[petsc-users] Ordering of preallocation and OwnershipRange

[Josh Hykes]

Hello,

I'm just starting to experiment with PETSc (v3.1), and I like the Python
bindings provided by petsc4py (v1.1.2). So far things seem fairly
straightforward, but I'm stumped on a small issue.

While creating a parallel AIJ matrix, I'd like to preallocate it using
arrays d_nnz and o_nnz. As I understand it, these arrays correspond to the
processor's local rows.

Currently I specify the global matrix size, and let PETSc decide on the
decomposition of the rows. I'd like to ask PETSc what rows each processor
has with the getOwnershipRange() function, and then do the preallocation.
However, according to the error message

> [1] MatAnyAIJSetPreallocation() line 393 in
petsc4py-1.1.2/src/include/custom.h
> [1] Operation done in wrong order
> [1] matrix is already preallocated

I'm not allowed to do it in this order.

Thus, my question is: is it possible to let PETSc figure out the row
decomposition while still using d_nnz and o_nnz for the preallocation? I
figure that I could resolve the problem by doing my own decomposition, but
it'd be nice if I could let those details up to PETSc.

I'm including an example using petsc4py of what I'd like to do, run with 2
MPI processes.

I apologize if this is a dumb question. Thank you for your help.

-Josh

# -----------------------------------------------
from petsc4py import PETSc as petsc

M, N = 4, 6

global_d_nnz = [2, 1, 1, 2]
global_o_nnz = [1, 3, 2, 1]

 A = petsc.Mat()
A.create(petsc.COMM_WORLD)
A.setSizes([M, N])
A.setType('aij')

i_start, i_end = A.getOwnershipRange()

A.setPreallocationNNZ([global_d_nnz[i_start:i_end],
                       global_o_nnz[i_start:i_end]]) # error occurs here
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