* the two level results tell us that MG is not doing well on the coarse grids. So the coarse grids are the problem.
* Do not worry about timing now. Get the math correct. The two level solve is not meant to be a solution just a diagnostic so don't try to optimize it by squaring the graph. Use -pc_gamg_square_graph 0. * It looks like you don't need 4 smoothing steps but lets keep it and we can dial it back later. * This table is interesting. First, you had about 12 iterations earlier and I think your rtol was tighter than the default (so the iteration could should go down not up). Do you know what change here? Note, even though -mg_levels_ksp_max_it is not in the ksp_view it does work. It is syntactic sugar to just add it to all levels like you did manually. Anyway, these number look reasonable. It is interesting that 3 levels ran well but the 4th level ran poorly. This implies we want to slow down coarsening on these levels, but ... First can you please rerun this experiment with -pc_gamg_square_graph 0. Also, please run with -info. This is very noisy but you can grep on "GAMG" and send that output to us (about 15 lines). Thanks, Mark On Mon, Oct 29, 2018 at 3:34 PM Manav Bhatia <bhatiama...@gmail.com> wrote: > Barry, > > Here are some quick numbers with the following options on 4 CPUs and > 543,606 dofs: > > -mg_levels_ksp_max_it 4 -pc_gamg_square_graph 1 -pc_gamg_threshold 0. > > #levels | #KSP Iters > ——————————— > 2 | 18 > 3 | 18 > 4 | 40 > 5 | 59 > > -Manav > > > On Oct 29, 2018, at 2:06 PM, Smith, Barry F. <bsm...@mcs.anl.gov> wrote: > > > Exactly how much does it increase with number of levels? Send a chart > number of levels and number of iterations. With say -mg_levels_ksp_maxit 4 > > Thanks > > Barry > > > > > On Oct 29, 2018, at 12:59 PM, Manav Bhatia <bhatiama...@gmail.com> wrote: > > Thanks for the clarification. > > I also observed that the number of KSP iterations increases with an > increase in the levels of AMG. Is this true, in general, for all/most > applications? > > -Manav > > On Oct 29, 2018, at 12:53 PM, Jed Brown <j...@jedbrown.org> wrote: > > Manav Bhatia <bhatiama...@gmail.com> writes: > > Thanks, Jed. > > The description says: “ Square the graph, ie. compute A'*A before > aggregating it" > > What is A here? > > > The original matrix, or its "graph" (your 6x6 blocks condensed to scalars). > > What is the impact of setting this to 0, which led to a very significant > increase in the CPU time in my case? > > > The aggregates are formed on the connectivity of your original matrix, > so root nodes are aggregated only with their first neighbors, resulting > in slower coarsening. > > > > >