On Sun, Sep 18, 2022 at 6:19 PM Barry Smith <bsm...@petsc.dev> wrote:
> > Mark, > > Some how your changes to GAMG in June slowed down the time to compute > the MIS dramatically. I cannot figure out what options to use to > get the exact same performance as an older branch. -mat_coarsen_type mis > -pc_gamg_threshold 0 result in longer times than the older code with its > default options. > The new MIS-2 folds in the square graph with the MIS. Before the square graph was in a separate method that created an squared graph explicitly. So don't use aggressive coarsening (you will see a PtAP if you use aggressive coarsening in the new code) And -pc_gamg_threshold 0 will filter (zeros only). Use < 0 for no filtering. The old code also had this optimization to not create a graph for bs==1 and no filter, Mark > > > > On Sep 18, 2022, at 4:21 PM, Mark Adams <mfad...@lbl.gov> wrote: > > > > On Sun, Sep 18, 2022 at 4:02 PM Barry Smith <bsm...@petsc.dev> wrote: > >> >> Mark, >> >> Do all MIS algorithms in PETSc require a symmetric graph structure? >> And parallel ones can hang if not structurally symmetric? >> > > Yes, > > >> >> When used sequentially I guess it never hangs but it may not produce a >> "correct" MIS if the matrix structure is not symmetric? > > > It is fine in serial and it is not necessarily an MIS of the > symmetrized graph. > If there is a one way edge between two vertices and the order of the > greedy MIS process picks the root of the edge it is an MIS of the > symmetrized graph, otherwise both vertices could get selected. > > >> But like the MIS is fine for GAMG in this circumstance? >> > > It will be fine for GAMG. The MIS is just a heuristic. > > >> >> Barry >> >> >
