CSC/ECE 506 Spring 2015/37 mr

A (simple) Introduction to OpenMP

How to run the samples on EOS

gcc -fopenmp hello.c -o hello

Question 1

Parallelize the following code using OpenMP

---

The code to parallelize:

int main()

{

      omp_set_num_threads(6);
      long sum = 1;
      int size = 5;
      int i, j, rowsum;
      int first[5][5] = { { 1, 2, 3, 4, 5 }, { 6, 7, 8, 9, 10 }, { 11, 12, 13, 14, 15 }, {16, 17, 18, 19, 20}, {21, 22, 23, 24, 25} };
      for ( i = 0 ; i < size ; i++ ){
              rowsum = 0;
              for ( j = 0 ; j < size ; j++ ){
                      rowsum = rowsum + first[i][j];
              }
              sum = rowsum*sum;
      }
      printf("sum: %d\n", sum);
   return 0;

}

Answer 1

int main()
{
       omp_set_num_threads(6);
       long sum = 1;
       int size = 5;
       int i, j, rowsum;
 #pragma omp parallel shared(rowsum, i, sum) private(j)
 {
       int first[5][5] = { { 1, 2, 3, 4, 5 }, { 6, 7, 8, 9, 10 }, { 11, 12, 13, 14, 15 }, {16, 17, 18, 19, 20}, {21, 22, 23, 24, 25} };
       #pragma omp for
       for ( i = 0 ; i < size ; i++ ){
               rowsum = 0;
               for ( j = 0 ; j < size ; j++ ){
                       rowsum = rowsum + first[i][j];
               }
               #pragma omp critical
               sum = rowsum*sum;
       }
 }
       printf("sum: %d\n", sum);
    return 0;
}

Question 2

Parallelize the code below using loop distribution

---

for (i=0; i<n; i++) { 
S1:  a[i] = b[i-1] * a[i+1];
S2:  b[i] = b[i] * 3;
S3:  c[i] = .2 * (c[i-1] * c[i]);
} 

After loop distribution

---

for(i=0; i < n; i++)  
 S1: a[i] = b[i-1] * a[i+1]; 
for(i=0; i < n; i++) 
 S3: c[i] = .2* c[i-1] * c[i];
for(i=0; i < n; i++) 
 S2: b[i] = b[i] * 3;

---

Now write the parallelized code in openMP using C++

---

#pragma omp parallel shared(a,b,c)private(i) {
 #pragma omp sections nowait
 {
       #pragma omp section
       for(i=0; i < n; i++)
         a[i] = b[i-1] * a[i+1];
       #pragma omp section
       for(i=0; i<n; i++)
        c[i] = .2* c[i-1]*c[i];
       #pragma omp section
       for(i=0; i < n; i++)
        b[i] = b[i] * 3;
  }//end omp sections
} //end omp parallel