I'm testing the performance speedup of some algorithms when using OpenMP and one of then is not scaling. Am I doing something wrong?
PC Details:
- Memory: 7,7 GiB
- Processor: Intel® Core™ i7-4770 CPU @ 3.40GHz × 8
- OS: Ubuntu 15.04 64-bit
- gcc: gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
Code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <omp.h>
int main(int argc, char **argv) {
int test_size, i;
double *vector, mean, stddeviation, start_time, duration;
if (argc != 2) {
printf("Usage: %s <test_size>\n", argv[0]);
return 1;
}
srand((int) omp_get_wtime());
test_size = atoi(argv[1]);
printf("Test Size: %d\n", test_size);
vector = (double *) malloc(test_size * sizeof(double));
for (i = 0; i < test_size; i++) {
vector[i] = rand();
}
start_time = omp_get_wtime();
mean = 0;
stddeviation = 0;
#pragma omp parallel default(shared) private(i)
{
#pragma omp for reduction(+:mean)
for (i = 0; i < test_size; i++) {
mean += vector[i];
}
#pragma omp single
mean /= test_size;
#pragma omp for reduction(+:stddeviation)
for (i = 0; i < test_size; i++) {
stddeviation += (vector[i] - mean)*(vector[i] - mean);
}
}
stddeviation = sqrt(stddeviation / test_size);
duration = omp_get_wtime() - start_time;
printf("Std. Deviation = %lf\n", stddeviation);
printf("Duration: %fms\n", duration*1000);
return 0;
}
Compilation line
gcc -c -o main.o main.c -fopenmp -lm -O3
gcc -o dp main.o -fopenmp -lm -O3
Results
$ OMP_NUM_THREADS=1 ./dp 100000000
166.224199ms
$ OMP_NUM_THREADS=2 ./dp 100000000
157.924034ms
$ OMP_NUM_THREADS=4 ./dp 100000000
159.056189ms
