Development of exascale fusion plasma turbulence simulations for post-K

Idomura, Yasuhiro   ; Ina, Takuya*; Obrejan, K.; Asahi, Yuichi*   ; Matsuoka, Seikichi* ; Imamura, Toshiyuki*

Under the post-K project, we have developed computing techniques of the Gyrokinetic Toroidal 5D full-f Eulerian code GT5D towards the next generation computing platforms based on many core processors. We discuss computational challenges related to complicated intra-processor memory hierarchy and limited inter-node communication performance compared with accelerated computation. The former issue is addressed by optimizing data access patterns of a stencil kernel on each many core architecture, and high performance gains are obtained. The latter issue is resolved by using advanced communication avoiding Krylov methods, which enables an order of magnitude reduction of collective communications and improves arithmetic intensity of main computing kernels. By applying these novel computing techniques, the performance of GT5D is dramatically improved on the latest many core platforms, and excellent strong scaling up to the full system size of the Oakforest-PACS (8,192 KNLs) is achieved.