[petsc-users] Use of MatRestrict/MatInterpolate with PCMG.

[Barry Smith]

  Vijay,

    The use of M>N in MatRestrict and MatInterpolate was always a bit cheesy since it has this broken case that you reported. I will change it to do as you suggest and use the size of the vectors in determining which way to apply. But note I will do this in petsc-dev http://www.mcs.anl.gov/petsc/petsc-as/developers/index.html not petsc-3.1 so you'll need to switch if you are not using petsc-dev.

   I'll try to get it down in the next few hours but it may take a little longer.


   Barry

On Dec 15, 2010, at 6:06 PM, Vijay S. Mahadevan wrote:

> Hi,
> 
> I have an implementation issue with the MatRestrict/Interpolate
> functions. The problem is that one of my coarser levels (with PCMG)
> has higher dofs than the finest level. This does not always happen and
> requires a weird fine mesh system (in a sense) that uses multi-grid,
> but the idea is that the finest level problem has a high order (HO)
> discretization while the lower level mesh has a linear tesselation of
> the finest HO level (which I can optimize) and then adaptively
> coarsened levels beyond that. Since the number of columns in this case
> is larger than the number of rows, MatRestrict invariably calls
> MatMultTranspose to multiply instead of MatMult and vice-versa while
> calling  MatInterpolate. These result in assertion errors while
> comparing the length of Mat and Vec. The chosen method is based on
> whether (M>N) which seems to act against what I am doing here...
> 
> I can always implement a shell matrix to replicate
> Restrict/Interpolate actions but my question is whether if such
> discretization will yield a consistent convergence in MG algorithm ?
> Is there a strong reason for checking if (M>N) rather than just doing
> (mat->rmap->N==y->map->N && mat->cmap->N==x->map->N) ? I would
> appreciate any detailed answer that you can provide for this and any
> suggestions to use the existing methods (without implementing the
> shell restriction) is very welcome.
> 
> Thanks,
> vijay