how do I get consistent execution times?-CodePudding

I am preparing for a coding challenge, where the fastest calculation of pi to the 10000th digit wins. all calculations will be run on a raspberry Pi4 running linux during competition. I want to know which code runs the fastest, so I can know which function to submit.

so I wrote a little program named "lol" to try and establish a baseline around a known time.

//lol....lol is an exe which calls usleep()
#include <unistd.h>
using namespace std;
int main(){
    usleep(100);
    return 0;
}

then to measure execution time, I wrote this:

#include <chrono>
#include <stdlib.h>
#include <iostream>
using namespace std::chrono;
using namespace std;
int main(int argc, char **argv){
    //returns runtime in nanoseconds
    //useage: runtime <program>
    //caveates: I put the exe in /home/$USER/bin
    //start timing
    auto start = high_resolution_clock::now();
    //executable being  timed:
    system(argv[1]);
    // After function call
    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<nanoseconds>(stop - start);
    cout << argv[1] << " " << duration.count() << endl;
    return 0;
    }

my issue is that the run time seems to be wildly variant. Is this because I'm running in userspace and my system is also doing other things? why am I not getting more consistent run times?

$ ./run_time lol
lol 13497886
$ ./run_time lol
lol 11175649
$ ./run_time lol
lol 3340143
./run_time lol
lol 3364727
$ ./run_time lol
lol 3372376
$ ./run_time lol
lol 1981566
$ ./run_time lol
lol 3385961

CodePudding user response：

instead of executing a program, measure a function completion in a single program:

    
    auto start = high_resolution_clock::now();
    //function being  timed:
    my_func();
    // After function call
    auto stop = high_resolution_clock::now();

you are using chrono header. so why usleepwhen you can use sleep_for: https://en.cppreference.com/w/cpp/thread/sleep_for

The merits of this contest is not how you micro-optimize to save 1ns. It`s about choosing the right algorithm to calculate pi.