  In this paper, the OpenACC heterogeneous parallel programming model is
successfully applied to modification and acceleration of the three-dimensional
Tokamak magnetohydrodynamical code (CLTx). Through combination of OpenACC and
MPI technologies, CLTx is further parallelized by using multiple-GPUs.
Significant speedup ratios are achieved on NVIDIA TITAN Xp and TITAN V GPUs,
respectively, with very few modifications of CLTx. Furthermore, the validity of
the double precision calculations on the above-mentioned two graphics cards has
also been strictly verified with m/n=2/1 resistive tearing mode instability in
Tokamak.
