Runtime adjustment of parallel nested loops

OpenMP allows programmers to specify nested parallelism in parallel applications. In the case of scientific applications, parallel loops are the most important source of parallelism. In this paper we present an automatic mechanism to dynamically detect the best way to exploit the parallelism when having nested parallel loops. This mechanism is based on the number of threads, the problem size, and the number of iterations on the loop. To do that, we claim that programmers must specify the potential application parallelism and give the runtime the responsibility to decide the best way to exploit it. We have implemented this mechanism inside the IBM XL runtime library. Evaluation shows that our mechanism dynamically adapts the parallelism generated to the application and runtime parameters, reaching the same speedup as the best static parallelization (with a priori information).