[petsc-dev] Seeking Advice on Petsc Preconditioners to Try

[Dave Nystrom]

Mark F. Adams writes:
 > It sounds like you have a symmetric positive definite systems like du/dt -
 > div(alpha(x) grad)u.  The du/dt term makes the systems easier to solve.
 > I'm guessing your hard system does not have this mass term and so is
 > purely elliptic.  Multigrid is well suited for this type of problem, but
 > the vector nature requires some thought.  You could use PETSc AMG -pc_type
 > gamg but you need to tell it that you have a system of two dof/vertex.
 > You can do that with something like:
 > 
 > ierr = MatSetBlockSize( mat, 2 );      CHKERRQ(ierr);

Best I can tell, the matrix block size cannot be set using a command line
option.  It that correct?

Thanks,

Dave

 > For the best results from GAMG you need to give it null space information
 > but we can worry about that later.
 > 
 > Mark