[petsc-users] converting parallel matrix MATMPIJ to MATAIJ

[Lukasz Kaczmarczyk]

On 6 Sep 2013, at 23:00, Barry Smith <bsmith at mcs.anl.gov> wrote:

> 
>  I would use MatGetRedundantMatrix()
> 
> 
> On Sep 6, 2013, at 4:52 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
>> On Fri, Sep 6, 2013 at 4:25 PM, Lukasz Kaczmarczyk <Lukasz.Kaczmarczyk at glasgow.ac.uk> wrote:
>> Hello,
>> 
>> I solve system of eq. generated by finite element method.
>> 
>> I  apply some projection matrix to stiffness matrix K,
>> P=I-CT[(CTC)^-1]C
>> where C  is some not square matrix.
>> 
>> Resulting stiffness matrix K' has form
>> K' = PT K P,
>> with that at hand I solve problem K' *x = f'
>> 
>> I manage to build shell matrix where I use sub ksp solver to get solution for (CTC)*b = C*x, where [ b = (CTC^-1*C*x)] . Using penalised matrix for preconditioner, where  K_prec = alpha*CCT + K, where alpha is penalty I can get solution in efficient way.
>> 
>> Now I like to avoid penalty parameter, in order to do that I will need to apply penalty matrix for each individual finite element  matrix before it is assembled into K. No problem with that, using scattering it can be done.
>> 
>> Problem is with  solution (CTC)*b = C*x, C and CTC matrices are parallel, since I have parallelised assembly functions, problem (CTC)*b = C*x need to be solved on each processor independently without communication. It is not problem, but to do that I need to transform C and CTC matrix form MATMPIAIJ to MATAIJ.
>> 
>> I think this might be what you want:
>> 
>>  http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetSubMatrices.html
>> 
>>   Thanks,
>> 
>>      Matt
>> 
>> I know that MatConvert will not do it. I wonder it is any other way that form very beginning to assemble matrix C as a serial matrix.
>> 
>> Regards,
>> Lukasz
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> -- 
>> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
>> -- Norbert Wiener
> 

Thanks Matt and Barry,

Sorry for taking your time, I should find that by myself. 
How to use MatGetSubMatrices is clear. 

I have feeling that MatGetRedundantMatrix could be more efficient. However,  apologise my ignorance, I have problem with MatGetRedundantMatrix,

*) is this should be equal to number of process  in the communicator group, in my case 1 since I like to use PETSC_COMM_SELF?
nsubcomm	- the number of subcommunicators (= number of redundant parallel or sequential matrices) 
subcomm	- MPI communicator split from the communicator where mat resides in

*) it could be equal to total number of rows in MPIAIJ? What if this number is smaller, the first mlocal_red are stored in redundant matrix?
mlocal_red	- number of local rows of the redundant matrix


Regards,
Lukasz