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

[Vijay S. Mahadevan]

Barry,

Thanks for the prompt change ! I do not work on the development
version but I can update these matrix routines alone.

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

Why would it glitch if the restricted size is the same as the original
size though ? I dont see a case where your check (M==Ny) would fail.
Can you please elaborate more on this ?

Vijay

On Wed, Dec 15, 2010 at 8:04 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>  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
>>
>
>