Abstract of REFMUL3+ Project

Finite-difference time-domain (FDTD) methods are some of the most popular numerical techniques used to simulate reflectometry. They offer a comprehensive description of the plasma phenomena. However, this method requires a fine spatial grid discretization to keep the error to a minimum, which in turn implies a high resolution time discretization to comply with the CFL numerical stability condition. Simulations in three-dimensions (3D) are therefore very demanding computationally, both in terms of floating point operations, as well as memory resources. They become only possible if the problem can be efficiently distributed over large number of resources. The REFMUL3 code was developed during 2016 within a collaboration between the Instituto de Plasmas e Fusão Nuclear (IPFN-IST) in Lisbon and the HLST, precisely to meet this goal [1]. It is a 3D full-wave code using a FDTD Yee scheme [2] with full polarisation that copes simultaneously with o- and x-modes, supports a general external magnetic field and a dynamic plasma. REFMUL3 employs a hybrid MPI/OpenMP parallelisation using explicit 3D domain decomposition, which yields very good scaling properties up to a few thousands of cores. This has opened the door to the simulation of much larger domains (gridcounts), which, as expected, exposed new challenges regarding data input/output (I/O) operations. This challenge has been addressed within the currently running HLST-REFMULIO project (2018), by adding parallel I/O capabilities and a basic check-point / restart file infrastructure to the code. The present proposal aims at further optimising the parallel I/O performance of REFMUL3. The effort should focus mainly on the performance tuning of the existing parallel HDF5 I/O infrastructure. Additionally, the generalisation of the I/O infrastructure using the Portable Data Interface (PDI) library is envisaged [3]. If time allows, the optimisation of the MPI communication by exploring the concept of overlapping communication with computation is also welcome.

[1] T. Ribeiro, Final report on HLST project REFMUL3 (2017)
[2] K. S. Yee, IEEE Trans. on Antennas and Propagation, 14:302-307, 1966.
[3] PDI, the portable data interface, https://gitlab.maisondelasimulation.fr/jbigot/pdi