Thread: claculate time of function

Only one comment: I think that to perform correctly the calculation of the elapsed time, the frequency scaling of the CPU must be disabled.

Originally Posted by zedd

Only one comment: I think that to perform correctly the calculation of the elapsed time, the frequency scaling of the CPU must be disabled.

Thanks for the reply, but how we disable this frequency ?

Hi,
i used the clock_t to calculate the run time of another function
i get "0 seconde"?
i say maybe it's in millysecond
i used

Code:

 cout << (double(clock() - timestart) / CLOCKS_PER_SEC) * 1000<< " seconds." << endl;

i get as the first one '0 seconde"
?
Note: there is many simple instruction , as sqrt, min, ....

I used this

Code:

 
clock_t start, stop;
 
 
    start = clock();
//
// block of instruction that i want calculate it's running  time
//
stop = clock();
 
    printf("Approx seconds, tenths, hundredths and milliseconds: %.3f\n", 
        ((double)(stop - start) / CLOCKS_PER_SEC));

I get 0.000 i will be crazy how many instructions give me 0 sec? it's impossible no?

[QUOTE]Thanks for the reply, but how we disable this frequency ?[/QUOTE]

Three possibilities:
- some BIOS usually permits this configuration
- Linux provides a specific widget
- the easiest way is to perform a number of operations that load the CPU before the timing of the function

Not really. I think you underestimate how fast computers can be. Throw in a few loops and take the average.

but i what i need is just the run time of my function , if i get 0 many time
i can't write it

Repeat what you want to time several times, then take the average. That way you get an approximation of the "real" time.

what you mean here ?
did you mean i run my app several time ? because i made that and always i get "0.000"

Originally Posted by Elysia

Code:

    namespace stdc = std::chrono;
    auto start = stdc::high_resolution_clock::now();
    constexpr int LoopCount = 1000000;
    for (int i = 0; i < LoopCount; i++)
    {
        // My code here
    }
    auto end = stdc::high_resolution_clock::now();
    std::cout << stdc::duration_cast<stdc::milliseconds>((end - start) / LoopCount).count() << "\n";

With this code i can get the "real" time?
can you please explain to me why without loop it does not work, and why i must use loop and excute many time the same code