> 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.
