[petsc-users] Determining the correct Preallocation

[Matthew Knepley]

On Mon, Oct 22, 2012 at 6:18 PM, Andrew Spott <andrew.spott at gmail.com>wrote:

> I want to preallocate a MPIAIJ matrix correctly, using
>
> MatMPIAIJSetPreallocation(A, PETSC_NULL, dnnz, PETSC_NULL, onnz);
>
> The problem is that I don't know how many local rows, or where the local
> columns are until I run said function, but I need to know the number of
> local rows and local columns so I can determine the correct preallocation.
>
> So how am I supposed to do a proper preallocation?  Is there something I'm
> missing, or do I just guess and correct?  If I know that my matrix is
> evenly sparse, should I just assume that it is evenly split up among
> processors, and go from there?  Or should I "MatSetUp", determine the
> correct preallocation and then do a MatMPIAIJSetPreallocation?


I think you are looking for


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscSplitOwnership.html

If you do not specify the layout, this function is called to partition your
matrix.

  Thanks,

     Matt


> -Andrew

-- 
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
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