Section: New Results
High performance fast multipole method for N-body problems
Task-based fast multipole method
With the advent of complex modern architectures, the low-level paradigms long considered sufficient to build High Performance Computing (HPC) numerical codes have met their limits. Achieving efficiency, ensuring portability, while preserving programming tractability on such hardware prompted the HPC community to design new, higher level paradigms. The successful ports of fully-featured numerical libraries on several recent runtime system proposals have shown, indeed, the benefit of task-based parallelism models in terms of performance portability on complex platforms. However, the common weakness of these projects is to deeply tie applications to specific expert-only runtime system APIs. The OpenMP specification, which aims at providing a common parallel programming means for shared-memory platforms, appears as a good candidate to address this issue thanks to the latest task-based constructs introduced as part of its revision 4.0. The goal of this paper is to assess the effectiveness and limits of this support for designing a high-performance numerical library. We illustrate our discussion with the ScalFMM library, which implements state-of-the-art fast multipole method (FMM) algorithms, that we have deeply re-designed with respect to the most advanced features provided by OpenMP 4. We show that OpenMP 4 allows for significant performance improvements over previous OpenMP revisions on recent multicore processors. We furthermore propose extensions to the OpenMP 4 standard and show how they can enhance FMM performance. To assess our statement, we have implemented this support within the Klang-OMP source-to-source compiler that translates OpenMP directives into calls to the StarPU task-based runtime system. This study,  shows that we can take advantage of the advanced capabilities of a fully-featured runtime system without resorting to a specific, native runtime port, hence bridging the gap between the OpenMP standard and the very high performance that was so far reserved to expert-only runtime system APIs.
Task-based fast multipole method for clusters of multicore processors
Most high-performance, scientific libraries have adopted hybrid parallelization schemes - such as the popular MPI+OpenMP hybridization - to benefit from the capacities of modern distributed-memory machines. While these approaches have shown to achieve high performance, they require a lot of effort to design and maintain sophisticated synchronization/communication strategies. On the other hand, task-based programming paradigms aim at delegating this burden to a runtime system for maximizing productivity. In this article, we assess the potential of task-based fast multipole methods (FMM) on clusters of multicore processors. We propose both a hybrid MPI+task FMM parallelization and a pure task-based parallelization where the MPI communications are implicitly handled by the runtime system. The latter approach yields a very compact code following a sequential task-based programming model. We show that task-based approaches can compete with a hybrid MPI+OpenMP highly optimized code and that furthermore the compact task-based scheme fully matches the performance of the sophisticated, hybrid MPI+task version, ensuring performance while maximizing productivity. In  we illustrate our discussion with the ScalFMM FMM library and the StarPU runtime system.