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

[Barry Smith]

  I have pushed this change to petsc-dev and it is ready for use.

   Barry

  Note it can still glitch if the restricted size is exactly the original size. :-(


On Dec 15, 2010, at 7:53 PM, Barry Smith wrote:

> 
>  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
>