[petsc-users] V-cycle multigrid with matrix shells

[Sylvain Barbot]

Hi Barry,

Thanks for your comments.

>>       CALL MatShellSetOperation(c%sA(l+1),MATOP_MULT,matrixsmoothfv,ierr);
>
>    You say this is a smoother but you register it as a MATOP_MULT? What is the calling sequence of matrixsmoothfv() and what does it do? Smooth or multiply? MATOP_MULT is for registering a multiple MATOP_SOR (or MATOP_RELAX in older versions of PETSc) is for providing a smoother.

My function matrixsmoothfv indeed performs the smoothing operation so
it not a multiplication. I can see no documentation about MATOP_SOR,
is there an example somewhere online? Assigning MATOP_SOR to my
smoothing matrix now generates the following runtime error message:

[0]PETSC ERROR: MatShellSetOperation_Fortran() line 107 in
src/mat/impls/shell/ftn-custom/zshellf.c
application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0
rank 0 in job 37  catalina.gps.caltech.edu_50093   caused collective
abort of all ranks
  exit status of rank 0: return code 1

>> I also provide work space...
>> Despite that, I have a crash upon running KSPSetup() with "[0]PETSC
>> ERROR: PCSetUp_BJacobi()".
>
> It is trying to use bjacobi because that is the default solver, you need to tell it to use some other solver explicitly if you do not want it to use block Jacobi. So if you registered your matrixsmoothfv() as a MATOP_SOR then you can use -mg_levels_pc_type sor to have it use that smoother.  What solver options are you running with?

I setup the solver with:

CALL PCMGSetType(c%pc,PC_MG_MULTIPLICATIVE,ierr);

but this error might not be representative of the problem. upstream,
my smoothing matrices were not set up properly.

>> MatGetVecs() line 7265 in src/mat/interface/matrix.c
>> [0]PETSC ERROR: KSPGetVecs() line 806 in src/ksp/ksp/interface/iterativ.c
>> [0]PETSC ERROR: KSPSetUp_GMRES() line 94 in src/ksp/ksp/impls/gmres/gmres.c
>> [0]PETSC ERROR: KSPSetUp() line
>>
>     This is not an indication that it is trying to use a direct solver, this is an indication then you never provided ANY kind of matrix at this level. You need to provide a matrix operator on each level for it to get work vectors from etc. It would be help to have the COMPLETE error message here so I can see exactly what is happening instead of a fragment making me guess what the problem is.