For '-pc_type mg', we use '-mg_levels_0_pc_type redundant -mg_coarse_pc_redundant_number <n>' to control num of processors for each coarse-grid solve, n<=np and n=np is the default which solves coarse-grid sequentially.
See petsc/src/snes/examples/tutorials/runex48_4, for which you can add option '-mg_coarse_pc_redundant_number 2' to have two subcommunicators at coarse-grid level. Can you use pcredundant for gamg? Hong On Tue, Jun 27, 2017 at 9:46 AM, Mark Adams <[email protected]> wrote: > > > On Tue, Jun 27, 2017 at 8:35 AM, Matthew Knepley <[email protected]> > wrote: > >> On Tue, Jun 27, 2017 at 6:36 AM, Mark Adams <[email protected]> wrote: >> >>> In talking with Garth, this will not work. >>> >>> I/we am now thinking that we should replace the MG object with >>> Telescope. Telescope seems to be designed to be a superset of MG. Telescope >>> does the processor reduction, and GAMG does as well, so we would have to >>> reconcile this. Does this sound like a good idea? Am I missing anything >>> important? >>> >> >> I don't think "replace" is the right word. Telescope only does process >> reduction. It does not do control flow for solvers, >> or restriction/prolongation. You can see telescope interacting with MG >> here >> > > Oh it is not the answer at all! > > >> >> https://arxiv.org/abs/1604.07163 >> >> I think more of this should be "default", in that the options are turned >> on if you are running GMG on a large number of procs. >> >> I also think GAMG should reuse the telescope code for doing reduction, >> but I am not sure how hard this is. Mark? >> > > There is a little logic in there for selecting the number of processors on > the coarse grid. I think the way we integrate this is to integrate > parameters, if we want. I take a hint on the number of equations to try to > keep on a process (nnz would be better). I don't take hints on cluster > size, I don't reduce the number of processors by an integer. I could change > these to be more in line with Telescope, but that does not solve our > problem. > > Does PETSc now support matrices with an LHS and RHS communicator? I think > it does. I could just make sub communicators for each level in the GAMG > setup. I run PtAP and the see what I get and reduce the number of > processors (using something like MatGetSubMtrix as I recall, to aggregate > the matrix), and repartition if desired (one should). > > Would it make sense for me to do this in GAMG, and see if it breaks > anything in MG? > > >> >> Thanks, >> >> Matt >> >> >>> Mark >>> >>> On Tue, Jun 27, 2017 at 4:48 AM, Mark Adams <[email protected]> wrote: >>> >>>> Parallel coarse grid solvers are a bit broken at large scale where you >>>> don't want to use all processors on the coarse grid. The ideal thing might >>>> be to create a sub communicator, but it's not clear how to integrate this >>>> in (eg, check if the sub communicator exists before calling the coarse grid >>>> solver and convert if necessary). A bit messy. It would be nice if a >>>> parallel direct solver would not redistribute the matrix, but then it would >>>> be asking too much for it to reorder also, so we could have a crappy >>>> ordering. So maybe the first option would be best long term. >>>> >>>> I see we have MUMPS and PaStiX. Do either of these not redistribute if >>>> asked? >>>> >>> >>> >> >> >> -- >> What most experimenters take for granted before they begin their >> experiments is infinitely more interesting than any results to which their >> experiments lead. >> -- Norbert Wiener >> >> http://www.caam.rice.edu/~mk51/ >> > >
