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Gyrokinetic codes in plasma physics need outstanding computational resources to solve increasingly complex problems, requiring the effective exploitation of cutting-edge HPC architectures. This paper focuses on the enabling of ORB5, a state-of-the-art, first-principles-based gyrokinetic code, on modern parallel hybrid multi-core, multi-GPU systems. ORB5 is a Lagrangian, Particle-In-Cell (PIC), finite element, global, electromagnetic code, originally implementing distributed MPI-based parallelism through domain decomposition and domain cloning. In order to support multi/many cores devices, the code has been completely refactored. Data structures have been re-designed to ensure efficient memory access, enhancing data locality. Multi-threading has been introduced through OpenMP on the CPU and adopting OpenACC to support GPU acceleration. MPI is further used in combination with the two approaches. The performance results obtained using the full production ORB5 code on the Summit system at ORNL, on Piz Daint at CSCS and on the Marconi system at CINECA are presented, showing the effectiveness and performance portability of the adopted solutions: the same source code version was used to produce all results on all architectures.
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Stephan Brunner, Claudio Gheller, Emmanuel Lanti, Noé Thomas Elie Ohana, Aaron Lewis Scheinberg, Laurent Villard
mode-switching'' approach consisting in using an effective heat source computed from a gradient-driven simulation as an input for the flux-driven run is also tested and consistent results are obtained between gradient-driven, flux-driven, and
mode-switched'' simulations. Finally, the
heat source is characterized, along with variations, using a 2D phase-space diagnostics. It is shown
that in some cases, the use of this fixed heat source can lead to a sampling problem causing an
artificial source/sink of moments.