[petsc-users] Basic question about local submatrix from DMComposite

[Matthew Knepley]

On Mon, Sep 17, 2018 at 12:12 PM zakaryah <zakaryah at gmail.com> wrote:

> I'm still working on implementing a homotopy solver for systems of
> nonlinear equations.  It seems that everything is working in serial, but
> I'm having at least one issue in parallel.
>
> To form the (n+1) x (n+1) Jacobian, I use MatGetLocalSubMatrix, with the
> ISs from the DMComposite.  One of the submatrices is (globally) 1 x n.  I
> need to set one of its values, at global column j, to a particular value.
> My current attempt to do this involves getting the local to global
> mappings, with DMCompositeGetISLocalToGlobalMappings, then using
> ISGlobalToLocalMappingsApply with global index j.  If the processor has a
> locally mapped index, I call MatSetValuesLocal at the local index.
>
> So far, this works, in that it seems the value is set at the appropriate
> point.  My issue is that I must use ADD_VALUES, due to the nature of the
> algorithm for the other submatrices.  Therefore, I need a way to ensure
> that the correct value is set.  For example, if two processors map to the
> same global index, then I need a way to check this and use local values
> such that the sum is the correct global value.
>

This sounds wrong. I think you should fix the algorithm to use INSERT,
which is what you really want. You can usually partition it into
and ADD phase, then call Assembly with FLUSH, and then an INSERT phase.

  Thanks,

     Matt


> I suspect that this has a simple solution - hopefully someone has some
> hints.  Thanks!
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20180917/52aba584/attachment.html>