[petsc-users] PCSORSetSymmetric

[Barry Smith]

On Sep 16, 2011, at 10:58 AM, John Fettig wrote:

> I don't really understand what is the purpose of the "local" options
> in PCSOR. In serial for AIJ for example, SOR_FORWARD_SWEEP and
> SOR_LOCAL_FORWARD_SWEEP appear to follow the same path in the code.

  Yes, the local means each process does its own independent smoothing, when there is only one process, local and nonlocal are identical
> 
> In parallel for MPIAIJ, SOR_FORWARD_SWEEP isn't even handled by
> MATSOR_MPIAIJ().  It actually results in a segfault if you try it
> (this looks like a bug),

   Yes, this is a bug, thanks for reporting it, it should have printed a nice error message about lack of support.

> for example in ksp/ksp/examples/tutorials:
> "mpirun -np 2 ./ex2 -pc_type sor -pc_sor_forward"  So in parallel, the
> only valid options are the "local" ones, which makes sense since you
> haven't provided a true parallel SOR.
> 
> What is the purpose of the "local" options then, if there really isn't
> a "global" SOR?

   There could be a global SOR (for example using a coloring) we just haven't implemented them but maybe someday someone who is interested will.

   Barry

> 
> Thanks,
> John