[petsc-users] Assembling a matrix for a DMComposite vector

[Anush Krishnan]

Hi all,

I created a DMComposite using two DMDAs (representing the x and y
components of velocity in a 2-D staggered cartesian grid used for CFD
simulations). DMCompositeGetISLocalToGlobalMappings gives me the global
indices of the elements and ghost cells, which I can use to set up a matrix
that operates on the vector created with the DMComposite. I obtain the
correct global indices for all the interior points. But when I look at the
global indices of the ghost cells, the ones outside the domain from the
x-component DM return -1, but the ones outside the domain from the
y-component DM return a positive value (which seems to be the largest
global index of the interior points on Process 0). My question is: Why do
they not return -1? Wouldn't that make matrix assembly easier (since
MatSetValues ignores negative indices)? I have attached a code which
demonstrates the above.

On a related note: Is it possible to use MatSetValuesStencil for assembling
a block diagonal matrix that operates on a vector created using the above
DMComposite?

Thanks,
Anush
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