[petsc-users] Create a nest not aligned by processors

[Barry Smith]

   Perhaps if you provide a brief summary of what you would like to do and we may have ideas on how to achieve it. 

   Barry

Note: that MATNEST does require that all matrices live on all the MPI processes within the original communicator. That is if the original communicator has ranks 0,1, and 2 you cannot have a matrix inside MATNEST that only lives on ranks 1,2 but you could have it have 0 rows on rank zero so effectively it lives only on rank 1 and 2 (though its communicator is all three ranks).

> On Mar 17, 2023, at 12:14 PM, Berger Clement <clement.berger at ens-lyon.fr> wrote:
> 
> It would be possible in the case I showed you but in mine that would actually be quite complicated, isn't there any other workaround ? I precise that I am not entitled to utilizing the MATNEST format, it's just that I think the other ones wouldn't work.
> 
> ---
> Clément BERGER
> ENS de Lyon
> 
> 
> Le 2023-03-17 15:48, Barry Smith a écrit :
> 
>>  
>>    You may be able to mimic what you want by not using PETSC_DECIDE but instead computing up front how many rows of each matrix you want stored on each MPI process. You can use 0 for on certain MPI processes for certain matrices if you don't want any rows of that particular matrix stored on that particular MPI process.
>>  
>>   Barry
>> 
>> 
>>> On Mar 17, 2023, at 10:10 AM, Berger Clement <clement.berger at ens-lyon.fr> wrote:
>>> Dear all,
>>> 
>>> I want to construct a matrix by blocs, each block having different sizes and partially stored by multiple processors. If I am not mistaken, the right way to do so is by using the MATNEST type. However, the following code
>>> 
>>> Call MatCreateConstantDiagonal(PETSC_COMM_WORLD,PETSC_DECIDE,PETSC_DECIDE,4,4,2.0E0_wp,A,ierr)
>>> Call MatCreateConstantDiagonal(PETSC_COMM_WORLD,PETSC_DECIDE,PETSC_DECIDE,4,4,1.0E0_wp,B,ierr)
>>> Call MatCreateNest(PETSC_COMM_WORLD,2,PETSC_NULL_INTEGER,2,PETSC_NULL_INTEGER,(/A,PETSC_NULL_MAT,PETSC_NULL_MAT,B/),C,ierr)
>>> 
>>> does not generate the same matrix depending on the number of processors. It seems that it starts by everything owned by the first proc for A and B, then goes on to the second proc and so on (I hope I am being clear).
>>> 
>>> Is it possible to change that ?
>>> 
>>> Note that I am coding in fortran if that has ay consequence.
>>> 
>>> Thank you,
>>> 
>>> Sincerely,
>>> 
>>> -- 
>>> Clément BERGER
>>> ENS de Lyon

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