Title : Gysela5D, Adapting a GYrokinetic SEmi-LAgrangian code for current architectures and towards Exascale
Speaker: Julien Bigot
Abstract: In order to design and operates the future reactor for nuclear fusion such as ITER (tokamaks), physicists need to better understand the various types of instabilities that develop in the plasma and impact the confinement of heat. Simulation of Ion Temperature Gradient (ITG) instabilities based on the Vlasov equations require huge amounts of computational power with a discretization of both the spacial and velocity space (6D). The gyrokinetic approximation makes this kind of simulation possible by reducing this to “only” 5D. Up to now, the semi-Lagrangian code Gysela5D has been used to perform large simulations using a few thousands cores (8k to 16k cores typically). These simulations make the hypothesis that electrons are adiabatic but recent advances seem to indicate that some instabilities could only be explained by simulating kinetic electrons. In order to do that, the spacial mesh would have to be refined by a 60³ ratio and time steps by a 60 ratio. Such simulations would require Exascale capable machines. In this talk, I present some challenges identified in order to provide an Exascale-ready code as well as solutions recently implemented and work in progress to tackle these. I especially focus on three such piece of work:
- memory scalability optimization;
- I/O optimizations for both checkpoints and result writing;
- communication patterns optimization for big number of cores (Blue Gene/Q).
I will also present recent results that show that the code scales with good performance up to 1,835,008 threads (the complete Juqueen Blue Gene/Q at Jülich).