Measuring CPU Time

Measuring how long your program ran for is easy with std::chrono, but what if you need details about user and system space time? Easy! Use Boost CPU Timer library.  It’s another simple library from Boost with only few classes: auto_cpu_timer is like a RAII object; it starts the clock in its constructor, stops it in its destructor, and prints the elapsed time to standard output. cpu_timer class is for manual starting and stopping of the clock; it also allows you to retrieve the elapsed times (wall, user, and system) in nanoseconds.

In the below example, I create three threads: procUser spends 100% of its time in the user space calling std::sqrt function. procSystem spawns a lot of threads causing transitions into the kernel. And procTimer is just an illustration of cpu_timer usage.

#include <iostream>
#include <thread>
#include <cmath>
#include <cstdlib>
#include <boost/timer/timer.hpp>

using namespace std;
using namespace boost::timer;

int main()
{
	srand((unsigned int)time(NULL));

	auto_cpu_timer program_timer(3);

	auto procUser = [](long work)
	{
		for (long i = 0; i < work; ++i)
			sqrt(123.456L);
	};

	auto procSystem = [](long work)
	{
		for (long i = 0; i < work; ++i)
			thread([](){}).detach();
	};

	auto procTimer = [](long work)
	{
		cpu_timer timer;
		timer.start();

		for(long i = 0; i < work; ++i)
			rand();

		timer.stop();
		cout << "Thread timer:" << timer.format(3);
	};

	thread t1(procUser, 1000000000);
	thread t2(procSystem, 100000);
	thread t3(procTimer, 100000000);

	t1.join();
	t2.join();
	t3.join();

	cout << "Program timer:";

	return 1;
}

Program output:

Thread timer: 0.750s wall, 1.790s user + 0.850s system = 2.640s CPU (352.1%)
Program timer: 3.171s wall, 5.080s user + 2.980s system = 8.060s CPU (254.2%)