[petsc-users] Distribution of columns on mutliple ranks

[Florian Lindner]

Hello,

given I have this piece of python code:

rank = MPI.COMM_WORLD.Get_rank()
sizes = [ 4, 5]
n = sizes[rank]

A = PETSc.Mat()
A.create()
A.setSizes( ((n, PETSc.DETERMINE), (n, PETSc.DETERMINE)) ) # sets the local size, let petsc determine global size
A.setFromOptions(); A.setUp()

print("Global Size = ", A.getSize())
print("Local  Size = ", A.getLocalSize())


and I run it with mpirun -n 2 and -mat_type dense (this, I think, should not be relevant), getSize() returns 9x9 for both ranks of couse.

getLocalSize returns 4x4 and 5x5.

I understand that the A is distributed like that:

  /4x4  ?  \
  \ ?  5x5 /


When I would give ( (PETSC_DETERMINE, 9), (PETSC_DETERMINE, 9) ) to setSizes it would look like that:

 / 4 x 9 \
 \ 5 x 9 /

because PETSc uses row based partitioning.

What happens when I set local rows and local cols, where do the parts that I marked with ? live?

Thanks, ;-)

Florian