改变OpenMP并行区域的运行时间

Question

Whenever I run this code it shows me different run-time took by the parallel section. I tried with a constant number of threads according to my core but still the effort is futile. The program is to calculate the value of pi. Compiled with gcc -fopenmp .

#include <stdio.h>
#include <omp.h>

static long num_steps = 100000; double step;
//double omp_get_wtime(void);

int main (){
      int i;
      double x,pi,max_threads,start,time;
      double sum=0.0;
      step = 1.0/(double) num_steps;
    //omp_set_num_threads(4);       
      omp_get_max_threads();
      start=omp_get_wtime();

    #pragma omp parallel
   {

    #pragma omp for reduction(+:sum) schedule(static) private(x) //reduction to get local copy
            for (i=0;i<num_steps;i++){
            x=(i+0.5)*step;
            sum += 4.0/(1.0+x*x);
            }
    //max_threads=omp_get_max_threads();
    }
time=omp_get_wtime()-start;
pi=step*sum;
printf("pi=(%f)\t run_time(%f)\n",pi,time);//,max_threads);
return 0;
 }

Answer 1

The code runs only a few milliseconds (on my system 2-6 ms), the time is being dominated overhead eg for thread creation. The serial version runs <1 ms. It is normal for such short execution times to be very variable as it depends on the current state of the system, eg there is some 'warmup needed'.

In this case, just increase num_steps to get meaningful stable results. Eg with num_steps = 1000000000 , 10 executions are all between 4.332 and 4.399 seconds on my system.

Generally, if you do performance measurements, you should compile with the -O3 flag.