A massively space-time parallel N-body solver

Authors:
R. Speck;D. Ruprecht;R. Krause;M. Emmett;M. Minion;M. Winkel;P. Gibbon
Affiliations:
Università della Svizzera italiana, Lugano, Switzerland;Università della Svizzera italiana, Lugano, Switzerland and Heinrich-Heine-Universität, Germany;Università della Svizzera italiana, Lugano, Switzerland;Lawrence Berkeley National Laboratory, Berkeley;Stanford University, Stanford;Jülich Supercomputing Centre, Jülich, Germany;Jülich Supercomputing Centre, Jülich, Germany
Venue:
SC '12 Proceedings of the International Conference on High Performance Computing, Networking, Storage and Analysis
Year:
2012

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Abstract

We present a novel space-time parallel version of the Barnes-Hut tree code pepc using pfasst, the Parallel Full Approximation Scheme in Space and Time. The naive use of increasingly more processors for a fixed-size N-body problem is prone to saturate as soon as the number of unknowns per core becomes too small. To overcome this intrinsic strong-scaling limit, we introduce temporal parallelism on top of pepc's existing hybrid MPI/PThreads spatial decomposition. Here, we use pfasst which is based on a combination of the iterations of the parallel-in-time algorithm parareal with the sweeps of spectral deferred correction (SDC) schemes. By combining these sweeps with multiple space-time discretization levels, pfasst relaxes the theoretical bound on parallel efficiency in parareal. We present results from runs on up to 262,144 cores on the IBM Blue Gene/P installation JUGENE, demonstrating that the space-time parallel code provides speedup beyond the saturation of the purely space-parallel approach.