Abstract of FELTOR2 Project

FELTOR, the Full-F electromagnetic model in toroidal geometry, is both a numerical library and a collection of physical application codes built on top of it. The main physical targets are (gyro-)fluid simulations of the tokamak edge and scrape-off layer regions in two and three dimensions. The modular design of the library allows for a collaboration of various specialists focusing on different aspects of the project. Our goal is to continuously improve the execution times of our algorithms, to develop and adapt numerical methods and at the same time provide an easy to use interface for fast development and testing of physical model equations.

In this proposal, we apply for support on two important topics that are both concerned with the scalability of algorithms on single and multiple compute nodes using a hybrid MPI and OpenMP parallelization strategy. The first is the support of our current efforts to implement matrix-free multigrid algorithms within the library. These will in general reduce the necessary number of iterations compared to conjugate gradient methods and provide improved scalability by avoiding scalar products and the associated global synchronization points. The second task is to review the parallel implementation of the flux coordinate independent approach, which involves irregular MPI communication patterns stemming from a "compressed sparse row"-type sparse matrix-vector multiplication. The problem is to identify and remove performance bottlenecks and ensure the scalability of the algorithm.

Overall, we hope that with the support of the HLST team we will be able to conduct parameter studies with three-dimensional simulations efficiently and to provide improved statistics in long-term turbulence studies.