How to check time performances in a C++ program on

问题:

I have implemented a C++ code on a Zedboard. It compiles and runs perfectly, but now i would like to check the performances in order to optimize some functions. I have checked some threads here (Testing the performance of a C++ app) and here (Timer function to provide time in nano seconds using C++), but i don't really understand how to apply it mon code ...

To make things clear : I'm not good at C++, I have never really learned the language formally but only used it several times with specific libraries. I am not even the author of the code I'm using (given to me by the professors).

My goal here is to check the time spent on each functions and globally when I execute the program on the Zedboard. The code is on Linux image on an SD card, the board booting on this image. It is using the opencv library for anImage processing application. I'm using g++ 4.6.3 as a compiler.

Thanks in advance for your answer !

回答1:

You can create a simple timer class using the <chrono> header. Something like this:

class Timer
{
public:
    using clock = std::chrono::steady_clock;

    void clear() { start(); tse = tsb; }
    void start() { tsb = clock::now(); }
    void stop()  { tse = clock::now(); }

    auto nsecs() const
    {
        using namespace std::chrono;
        return duration_cast<nanoseconds>(tse - tsb).count();
    }

    double usecs() const { return double(nsecs()) / 1000.0; }
    double msecs() const { return double(nsecs()) / 1000000.0; }
    double  secs() const { return double(nsecs()) / 1000000000.0; }

    friend std::ostream& operator<<(std::ostream& o, Timer const& timer)
    {
        return o << timer.secs();
    }

private:
    clock::time_point tsb;
    clock::time_point tse;
};

You can use it simply like this:

Timer timer;

timer.start();

// do some stuff
std::this_thread::sleep_for(std::chrono::milliseconds(600));

timer.stop();

std::cout << timer << " seconds" << '\n';

EDIT: On POSIX systems you can use clock_gettime() if <chrono> is not available:

class Timer
{
public:
    void clear() { start(); tse = tsb; }
    void start() { clock_gettime(CLOCK_MONOTONIC, &tsb); }
    void stop() { clock_gettime(CLOCK_MONOTONIC, &tse); }

    long nsecs() const
    {
        long b = (tsb.tv_sec * 1000000000) + tsb.tv_nsec;
        long e = (tse.tv_sec * 1000000000) + tse.tv_nsec;
        return e - b;
    }

    double usecs() const { return double(nsecs()) / 1000.0; }
    double msecs() const { return double(nsecs()) / 1000000.0; }
    double  secs() const { return double(nsecs()) / 1000000000.0; }

    friend std::ostream& operator<<(std::ostream& o, Timer const& timer)
    {
        return o << timer.secs();
    }

private:
    timespec tsb;
    timespec tse;
};

回答2:

I have found an unsatisfying solution, but I thought I could still post it if it can be of any help.

I made use of the gettimeofday() function defined in <time.h>. It's pretty simple to use but has flaws which i may explain later :

timeval t1, t2;
gettimeofday(&t1, NULL);
/* some function */
gettimeofday(&t2, NULL);
double time;
time = (t2.tv_sec - t1.tv_sec)*1000.0 + (t2.tv_usec - t1.tv_usec)/1000.0; 
cout << time << "ms" << "\n";

This way I measure a time in milliseconds and display it on screen. However gettimeofday is not based on the computer clock but on the actual time. To be clear, the time elapsed between 2 calls contains indeed my function, but also all the processes running in the background on Ubuntu. In other terms, this does not give me the precise time taken by my function to execute, but a value slightly higher.

EDIT : I found another solution again, using the clock() function from <time.h> and the result seems correct in comparison of the ones i get with the previous method. Unfortunately, the precision is not enough since it gives a number in seconds with only 3 digits.