[petsc-users] Understanding preallocation for MPI

[Florian Lindner]

Hello,

I'm having some struggle understanding the preallocation for MPIAIJ matrices, especially when a value is in off-diagonal
vs. diagonal block.

The small example program is at https://pastebin.com/67dXnGm3

In general it should be parallel, but right now I just run it in serial.

According to my understanding of

http://www.mcs.anl.gov/petsc/petsc-3.7/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html

a entry is in the diagonal submatrix, if its row is in the OwnershipRange and its column is in OwnershipRangeColumn.
That also means that in a serial run, there is only a diagonal submatrix.

However, having MAT_NEW_NONZERO_ALLOCATION_ERR set, I get an error when

Inserting 6 elements in row 2, though I have exactly

2 o_nnz = 0, d_nnz = 6 (means 6 elements allocated in the diagonal submatrix of row 2)

Error is:

[0]PETSC ERROR: Argument out of range
[0]PETSC ERROR: New nonzero at (2,5) caused a malloc
Use MatSetOption(A, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE) to turn off this check


What is wrong with my understanding?

Thanks,
Florian