Optimizations of a spectral/finite difference gyrokinetic code for improved strong scaling toward million cores

Maeyama, Shinya; Watanabe, Tomohiko*; Idomura, Yasuhiro   ; Nakata, Motoki; Nunami, Masanori*; Ishizawa, Akihiro*

Plasma turbulence is multi-scale physics including spatio-temporal scales of electrons and ions, whose simulations require huge computations. In this work, we present optimizations of the gyrokinetic Eulerian code GKV, which performs 5D computational fluid dynamics calculations by means of spectral and finite difference methods, on massively parallel platforms. First, a segmented rank mapping on the 3D torus network utilizes the bi-section bandwidth and reduces the collisions of the messages for simultaneous point-to-point communications, and therefore, reduces communication cost. Second, computation-communication overlaps with pipelining methods effectively mask the communication cost. Thanks to the optimizations, GKV achieves excellent strong scaling toward million cores with high parallel efficiency, and enables us to address the first multi-scale plasma turbulence simulations employing the real ion-to-electron mass ratio.