[petsc-users] Clarification on MatMPIBAIJSetPreallocation (d_nnz and o_nnz)

[Edoardo alinovi]

Hello Barry,

I am doing some preliminary work to add a coupled solver in my code.  I am
reading some documentation on MPIBAIJ as the matrix will be composed of
blocks of size 3x3 in 2D and 4x4 in 3D for each cell in the domain.

I can't quite understand how to set d_nnz and o_nnz. What is their size?
Should I provide a number of non-zero entries for each block or should I do
it line by line as in MATAIJ?

Would you be able to provide me with a silly example?

Thank you!
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