C++ timing with millisecond accuracy

How do you know if you have efficiently implemented a routine. A simple routine from Boost can tell you. More specifically the boost::timer function.
Boost is a set of free software libraries that extend the functionality of C++, in the Wikipedia discription. There are tons of very useful features you'd expect to be native to an advanced programming language as C++ but are not.

How to get boost? Usually on Linux you can just say something like apt-get install libboost or yum install libboost. Let me know if you want a tutorial to compile or install boost. The examples below compile with both Intel and Gnu C++ compilers and even Nvidia compilers (I tried NVCC 4.0).

Here is a code example of how to time a routine with boost:

#include <boost/timer.hpp>      // For boost::timer class
#include <iostream>             // For printing


// Some constants
unsigned int x = 20000;
bool flag = true;
unsigned int k = 0;

int main() {
        // Start boost timer.
        boost::timer t;

        // Do a double loop that takes some time.
        for (unsigned int i = 0; i < x; i++) {
                for (unsigned int j = 0; j < x; j++) {
                        k = i + j;
                        if (k % 2 == 0) {
                                flag = false;
                        } else {
                                flag = true;
                        }
                }
        }

        // Store elapsed time from boost
        double elapsed = t.elapsed();

        // Print elapsed time from the t object.
        std::cout << "boost/timer says:" << std::endl;
        std::cout << "Total time    = " << elapsed 
                << " seconds" << std::endl;
        std::cout << "Avg time/iter = " << elapsed / (x * x) 
                << " seconds" << std::endl;

        return 0;
}

I get the following output on my Core i5-760 box and the icpc compiler:

boost/timer says:
Total time    = 0.35 seconds
Avg time/iter = 8.75e-10 seconds


You can use t.reset() if you want to reset the timer in your program.


If you want higher accuraccy, you can use the gettimeofday() function from sys/time.h. This example does not require boost either, but the code is way uglier. This is done very much like in the above code:

#include <sys/time.h>           // For gettimeofday()
#include <iostream>             // For printing


// Some constants
unsigned int x = 20000;
bool flag = true;
unsigned int k = 0;

int main() {

        // Create objects for gettimeofday() timing
        struct timeval start_time;
        struct timeval end_time;

        // Get the time of day -- NULL means we don't care about
        // time zones. Store in start_time.
        gettimeofday(&start_time, NULL);


        // Do a double loop that takes some time.
        for (unsigned int i = 0; i < x; i++) {
                for (unsigned int j = 0; j < x; j++) {
                        k = i + j;
                        if (k % 2 == 0) {
                                flag = false;
                        } else {
                                flag = true;
                        }
                }
        }

        //Store time of day in end time
        gettimeofday(&end_time, NULL);

        // Calculate the elapsed time with micro second accuracy from gettime of day
        double start = start_time.tv_sec*1000000 + (start_time.tv_usec);
        double end   = end_time.tv_sec*1000000 + (end_time.tv_usec);
        elapsed     = end - start;

        // Print elapsed time from gettimeofday().
        std::cout << "gettimeofday says:" << std::endl;
        std::cout << "Total time    = " << elapsed 

                << " microseconds" << std::endl;
        std::cout << "Avg time/iter = " << elapsed / (x * x) 

                << " microseconds" << std::endl;

        return 0;
}

This gives me the following output:

gettimeofday says:
Total time    = 366208 microseconds
Avg time/iter = 0.00091552 microseconds

Happy coding! Let me know if you have questions or comments.