Parallel Multigrid Solver for 3D Unstructured Finite Element Problems

Multigrid is a popular solution method for the system of linear algebraic equations that arise from PDEs discretized with the finite element method. The application of multigrid to unstructured grid problems, however, is not well developed. We discuss a method, that uses many of the same techniques as the finite element method itself, to apply standard multigrid algorithms to unstructured finite element problems. We use maximal independent sets (MISs) as a mechanism to automatically coarsen unstructured grids; the inherent flexibility in the selection of an MIS allows for the use of heuristics to improve their effectiveness for a multigrid solver. We present parallel algorithms, based on geometric heuristics, to optimize the quality of MISs and the meshes constructed from them, for use in multigrid solvers for 3D unstructured problems. We conduct scalability studies that demonstrate the effectiveness of our methods on a problem in large deformation elasticity and plasticity of up to 40 million degrees of freedom on 960 processor IBM PowerPC 4-way SMP cluster with about 60% parallel efficiency.