0 - Delta
Use a delta function to compute time differences:
auto start = std::chrono::steady_clock::now();
std::cout << "Elapsed(ms)=" << since(start).count() << std::endl;
since accepts any timepoint and produces any duration (milliseconds is the default). It is defined as:
template <
class result_t = std::chrono::milliseconds,
class clock_t = std::chrono::steady_clock,
class duration_t = std::chrono::milliseconds
>
auto since(std::chrono::time_point<clock_t, duration_t> const& start)
{
return std::chrono::duration_cast<result_t>(clock_t::now() - start);
}
Demo
1 - Timer
Use a timer based on std::chrono:
Timer clock; // Timer<milliseconds, steady_clock>
clock.tick();
/* code you want to measure */
clock.tock();
cout << "Run time = " << clock.duration().count() << " ms\n";
Demo
Timer is defined as:
template <class DT = std::chrono::milliseconds,
class ClockT = std::chrono::steady_clock>
class Timer
{
using timep_t = typename ClockT::time_point;
timep_t _start = ClockT::now(), _end = {};
public:
void tick() {
_end = timep_t{};
_start = ClockT::now();
}
void tock() { _end = ClockT::now(); }
template <class T = DT>
auto duration() const {
gsl_Expects(_end != timep_t{} && "toc before reporting");
return std::chrono::duration_cast<T>(_end - _start);
}
};
As Howard Hinnant pointed out, we use a duration to remain in the chrono type-system and perform operations like averaging or comparisons (e.g. here this means using std::chrono::milliseconds). When we just do IO, we use the count() or ticks of a duration (e.g. here number of milliseconds).
2 - Instrumentation
Any callable (function, function object, lambda etc.) can be instrumented for benchmarking. Say you have a function F invokable with arguments arg1,arg2, this technique results in:
cout << "F runtime=" << measure<>::duration(F, arg1, arg2).count() << "ms";
Demo
measure is defined as:
template <class TimeT = std::chrono::milliseconds,
class ClockT = std::chrono::steady_clock>
struct measure
{
template<class F, class ...Args>
static auto duration(F&& func, Args&&... args)
{
auto start = ClockT::now();
std::invoke(std::forward<F>(func), std::forward<Args>(args)...);
return std::chrono::duration_cast<TimeT>(ClockT::now()-start);
}
};
As mentioned in (1), using the duration w/o .count() is most useful for clients that want to post-process a bunch of durations prior to I/O, e.g. average:
auto avg = (measure<>::duration(func) + measure<>::duration(func)) / 2;
std::cout << "Average run time " << avg.count() << " ms\n";
+This is why the forwarded function call.
+The complete code can be found here
+My attempt to build a benchmarking framework based on chrono is recorded here
+Old demo
