Optimization of the sparse matrix-vector products of an IDR Krylov iterative solver in EMGeo for the Intel KNL manycore processor

Abstract

In geophysical-imaging, medium properties can be studied by performing scattering experiments using electromagnetic or seismic waves. Quantities such as densities, elasticities, stress etc. can be obtained from fitting the observed measurements to the results predicted by a simulation. The EMGeo software performs these simulations and solves the inverse scattering problem in the Laplace-Fourier domain. In this paper, we focus on the Seismic part and forward step of the inverse scattering problem, which involves inverting a large sparse matrix. For this purpose, EMGeo uses an Induced Dimensional Reduction (IDR) Krylov subspace solver. The Sparse Matrix Vector (SpMV) product is responsible for more than half of the total runtime. We demonstrate how we use spatial and multiple Right Hand Side (RHS) blocking cache optimizations to increase arithmetic intensity and thus the performance, as SpMV product is memory bandwidth-bound. Our optimizations achieve 5.0× and 4.8× speedup in the SpMV product in Haswell and KNL processors, respectively. We also achieve 1.8× and 3.3× speedup in the overall IDR solver in Haswell and KNL processors, respectively. We also give an outlook over possible future optimizations.