如何使用openMP将顺序程序转换为并行程序？

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];

How can i make this for parallel?

Answer 1

I recommend you start by checking out the following link: http://bisqwit.iki.fi/story/howto/openmp/ . It gives a brief overview of what you can achieve using OpenMP.

For your code fragment parallelising can be as easy as writing one pragma:

#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];

That is the idea behind OMP: you annotate your program with messages, that allow the code to be compiled and linked with OMP and then ran in parallel, or compiled ignoring the pragmas in which case the program should remain a valid sequential program.

In this case the pragma leaves the decision as to how many threads to run to the runtime. The runtime generally makes a decision based on the amount of cores in the machine. The outer loop will be parallelized and each i iteration will conceptually be performed by a different thread. This is important because you have data dependencies between various j iterations, and communication/synchronization in parallel is tricky. Keeping the inner loop within a thread deals with this problem. The shared section could be left out because things are shared by default. But for this reason exactly you shouldn't leave it out: be explicit about what you want shared and what you want private. This is a good way to avoid many bugs that happen when writing parallel code.