Optimizations of the eigensolvers in the ELPA library

The solution of (generalized) eigenvalue problems for symmetric or Hermitian matrices is a common subtask of many numerical calculations in electronic structure theory or materials science. Solving the eigenvalue problem can easily amount to a sizeable fraction of the whole numerical calculation. For researchers in the field of computational materials science, an efficient and scalable solution of the eigenvalue problem is thus of major importance. The ELPA-library is a well-established dense direct eigenvalue solver library, which has proven to be very efficient and scalable up to very large core counts. In this paper, we describe the latest optimizations of the ELPA-library for new HPC architectures of the Intel Skylake processor family with an AVX-512 SIMD instruction set, or for HPC systems accelerated with recent GPUs. We also describe a complete redesign of the API in a modern modular way, which, apart from a much simpler and more flexible usability, leads to a new path to access system-specific performance optimizations. In order to ensure optimal performance for a particular scientific setting or a specific HPC system, the new API allows the user to influence in straightforward way the internal details of the algorithms and of performance-critical parameters used in the ELPA-library. On top of that, we introduced an autotuning functionality, which allows for finding the best settings in a self-contained automated way. In situations where many eigenvalue problems with similar settings have to be solved consecutively, the autotuning process of the ELPA-library can be done "on-the-fly". Practical applications from materials science which rely on so-called self-consistency iterations can profit from the autotuning. On some examples of scientific interest, simulated with the FHI-aims application, the advantages of the latest optimizations of the ELPA-library are demonstrated