2014-12-07-cpp-chrono.md

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post	C++ chrono	cpp-chrono	cpp	cpp

Chrono high resolution dive

I have been toying with the C++ chrono functionality over the last few hours. The high-resolution functionality is very welcomed compared to the non-portable equivalents (or boost) of times past. Note there are 2 clocks available to you in this namespace, system_clock and steady_clock.

• system_clock: wall clock

• steady_clock: monotonically increasing clock. Immune against NTP changes

In the below example, the code computes the latency on hitting the processor cache. The chrono class, high-resolution clock is used for this.

#include <iostream>
#include <chrono>
#include <ctime>
#include <cmath>
#include <cstdlib>
#include <cstdio>

typedef std::chrono::high_resolution_clock::time_point point;
typedef std::chrono::high_resolution_clock Time;
typedef std::chrono::milliseconds ms;
typedef std::chrono::duration<float> fsec;

std::chrono::high_resolution_clock::time_point get_now() {
 	return Time::now();
}

ms get_delta(point pA, point pB) {
	return std::chrono::duration_cast<ms>(pB - pA);
}

void print_delta(point pA, point pB) {
	ms diff = get_delta(pA, pB);
	std::cout << diff.count() << std::endl;
}

int main(int argc, char * argv[]) {
  for(int i = 0; i < 18; ++i) {
  	int size = std::pow(2.0, i) * 1024;
  	int * tab = new int[size];
  	float average = 0.0;
  	for(int j = 0; j < 4; ++j) {
  		auto t0 = get_now();		
  		for(int k = 0; k < (100*1024*1024); ++k) {
  			tab[(k*16)%(size/4)]++;
  		}
  		average += get_delta(t0, get_now()).count();
  	}
  	printf("size=%d kB, time:%g ms\n", size/1024, average/4);
  	free(tab);
  }
}

{: .language-cpp}

In this example, the clock hasn't been checked to see if it is truly a steady state clock so time adjustments may yeild the measurement inaccurate. In order to correct for this, we should check to see that:

  std::chrono::high_resolution_clock::is_steady == true

{: .language-cpp}  

Checking on my OSX system:

CLOCK KIND	STEADY
system_clock	false
high_resolution_clock	true
steady_clock	true

  chrono reference

PS: So much better than in assembly