HTS: A Threaded Multilevel Sparse Hybrid Solver

Large shared-memory many-core nodes have become the norm in scientific computing, and therefore the sparse linear solver stack must adapt to the multilevel structure that exists on these nodes. One adaption is the development of hybrid-solvers at the node level. We present HTS as a hybrid threaded solver that aims to provide a finer-grain algorithm to keep an increased number of threads actively working on these larger shared-memory environments without the overheads of message passing implementations. Additionally, HTS aims at utilizing the additional shared memory that may be available to improve performance, i.e., reducing iteration counts when used as a preconditioner and speeding up calculations. HTS is built around the Schur complement framework that many other hybrid solver packages already use. However, HTS uses a multilevel structure in dealing with the Schur complement and allows for fill-in in certain off-diagonal submatrices to allow for a faster and more accurate solve phase. These modifications allow for a tasking thread library, namely Cilk, to be used to speed up performance while still reducing peak memory by more than 20% on average compared to an optimized direct factorization method. We show that HTS can outperform the MPI-based hybrid solver ShyLU on a suite of sparse matrices by as much as 2×, and show that HTS can scale well on three-dimensional finite difference problems.