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

[Edoardo alinovi]

Thank you Jed for the hint.

So just to understand it with an example. Say I have this matrix here,
which has 4 3x3 blocks

1 2 0 | 0 5 0 |
0 2 3 | 0 0 1 |       <---- Proc 1
0 0 1 | 0 2 2 |
--------|--------|
1 2 0 | 0 5 0 |
0 2 0 | 0 0 1 |       <---- Proc 2
0 0 1 | 0 0 2 |
-------|---------|

This can be represented as a collection of submatrices like:
A  B
C  D

A and D are the diagonal blocks, while B and C are the off-diagonal ones.
How should I set d_nnz and o_nnz in this case?
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