OpenMP C++ GCC Basic routine

Question

I'm running a very simple routine in C++ with openMP and measuring the elapsed time... the code goes at reads,

#include <iostream>
#include <math.h>
#include "timer.h"
#include <omp.h>



int main ()
{
    double start,finish;
    int i;
    int n=8000;
    double a[n];
    double b[n];
    double c[n];



    GET_TIME(start);
#pragma omp parallel private(i,a) shared(b,c,n)
    {
#pragma omp for 
        for (i=0; i<n-1; i++)
        b[i] += (a[i] + a[i+1])/2;
#pragma omp for
        for (i=0; i<n-1; i++)
            c[i] += (a[i] + a[i+1])/2;
    } 
    GET_TIME(finish);
    std::cout<< "Elapsed time is" <<(finish-start)<<"seconds";
    return 0;
}

Code with I'm compiling with the following bash script (observe that threads are defined in the environment variable OMP_NUM_THREADS=$n):

#!/bin/bash

clear

g++ -O3 -o test test.cpp -fopenmp 

for n in $(seq 1 8); do
  export OMP_NUM_THREADS=$n
   ./test
    echo threads=$n
done

As a result, a general trend of decreasing the performance with increasing the number of threads is observed as follows: (Of course the numbers can change)...

Elapsed time is0.000161886secondsthreads=1
Elapsed time is0.00019002secondsthreads=2
Elapsed time is0.00226498secondsthreads=3
Elapsed time is0.000210047secondsthreads=4
Elapsed time is0.000212908secondsthreads=5
Elapsed time is0.00920105secondsthreads=6
Elapsed time is0.00937104secondsthreads=7
Elapsed time is0.000834942secondsthreads=8

Any suggestions for increasing the performance (instead of decreasing it)? Thank you very much!.

Answer 1

You can do this instead, it will increase the operation done by each thread. This is to overcome the overhead needed to start a new thread by actually having the thread do some more work. Also, there is no need to declare the b, c or n as shared.

#pragma omp parallel private(i,a,b,c,n)
{
#pragma omp for schedule(static)
    for (i=0; i<n-1; i++){
        b[i] += (a[i] + a[i+1])/2;
        c[i] += (a[i] + a[i+1])/2;}
}