Direct solution of larger coupled sparse/dense linear systems using low-rank compression on single-node multi-core machines in an industrial context

While hierarchically low-rank compression methods are now commonly available in both dense and sparse direct solvers, their usage for the direct solution of coupled sparse/dense linear systems has been little investigated. The solution of such systems is though central for the simulation of many important physics problems such as the simulation of the propagation of acoustic waves around aircrafts. Indeed, the heterogeneity of the jet flow created by reactors often requires a Finite Element Method (FEM) discretization, leading to a sparse linear system, while it may be reasonable to assume as homogeneous the rest of the space and hence model it with a Boundary Element Method (BEM) discretization, leading to a dense system. In an industrial context, these simulations are often operated on modern multicore workstations with fully-featured linear solvers. Exploiting their low-rank compression techniques is thus very appealing for solving larger coupled sparse/dense systems (hence ensuring a finer solution) on a given multicore workstation, and - of course - possibly do it fast. The standard method performing an efficient coupling of sparse and dense direct solvers is to rely on the Schur complement functionality of the sparse direct solver. However, to the best of our knowledge, modern fully-featured sparse direct solvers offering this functionality return the Schur complement as a non compressed matrix. In this paper, we study the opportunity to process larger systems in spite of this constraint. For that we propose two classes of algorithms, namely multi-solve and multi-factorization, consisting in composing existing parallel sparse and dense methods on well chosen submatrices. An experimental study conducted on a 24 cores machine equipped with 128 GiB of RAM shows that these algorithms, implemented on top of state-of-the-art sparse and dense direct solvers, together with proper low-rank assembly schemes, can respectively process systems of 9 million and 2.5 million total unknowns instead of 1.3 million unknowns with a standard coupling of compressed sparse and dense solvers.