns as reported by clock_getres.

However, you probably want to take into account the overhead of calling the now() function and not just the resolution of the underlying API. The obvious solution is to time our timer.

This can be accomplished with the following simple program (godbolt):

#include <fmt/chrono.h>

int main() {
  const int num_measurements = 100;
  using clock = std::chrono::steady_clock;
  clock::time_point measurements[num_measurements];
  for (int i = 0; i < num_measurements; ++i) {
    measurements[i] = clock::now();
  }
  auto min_duration = clock::duration::max();
  for (int i = 1; i < num_measurements; ++i) {
    auto duration = measurements[i] - measurements[i - 1];
    if (duration != clock::duration() && duration < min_duration)
      min_duration = duration;
  }
  fmt::print("{}\n", min_duration);
}

On my M1 MacBook Pro with libc++ it gives ~41ns:

% ./clock
41ns

And on godbolt’s Linux machine with libstdc++ it gives ~20ns.

I also posted a shorter version of this as a StackOverflow answer.

If you know a better way to do this please let me know in a comment.

Happy timing!