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

[Barry Smith]

  If you explicitly provide both then you are ok. If you provide one and not the other it will fail silently with a bad algorithm.


  Barry

On Dec 15, 2010, at 9:12 PM, Vijay S. Mahadevan wrote:

>>  Well if they happen to be equal then it will never apply the transpose thus giving a bad algorithm and garbage.
> 
> hmm, in the case when I provide a matrix for both
> Interpolate/Restrict, I have (M==N==Nx==Ny),  this would always call
> the MatMult routine. As long as I provide both these
> operators/matrices explicitly, there is still no problem. A possible
> issue is only when someone provides just the restriction or
> prolongation. But I understand that when this happens, the other
> operator is computed explicitly as its transpose. If this is actual
> implementation, I still dont see a problem. Although, if the
> restriction/prolongation operator are implicitly assumed to be
> transpose of the other, then it will quite horribly fail since only
> MatMult is called for both. I am not completely sure about the mode
> currently used in petsc but it would be great if you can help me
> understand.
> 
> Note: I probe more on this since my linear tesselation (most often)
> results in the same number of dofs on the coarser level
> (p-coarsened/h-refined) and I dont want a glitch to come back and bite
> me later on..
> 
> Vijay
> 
> On Wed, Dec 15, 2010 at 8:36 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> 
>> 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
>>>>> 
>>>> 
>>>> 
>> 
>>