[petsc-dev] PETSc MG Solvers vs agmg

[Barry Smith]

On May 1, 2012, at 8:34 AM, Dave Nystrom wrote:

> I have a 2d resistive mhd code that has an interface to both agmg and various
> multigrid solvers available from PETSc including gamg, hypre and ml.  I'm not
> that familiar with multigrid and so it is difficult for me to know how to
> experiment with the various mg packages and tune them to my particular
> problems.  In my code, agmg is a stand alone solver.  I have a couple of
> questions.
> 
> 1.  Is it possible to use the PETSc mg solvers effectively without an
> iterative method by choosing preonly as the iterative method - like I do for
> the PETSc direct solves?

   Use -ksp_type richardson     Note that using -ksp_type preonly would result in only 1 iteration of the multigrid method and hence not what you want. 

   Note that algebraic multigrid methods are generally designed to be used with a Krylov accelerator and convergence may suffer (i.e. it will take longer or not even converge) without a Krylov accelerator. Though, of course, experimenting to find the fastest is a good idea.

> 
> 2.  Is anyone on this list sufficiently familiar with agmg and the other
> PETSc mg solvers to know how to configure the PETSc mg solvers to work more
> like agmg?  It seems that agmg gives better performance than the PETSc mg
> solvers but I also have issues with agmg including fragility.

   Please send a link for information on agmg; I've never heard of that.

   Barry

> 
> Thanks,
> 
> Dave