OpenMP: Assign threads one iteration at a time

Question

Say you have a loop containing a varying number of iterations and 4 cores

I understand that

#pragma omp parallel for

will basically divide the iterations in like this with chunks of size/4 length

| T1 | T2 | T3 | T4 |

However, in my particular situation, this behavior would be more advantageous. Where each chunk is size/size length. So thread 1 would not get iterations 0..size/4, but instead iterations 0,size/4,2*size/4,3*size/4

|T1|T2|T3|T4|T1|T2|T3|T4|T1|T2|T3|T4|T1|T2|T3|T4|

How can I have my code execute like this when the number of iterations is not known until runtime?

Answer 1

What you are describing -- assuming that your heuristic is size/total threads -- is a round-robin scheduling (i.e., static scheduling) with chunk_size = 1. For that you simply need :

#pragma omp parallel for schedule(static,1)

In this case, it makes no difference if the number of iterations is known (or not) at runtime.