GAMG is almost algorithmically invariant but the graph coarsening is not invariant not deterministic. You should not see much difference in teration could but a little decay is expected.
On Wed, Apr 17, 2019 at 12:36 PM Matthew Knepley via petsc-users < petsc-users@mcs.anl.gov> wrote: > On Wed, Apr 17, 2019 at 11:59 AM Marian Greg via petsc-users < > petsc-users@mcs.anl.gov> wrote: > >> Thanks Satish for the reply. However, I also observed the same behavior >> with gamg and sor preconditioners and ksp_type bcgs as well as gmres. Could >> you tell which solver and preconditioners would behave same on whatever >> number of mpi I use? > > > 1) SOR is parallel will also be Block Jacobi-SOR > > 2) Jacobi will be invariant > > 3) Chebyshev will be invariant > > 4) GAMG will be invariant if you have an elliptic equation. So for > instance you can use GAMG on SNES ex5 or ex12 and the iterates will not > increase > > Thanks, > > Matt > > >> Thanks, Mari >> >> >> On Wednesday, April 17, 2019, Balay, Satish <ba...@mcs.anl.gov> wrote: >> >>> Yes - the default preconditioner is block-jacobi - with one block on >>> each processor. >>> >>> So when run on 1 proc vs 8 proc - the preconditioner is different >>> (with 1block for bjacobi vs 8blocks for bjacobi)- hence difference in >>> convergence. >>> >>> Satish >>> >>> On Wed, 17 Apr 2019, Marian Greg via petsc-users wrote: >>> >>> > Hi All, >>> > >>> > I am facing strange behavior of the ksp solvers with increasing number >>> of >>> > MPI. The solver is taking more and more iterations with increase in >>> number >>> > of MPIs. Is that a normal situation? I was expecting to get the same >>> number >>> > of iteration with whatever number of MPIs I use. >>> > >>> > E.g. >>> > My matrix has about 2 million dofs >>> > Solving with np 1 takes about 3500 iteration while solving with np 4 >>> takes >>> > 6500 iterations for the same convergence criteria. >>> > >>> > Thanks >>> > Mari >>> > >>> >>> > > -- > 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 > > https://www.cse.buffalo.edu/~knepley/ > <http://www.cse.buffalo.edu/~knepley/> >