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Idomura, Yasuhiro; Watanabe, Tomohiko*; Todo, Yasushi*
Shimyureshon, 38(2), p.79 - 86, 2019/06
We promote the research and development of exascale fusion plasma simulations on Post-K towards estimation and prediction of core plasma performance, and exploration of improved operation scenarios on the next generation fusion experimental reactor ITER. In this paper, we review developed exascale simulation technologies and outcomes from validation studies on existing experimental devices, and discuss perspectives on exascale fusion plasma simulations on Post-K.
Idomura, Yasuhiro; Ina, Takuya*; Mayumi, Akie; Yamada, Susumu; Imamura, Toshiyuki*
Lecture Notes in Computer Science 10776, p.257 - 273, 2018/00
Times Cited Count：2 Percentile：50.36(Computer Science, Artificial Intelligence)A preconditioned Chebyshev basis communication-avoiding conjugate gradient method (P-CBCG) is applied to the pressure Poisson equation in a multiphase thermal-hydraulic CFD code JUPITER, and its computational performance and convergence properties are compared against a preconditioned conjugate gradient (P-CG) method and a preconditioned communication-avoiding conjugate gradient (P-CACG) method on the Oakforest-PACS, which consists of 8,208 KNLs. The P-CBCG method reduces the number of collective communications with keeping the robustness of convergence properties. Compared with the P-CACG method, an order of magnitude larger communication-avoiding steps are enabled by the improved robustness. It is shown that the P-CBCG method is 
and 
faster than the P-CG and P-CACG methods at 2,000 processors, respectively.
Idomura, Yasuhiro
no journal, ,
The research plan and the current status of fusion plasma code development in the Post-K priority issue 6 "Development of Innovative Clean Energy" [Core Design of Fusion Reactor] are reviewed. This project develops first principles based plasma analysis codes on plasma turbulence phenomena and MHD phenomena, which are taken into account in the design of operation scenarios of fusion reactors. Because of the spatio-temporal scales in ITER core plasmas and the complexity of their physics models including multi-species ions and burning processes, these fusion plasma analysis codes require new simulation techniques. In this talk, we discuss the development of Exa-scale computating technologies such as many-core optimization, and new physics and numerical models to enable long time scale simulations.
Idomura, Yasuhiro
no journal, ,
A communication-avoiding generalized minimal residual (CA-GMRES) method is applied to the gyrokinetic toroidal five dimensional Eulerian code GT5D, and its performance is compared against the original code with a generalized conjugate residual (GCR) method on the Oakforest-PACS (KNL). The CA-GMRES method has less memory access and collective communications than the GCR method, and thus, is suitable for future Exa-scale architectures with limited memory and network bandwidths. It is shown that compared with the original GCR version, the CA-GMRES version is accelerated by 1.32x, and the cost of data reduction communication is reduced from ~13% to ~1% of the total cost at 1,280 nodes.
Idomura, Yasuhiro
no journal, ,
This talk reviews exascale computing technologies in fusion plasma simulations developed under the Post-K priority issue. Burning plasmas in ITER consists of multi-species ions, and their spatio-temporal scales are more than an order of magnitude larger than existing devices. Therefore, burning plasma simulations in ITER require exascale computing. To this end, we have developed novel computing technologies, which enables highly efficient computation on latest many core processors and reduces the inter-node communication, in the five dimensional fusion plasma turbulence code GT5D, and their performances were demonstrated on the Oakforest-PACS, which consists of 8,208 XeonPhi7250 (KNL) processors.
Idomura, Yasuhiro
no journal, ,
This talk reviews computing technologies developed for extreme scale nuclear CFD simulations on latest many core computing platforms. At JAEA, there are needs for extreme scale CFD simulations for analyzing critical issues such as melt relocation behavior of nuclear reactors at severe accidents and environmental dynamics of radioactive substances. Although the latest many core platforms offer promising solutions for such high computing needs, accelerated computation reveals severe bottlenecks of inter-node communication and data I/O. To resolve these issues, we have developed novel communication-avoiding matrix solvers and an In-Situ visualization system for the three dimensional multi-phase and multi-component thermal hydraulic core, JUPITER, and their performances were demonstrated in on the Oakforest-PACS, which consists of 8,208 XeonPhi7250 (KNL) processors.
Idomura, Yasuhiro; Ina, Takuya*; Mayumi, Akie; Yamada, Susumu; Matsumoto, Kazuya*; Asahi, Yuichi*; Imamura, Toshiyuki*
no journal, ,
We propose a modified communication-avoiding generalized minimal residual (CA-GMRES) method, which reduces both computation and memory access by 30% with keeping the same CA property as the original CA-GMRES method. These numerical properties, less communication and computation with higher arithmetic intensity, are promising features for future exascale machines with limited memory and network bandwidths. The modified CA-GMRES method is applied to a large scale non-symmetric matrix in an implicit solver of the gyrokinetic toroidal five dimensional Eulerian code GT5D, and its performance is estimated on the Oakforest-PACS (KNL). The numerical experiment shows that compared with the generalized conjugate residual method, computing kernels are accelerated by 1.5x, and the cost of data reduction communication is reduced from 12.5% to 1% of the total cost at 1,280 nodes.
Mayumi, Akie; Idomura, Yasuhiro; Ina, Takuya*; Yamada, Susumu; Imamura, Toshiyuki*
no journal, ,
To improve the convergence property of the communication avoiding conjugate gradient (CA-CG) method is needed for applying it to ill conditioned problems such as the pressure Poisson equation in the multiphase CFD code JUPITER. In the CA-CG method, one can avoid more communication by increasing the number of CA steps. However, this makes the CA-CG method less robust against numerical errors. To resolve this problem, we apply the Chebyshev basis CG (CBCG) method to JUPITER.
