openmp two consecutive loops, problem with reduction clause

Question

There is two consecutive loops and there is a reduction clause in the second loop.

#pragma opm parallel
{
#pragma omp for
    for (size_t i = 0; i < N; ++i)
    {
      
    }
#pragma omp barrier
#pragma omp for reduction(+ \
                    : sumj)
    for (size_t i = 0; i < N; ++i)
    {
        sumj = 0.0;
        for (size_t j = 0; j < adjList[i].size(); ++j)
        {
            sumj += 0;
        }
        Jac[i, i] = sumj;
    }
}

to reduce the creating threads overhead I wand to keep the threads and use them in the second loop, but I get the following error

lib.cpp:131:17: error: reduction variable ‘sumj’ is private in outer context
         #pragma omp for reduction(+ \
                 ^~~

how to fix that?

There is no need for a barrier after the first loop. Unless you explicitly remove it (by using the "nowait" clause on the "omp for") there is always a barrier at the end of an OpenMP for construct. — Jim Cownie, Nov 10 '20 at 10:19
You need to show us more code, since, as the compiler is telling you, the problem is that the variable sumj is not a shared variable. But we can't see any of the variable declarations, so can't tell what is going on. — Jim Cownie, Nov 10 '20 at 10:21

score 1 · Accepted Answer · edited Nov 10 '20 at 07:32

I'm not sure what you are trying to do, but it seems that something like this would do what you expect:

#pragma omp parallel
{
    #pragma omp for
    for (size_t i = 0; i < N; ++i)
    {
      
    }
    #pragma omp barrier
    #pragma omp for
    for (size_t i = 0; i < N; ++i)
    {
        double sumj = 0.0;
        for (size_t j = 0; j < adjList[i].size(); ++j)
        {
            sumj += 0;
        }
        Jac[i, i] = sumj;
    }
}

Reduce would be useful in the case of an "omp for" in the interior loop.

openmp two consecutive loops, problem with reduction clause

1 Answers1