Exploiting distributed and shared memory hierarchies with Hitmap

Current multicomputers are typically built as interconnected clusters of shared-memory multicore computers. A common programming approach for these clusters is to simply use a message-passing paradigm, launching as many processes as cores available. Nevertheless, to better exploit the scalability of these clusters and highly-parallel multicore systems, it is needed to efficiently use their distributed- and shared-memory hierarchies. This implies to combine different programming paradigms and tools at different levels of the program design. This paper presents an approach to ease the programming for mixed distributed and shared memory parallel computers. The coordination at the distributed memory level is simplified using Hitmap, a library for distributed computing using hierarchical tiling of data structures. We show how this tool can be integrated with shared-memory programming models and automatic code generation tools to efficiently exploit the multicore environment of each multicomputer node. This approach allows to exploit the most appropriate techniques for each model, easily generating multilevel parallel programs that automatically adapt their communication and synchronization structures to the target machine. Our experimental results show how this approach mimics or even improves the best performance results obtained with manually optimized codes using pure MPI or OpenMP models.