Idomura, Yasuhiro
no journal, ,
Turbulent transport is one of key issues in fusion science. To address this issue via a five dimensional (5D) gyrokinetic model, the Gyrokinetic Toroidal 5D full-f Eulerian code GT5D has been developed. On the K-computer, inter-node parallelization techniques such as multi-dimensional/-layer domain decomposition and communication-computation overlap were developed, and strong scaling of GT5D was improved up to 
million cores. This computing power enabled us to study ITER relevant issues such as the plasma size scaling of turbulent transport. However, extensions of GT5D towards burning plasmas including kinetic electrons and multi-species ions require exascale computing. Under the post-K project, we have developed computing techniques for the next generation computing platforms based on many core processors. In this talk, we discuss computational challenges related to complicated intra-processor memory hierarchy and limited inter-node communication performance compared with accelerated computation.
Idomura, Yasuhiro
no journal, ,
Under the Post-K project, a Gyrokinetic Toroidal 5D full-f Eulerian code GT5D has been developed towards exascale burning plasma simulations on the Post-K machine. In this talk, we review the present status on new computational techniques on GT5D, in which Krylov based sparse matrix solvers for a semi-implicit time integration method occupy the most of computational cost. Recently, we introduced communication-avoiding (CA) Krylov methods, and improved the scalability of GT5D on the latest many-core platforms. We also ported the CA-Krylov solver on the latest GPU platforms, and achieved speedup by more than an order of magnitude.
Idomura, Yasuhiro
no journal, ,
Under the Post-K project, a Gyrokinetic Toroidal 5D full-f Eulerian code GT5D has been developed towards exascale burning plasma simulations on the Post-K machine. In this talk, firstly, we review petascale computing techniques on current petascale machines such as the K-computer and their impacts on the progress of full-f gyrokinetic simulations. Secondly, we discuss extensions of simulation models towards burning plasma simulations and systematic experimental data analyses. Thirdly, we present new exascale computing techniques towards the Post-K machine, and demonstrate their computational performance on the latest many core platforms.
Idomura, Yasuhiro
no journal, ,
We review gyrokinetic simulations of turbulent fusion plasmas. First, the five dimensional (5D) gyrokinetic model is presented, and its numerical approaches, namely, Lagrangian and Eulerian approaches are explained. Second, the gyrokinetic toroidal 5D Eulerian code GT5D is presented, and novel exascale computing techniques on the latest many core and GPU platforms are discussed. Third, the current petascale numerical experiments on ion temperature gradient turbulence are presented, and the target issues in the future exascale numerical experiments on FUGAKU are discussed.
Idomura, Yasuhiro
no journal, ,
A gyrokinetic toroidal 5D Eulerien code GT5D resolves global torus plasma using 5D grids, and core plasma simulations of ITER require exascale computing on Fugaku. To this end, we developed a new communication avoiding (CA) Krylov solver with FP16 preconditioning for implicit finite difference computation, which occupies more than 80% of the total computing cost in GT5D. In this solver, a bottleneck of global collective communication is resolved using a CA Krylov subspace method. In addition, halo communication is reduced by improving the convergence property with FP16 preconditioning. The FP16 preconditioner was designed based on physics properties of the operator, and was implemented using FP16 SIMD operations. Compared with the conventional solver, the new solver improved the performance of ITER size simulations with ~100 billion grids by ~3.5x, and a good strong scaling was achieved up to 5,760 nodes.
Idomura, Yasuhiro
no journal, ,
Thanks to new technologies such as KNL and MCDRAM, Oakforest-PACS (OfP) achieved significantly high computing power and memory bandwidths against the conventional multi-core platforms, and played an important role as a prototype of exascale supercomputers. We developed extreme scale nuclear CFD simulations on OfP, where an important issue was to resolve communication bottlenecks revealed by accelerated computation. This issue was resolved by developing communication-avoiding (CA) matrix solvers based on CA Krylov subspace methods and CA multigrid methods, and high performance CFD simulations were enabled by using the full system size on OfP. In this talk, we review CA matrix solvers developed for the five dimensional plasma simulation code GT5D and the three dimensional multi-phase multi-component thermal-hydraulic code JUPITER.
Idomura, Yasuhiro
no journal, ,
The Gyrokinetic Toroidal 5D full-f Eulerian code GT5D was ported on Fugaku and Summit, which are state-of-the-art exascale supercomputers based on many core CPUs and GPUs. GT5D is based on a semi-implicit finite difference scheme, in which a stiff linear 4D convection operator is subject to implicit time integration, and the implicit finite difference solver for fast kinetic electrons occupies more than 80% of the total computing cost. The implicit solver was originally developed using a Krylov subspace method (GCR), in which global collective communication and halo data communication were bottlenecks on exascale supercomputers. This issue was resolved by a communication-avoiding Krylov subspace method (CA-GMRES), which reduces the number of global collective communication, and a FP16 preconditioner, which reduces the number of iterations and thus halo data communication. On Fugaku and Summit, the new CA-GMRES solver respectively achieved 
and 
speedups from the conventional GCR solver, and excellent strong scaling was obtained up to 5,760 CPUs/GPUs.
Hasegawa, Yuta
no journal, ,
We explain the research plan for the exascale computational fluid dynamics (CFD) in Japan Atomic Energy Agency, Center for Computational Science and e-Systems (JAEA-CCSE). JAEA-CCSE has been promoting the development of 1m scale real-time urban wind simulation code, "CityLBM." In this presentation, we show our past studies that accelerate the CFD code, and our recent research which introduces ensemble data assimilation toward the digital twin of the urban wind. The outlook for the exascale computing with them is also described.
