Hello, Before jumping into block AMG, I started testing AMG on the pressure equation.
I find that for simple systems, both serial and parallel version of AMG worked well. For a difficult system (variable coefficient laplacian operator), serial worked but AMG struggled when using 4 cores. I was using AMG as preconditioner to BiCGSTAB. Regular ILU0 preconditioner worked well both in serial and parallel. I am not sure why AMG struggled. Are there any tuning parameters for AMG? Does Petsc have SOR/Gauss Seidel/ILU0 smoother options? Or can I provide my own user smoother? Are there options to choose between unsmoothed aggregation vs smoothed aggregation? thanks, Raju. On Tue, Sep 22, 2020 at 10:23 PM Barry Smith <bsm...@petsc.dev> wrote: > > Raju, > > If you call MatSetBlockSize(mat,4) then GAMG will keep the 4 variables > together through all the levels. > > You might consider using the point block Jacobi smoother with it > instead of the default Jacobi smoother -mg_levels_pc_type pbjacobi > > There is also PCPATCH which supports Vanka smoothing (though it is to > well hidden) PCPatchSetConstructType(pc,PC_PATCH_VANKA) > -pc_patch_construct_type vanka > > > For coupled Navier stokes you might also consider using PCFIELDSPLIT > and GAMG just for the pressure solve inside it. > > Barry > > > On Sep 22, 2020, at 9:24 PM, Raju Mandhapati via petsc-users < > petsc-users@mcs.anl.gov> wrote: > > > > Hello, > > > > Does Petsc have an option to solve block matrices using AMG > solver/preconditioner. My block matrix is coming from solving u,v,w,p > (coupled Navier stokes) at each cell. > > > > thanks > > Raju. > >
