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

[Barry Smith]

On Dec 15, 2010, at 8:16 PM, Vijay S. Mahadevan wrote:

> 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 ?

  Well if they happen to be equal then it will never apply the transpose thus giving a bad algorithm and garbage.

  Barry

> 
> 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
>>> 
>> 
>>