Soutenance de thèse : Salli Moustafa

The goal of the research presented in this thesis was to study, and to propose a suitable solution, to the  challenges posed by the use of hierarchical massively parallel computers, for solving the neutron transport equation according to the discrete ordinates method (SN). The classic source iteration (SI) scheme used for solving this equation involves the so-called sweep operation on the spatial mesh. This sweep operation gathers the vast majority of computations in the SN method and it exposes a wavefront like progression over the spatial mesh.

We first proposed a strategy for designing an efficient parallel implementation of the sweep operation on modern architectures by combining the use of the SIMD paradigm and the emerging task-based runtime systems. We have shown that the PaRSEC framework, based on parametrized DAG model, produced the most efficient implementation for the Cartesian transport sweep, as compared to Intel TBB and StarPU versions. We designed an accurate parallelsweep simulator in order to determine the optimal parallel spatial decomposition of the transport sweep. We used this sweep simulator to justify the need for a task-based implementation of the sweep operation in order to maximize its performances on multicore-based architectures. Using optimal partitioning, the performance of the PaRSEC implementation of the sweep operation reaches 6.1 Tflop/s on 768 cores of the IVANOE supercomputer, which corresponds to 33.9% of the theoretical peak performance of this set of computational resources.

Then we studied the challenge of converging the source iterations in highly diffusive media such as the PWR cores. We have implemented and studied the convergence of a new acceleration scheme (PDSA) that naturally suits our Hybrid parallel implementation. The combination of all these techniques have enabled us to develop a massively parallel version of the Domino solver. It is capable of tackling the challenges posed by the neutron

transport simulations and compares favorably with state-of-the-art solvers such as Denovo. For a typical 26-group PWR calculations involving 1.02 × 10^12 DoFs, the time to solution required by the Domino solver is 45 min using 1536 cores. Consequently, this Domino solver can be used by nuclear power plant operators such as EDF for improving the efficiency and safety of nuclear power plants.