Increase speedup with OpenMP and reductions

Question

I would increase speedup of these code with OpenMP.

for (i = 1; i < length; i++)
{
   for (j = 1; j < i; j++)
     sum_c += c[j];
   c[i] = sum_c + W[i];
   sum_c = 0;
}

I try with these solution

for (i = 1; i < length; i++)
{
   #pragma omp parallel for num_threads(NTHREADS) reduction(+:sum_c)
   for (j = 1; j < i; j++)
     sum_c += c[j];
   c[i] = sum_c + W[i];
   sum_c = 0;
}

but I increase speedup only for

length=100000

Are there any better solutions? The code can be rewritten.

Answer 1

You are repeating work a lot using a inner loop. The same result can be obtained by the following single loop.

sum_c = 0;
for (i = 1; i < length; i++)
{
   c[i] = sum_c + W[i];
   sum_c += c[i];
}