Optimizing the parallel scheme of the Poisson solver for the reduced kinetic code TERESA^*

¹ CEA Cadarache, IRFM, F-13108 Saint-Paul-lez-Durance, France

Abstract

The parallelization performance of the TERESA Code for Trapped Element REduction in Semi lagrangian Approach is analyzed. TERESA is a kinetic code in four dimensions (4D), two in "real" space and two "energy" coordinates. It addresses the turbulent evolution of the distribution of ions governed by uctuations of the electric potential. The numerical scheme is split into four steps: the 4D advection of the distribution function with the Vlasov equation, the computation of the charge density, the calculation of the electric potential using the quasineutrality asymptotic limit of the Poisson equation and finally, the two high frequency averages of the electric potential in order to compute the advection field. Starting from an initial standard parallelization scheme we find that the Poisson solver accounts for most of the execution time of TERESA (up to 90% at aimed meshes) and that its scaling performance is poor. Two improvements of the Poisson solver have been implemented and tested. These show that performance levelling are due to interference phenomena when addressing computation resources as well as communication overheads when the loading of the CPUs with MPI processes / OpenMP threads is not optimized. Tests on two computers, Rheticus et Helios, with very similar architecture, allows one to better diagnose the interference phenomena. These developments lead to reduction by up to a factor 100 of the execution time and extends the scalability properties up to 1024 cores for large meshes cases.