Inaccurate time measurement in test.h

[cppascalinux]

In test.h, we use the following macros to measure the update and query time for each sketch:

#define DEFINE_TIMERS                                                          \
  auto timer = std::chrono::microseconds::zero();                              \
  auto tick = std::chrono::steady_clock::now();                                \
  auto tock = std::chrono::steady_clock::now();
#define START_TIMER tick = std::chrono::steady_clock::now();
#define STOP_TIMER                                                             \
  tock = std::chrono::steady_clock::now();                                     \
  timer += std::chrono::duration_cast<std::chrono::microseconds>(tock - tick);
#define TIMER_RESULT static_cast<int64_t>(timer.count())

Here the timer is measured in microseconds. However, in testUpdate, we measure the time for each update separately:

  DEFINE_TIMERS;
  for (auto ptr = begin; ptr != end; ptr++) {
    START_TIMER;
    ptr_sketch->update(ptr->flowkey,
                       cnt_method == Data::InLength ? ptr->length : 1);
    STOP_TIMER;
  }
  if (metric_vec.in(Metric::RATE))
    update[Metric::RATE] = 1.0 * (end - begin) / TIMER_RESULT * 1e6;

For many sketch algorithms (e.g. CMSketch), the update operation takes far less than 1us (~200ns on my machine), and the timer result is rounded to 0us. This causes significant inaccuracy for time measurement. The same problem exists in testInsert and testQuery, etc.

Changing microseconds to nanoseconds would be a solution.

Also notice that, std::chrono measures the wall clock time instead of CPU time. We might want to switch to std::clock(), which measures the CPU time and produces a more accurate result.