redistribution of vectors

[Matthew Knepley]

On Tue, Feb 10, 2009 at 2:11 PM, Stephan Kramer
<stephan.kramer at imperial.ac.uk> wrote:
> Hi,
>
> I'm trying to pick up a linear system (matrix and rhs) that I've written out
> in a previous parallel run using MatView and VecView in binary mode. Now
> when I've read the matrix and rhs vector the rows are evenly distributed by
> PETSc over the processes. The partioning previously used in the parallel run
> however doesn't in general correspond to this even distribution (the global
> node numbering is the same, but number of rows are not exactly equal per
> process due to other constraints). So what I'm trying to do is redistribute
> the read in matrix and vector to match the previously used partioning. I've
> found that for the matrix this is easy to do using MatGetSubMatrix(), but
> I'm a little stuck on how to do this for the vector. Is there a similar way
> of doing this for the vector? There seems to be no way of extracting vector
> values not stored on this processor.
>
> I've tried to look at src/ksp/ksp/examples/tutorials/example10.c where
> something similar is done for a repartioning created by MatPartioning. It
> does the redistribution of the matrix with MatGetSubMatrix() as I expected,
> but then seems to do nothing for the vector (there is only a comment:  /*
> need to move the vector also */ ).
>
> Does this mean I just have to extract the vector values locally and do some
> redistributing with MPI myself?

You can certainly use a VecScatter to redistribute a vector. You
already have the
indices from MatGetSubMatrix() so it should be easy.

However, you can also use VecLoadIntoVector() and provide an initial
partitioning.

  Matt

> Cheers
> Stephan
-- 
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