Speaker
Description
We discuss two related tools for exascale workflows with NekRS. The first
is parRSB, which is the parallel partitioner for Nek5000/RS that is based
on recursive coordinate and recursive spectral bisection. The second is a
new, communication-minimal, coarse-grid solve that is targeting exascale
platforms such as Frontier/Aurora. The pressure solvers in Nek5000/RS rely
on p-multigrid (pMG) with multilevel Schwarz smoothers for preconditioning
an outer GMRES iteration. The coarsest level in pMG corresponds to
the unstructured mesh comprising p=1 trilinear elements. For such meshes,
the number of unknowns is approximately equal to the number of elements, E, and
the number of unknowns per MPI rank is therefore approximately E/P. On
GPU-based platforms, E/P is typically around 10,000, which is too small to fully
occupy a GPU. Nek5000/RS mesh sizes, however, can be as large as E = 1 billion,
so the over-all coarse problem is not small. To reduce communication in the
coarse solve, we employ a two-level Schwarz method with a local solve that
requires only a single pair of nearest-neighbor communications and with a
global reduced-system solve that requires only 2 log_2 P messages. On
Frontier, the communication for the reduced-system solve is on par with a single MPI-allreduce.
Relevance for Nek [100 words max]
Paul
