[petsc-users] Question on assembling matrix in parallel

[Smith, Barry F.]

> On Mar 1, 2018, at 10:24 AM, Edoardo alinovi <edoardo.alinovi at gmail.com> wrote:
> 
> Dear All,
> 
> thanks to your suggestions and Petsc, I have finished to program my finite volume code for CFD that runs in parallel and this is great.
> 
> I am now tryig to improve its performace and I have a question on matrix in MPIAij format.
> 
> Basically after the discetization, I have one matrix for each velocity component and they are equal except for the diagonal elements which changes because of the boundary conditions. What I am doing now is:
> 
> - fill each matrix using matSetValues row by row 
> - finalize each matrix assembly with MatAssemblyBegin and MatAssemblyEnd
> 
> I am wondering if would be possible  to build just one matrix doing MatAssemblyBegin/MatAssemblyEnd  and for the  other two change the diagonal values locally, thus avoiding the parallel communications.

   You can use MatDiagonalSet() to add to or set the diagonal values of the matrix.

> 
> Also can I put a matrix object in a module and make it a global variable?

   You can.

> Can you provide/link me an example? I am using Fortran.

   There may be examples in the master branch of PETSc. It is straightforward. Just do it and if it doesn't work send us email with full bug report.


> 
> Thank you very much for your help,
> 
> Edoardo
> 
>