我如何理解我在openMP中的代码？

Question

I'm beginning in openMP and i want parallelize this portion of code :

for (i=0 ;i<n ;i++) 
    for (j=1 ;j<n ;j++)  
       A[i][j]+=A[i][j-1];

and i find this answer:

#pragma omp parallel for private(i, j) shared(A, n)
for (i = 0; i < n; ++i) 
  for (j = 1; j < n; ++j)  
    A[i][j] += A[i][j-1];

i have some questions:
- why does i private and not shared?
- about this answer if i have 4 threads so,each thread have (i = 0; i < n; ++i)and (j = 0; j < n; ++j) iteration? i need your help.

Answer 1

1) i is private because each thread needs to iterate from 0 to n separately; otherwise, if make i shared when your threads together will iterate over ONE loop from 0 to n .

2) Yes, in this code each thread has it's own copy of i and j variables and that's why they will operate separately.

3) I am not sure in this example but I can say you must avoid data dependency because it cause a problems in making code to work parallel: processors (or workers ) must do exactly one single job without dependency on another worker's state or result to bring more efficiency. See SIMD and try to find some vectorization information. Shortly, vectorization is a technique which helps much in paralellizing code because it implements SIMD paradigm. On modern cpu's like Intel Sandy Bridge and older architectures using this technique allows you to speed up very much your parallel computing by using AVX/AVX2 extensions.