OpenMP example;)

Original Sequential version of MXV in C

 42 #include <stdio.h>
 43 #include <stdlib.h>
 44
 45 void mxv(int m, int n, double * restrict a,
 46          double * restrict b, double * restrict c);
 47
 48 int main(int argc, char *argv[])
 49 {
 50    double *a,*b,*c;
 51    int i, j, m, n;
 52
 53    //printf("Please give m and n: ");
 54    //scanf("%d %d",&m,&n);
 55    printf("runnging 10000 * 10000 matrics x vector\n");
 56    m = 10000;
 57    n = 10000;
 58
 59    if ( (a=(double *)malloc(m*sizeof(double))) == NULL )
 60       perror("memory allocation for a");
 61    if ( (b=(double *)malloc(m*n*sizeof(double))) == NULL )
 62       perror("memory allocation for b");
 63    if ( (c=(double *)malloc(n*sizeof(double))) == NULL )
 64       perror("memory allocation for c");
 65
 66    printf("Initializing matrix B and vector c\n");
 67    for (j=0; j<n; j++)
 68       c[j] = 2.0;
 69    for (i=0; i<m; i++)
 70       for (j=0; j<n; j++)
 71          b[i*n+j] = i;
 72
 73    printf("Executing mxv function for m = %d n = %d\n",m,n);
 74    (void) mxv(m, n, a, b, c);
 75
 76    free(a);free(b);free(c);
 77    return(0);
 78 }
 79
 80 void mxv(int m, int n, double * restrict a, double * restrict b,
 81          double * restrict c)
 82 {
 83    int i, j;
 84
 85    for (i=0; i<m; i++)
 86    {
 87       a[i] = 0.0;
 88       for (j=0; j<n; j++)
 89          a[i] += b[i*n+j]*c[j];
 90    }

and OpenMP 

 42 #include <stdio.h>

 43 #include <stdlib.h>

 44

 45 void mxv(int m, int n, double * restrict a,

 46          double * restrict b, double * restrict c);

 47

 48 int main(int argc, char *argv[])

 49 {

 50    double *a,*b,*c;

 51    int i, j, m, n;

 52

 53    //printf("Please give m and n: ");

 54    //scanf("%d %d",&m,&n);

 55    printf("running 10000 * 10000 matrics x vectors\n");

 56    m = 10000;

 57    n = 10000;

 58

 59    if ( (a=(double *)malloc(m*sizeof(double))) == NULL )

 60       perror("memory allocation for a");

 61    if ( (b=(double *)malloc(m*n*sizeof(double))) == NULL )

 62       perror("memory allocation for b");

 63    if ( (c=(double *)malloc(n*sizeof(double))) == NULL )

 64       perror("memory allocation for c");

 65

 66    printf("Initializing matrix B and vector c\n");

 67    for (j=0; j<n; j++)

 68       c[j] = 2.0;

 69    for (i=0; i<m; i++)

 70       for (j=0; j<n; j++)

 71          b[i*n+j] = i;

 72

 73    printf("Executing mxv function for m = %d n = %d\n",m,n);

 74    (void) mxv(m, n, a, b, c);

 75

 76    free(a);free(b);free(c);

 77    return(0);

 78 }

 79

 80 void mxv(int m, int n, double * restrict a, double * restrict b,

 81          double * restrict c)

 82 {

 83    int i, j;

 84 #pragma omp parallel for default(none) \

 85         shared(m,n,a,b,c) private(i,j)

 86    for (i=0; i<m; i++)

 87    {

 88       a[i] = 0.0;

 89       for (j=0; j<n; j++)

 90          a[i] += b[i*n+j]*c[j];

 91    } /*-- End of omp parallel for --*/

 92 }

OpenMP will check your processors and will create # of threads.

in my case it creates 16 threads for doing mxv functions.

you can check # of threads with omp_get_number_threads();

and also set # of threads with omp_set_number_threads(#number);

or #pragma omp num_threads(#number)

results for those two sample codes are

 time ./fig3.5.exe

runnging 10000 * 10000 matrics x vector

Initializing matrix B and vector c

Executing mxv function for m = 10000 n = 10000

real    0m1.197s

user    0m0.688s

sys     0m0.508s

 time ./fig3.10.exe

running 10000 * 10000 matrics x vectors

Initializing matrix B and vector c

Executing mxv function for m = 10000 n = 10000

real    0m0.831s

user    0m3.700s

sys     0m0.456s