> On 27 Jul 2018, at 5:33 PM, Jed Brown <j...@jedbrown.org> wrote: > > Pierre Jolivet <pierre.joli...@enseeiht.fr> writes: > >>> On 27 Jul 2018, at 5:12 PM, Jed Brown <j...@jedbrown.org> wrote: >>> >>> Pierre Jolivet <pierre.joli...@enseeiht.fr> writes: >>> >>>> Everything is fine with GAMG I think, please find the (trimmed) -eps_view >>>> attached. The problem is that, correct me if I’m wrong, there is no easy >>>> way to redistribute data efficiently from within PETSc when using >>>> fieldsplit with unbalanced number of unknowns per field. For the other >>>> three fields, the solvers are still behaving somehow properly. Now if I’d >>>> like to optimize this some more, I’d probably need to switch from a >>>> fieldsplit to a MatNest, with submatrices from different communicators, so >>>> that I don’t have all processes handling the pressure space. But this is >>>> apparently not allowed. >>> >>> What if pressure is still on a global communicator, but all the degrees >>> of freedom are in a subset? Then MatMult and the like have nothing to >>> send or receive on the processes without any dofs. Since there are no >>> reductions in PCApply (there are in setup), it should complete >>> immediately for all the processes that don't have any dofs, right? >> >> My PC is PCKSP, with GMRES underneath, so there are reductions on the global >> communicator in PCApply. > > Why PCKSP in the pressure solve?
I need to solve (on the pressure space) Ax = b with A^-1 = B^-1+ C^-1. I do an additive PCCOMPOSITE, with two PCKSP. B is a mass matrix (PCJACOBI, no problem), C is a shifted Laplacian (PCGAMG, problem…).
