OpenMP slower than serial ( sometmes )

HI

I am new in Open MP and willing to learn . I am reading the Open MP documentation and this tutorial

https://computing.llnl.gov/tutorials/openMP/exercise.html

I am really new to parallel programming , I have been searching over diferent examples and at the end I am trying to create a simple loop faster in omp than without, here is my code:

#include <omp.h>
#include <stdio.h>
#define N 100000

main ()
{

int i;
float a[N], b[N], c[N];

/* Some initializations */
for (i=0; i < N; i++)
a[i] = b[i] = i * 1.0;

double start = omp_get_wtime();

omp_set_dynamic(0);
omp_set_num_threads(4);

#pragma omp parallel shared(a,b,c) private(i)
{

#pragma omp for schedule(static)
for (i=0; i < N; i++)
c[i] = a[i] + b[i];

} /* end of parallel section */

double end = omp_get_wtime();

printf(“%f\n”,start);
printf(“%f\n”,end);
printf(“%f\n”,end - start);

}

Same code but with out the #pragma sections , the problem is that is not always faster than serial code, some times serial code is even faster .

Any clue is more than welcome, if this is not he mailing list to post this please tell me. I am really interested to lear Open MP

Thanks a lot and regards