Parallel I/O optimization for micrometer-scale atomistic calculations under extreme conditions*

Atomistic calculations is an essential means for materials under extreme conditions; for example, fusion reactors are required to be designed without the necessary environment to test them. To better complement experiments and make predictions more useful, however, improvements are necessary in a quantitative sense against experimental measurements as well as theoretical references such as first principles calculations. One way is to develop accurate interatomic potentials, and the other is to break scaling limits.  In this study, we focus on how to extend molecular dynamics beyond the conventional length scale to what we call a micron virtual specimen that contains tens of billions of atoms. Common problems such as  a stall due to I/O bandwidth arise for such large calculations necessarily running on 68K+ CPU cores, which can be avoided  by assigning only a part of the allocated processes to I/O while the rest to calculations. We will report the current development status along with the preliminary results for irradiated-damage simulations on KISTI Nurion.