[petsc-users] MatMPIAIJSetPreallocation: "nnz cannot be greater than row length"

[David Knezevic]

Hi all,

I've implemented a solver for a contact problem using SNES. The sparsity
pattern of the jacobian matrix needs to change at each nonlinear iteration
(because the elements which are in contact can change), so I tried to deal
with this by calling MatSeqAIJSetPreallocation
and MatMPIAIJSetPreallocation during each iteration in order to update the
preallocation.

This seems to work fine in serial, but with two or more MPI processes I run
into the error "nnz cannot be greater than row length", e.g.:
nnz cannot be greater than row length: local row 528 value 12 rowlength 0

This error is from the call to
MatSeqAIJSetPreallocation(b->B,o_nz,o_nnz);
in MatMPIAIJSetPreallocation_MPIAIJ.

Any guidance on what the problem might be would be most appreciated. For
example, I was wondering if there is a problem with calling
SetPreallocation on a matrix that has already been preallocated?

Some notes:
- I'm using PETSc via libMesh
- The code that triggers this issue is available as a PR on the libMesh
github repo, in case anyone is interested:
https://github.com/libMesh/libmesh/pull/460/
- I can try to make a minimal pure-PETSc example that reproduces this
error, if that would be helpful.

Many thanks,
David
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