[petsc-users] newbie question on the parallel allocation of matrices

[Matthew Knepley]

On Fri, Dec 2, 2011 at 8:58 AM, Treue, Frederik <frtr at risoe.dtu.dk> wrote:

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> *From:* petsc-users-bounces at mcs.anl.gov [mailto:
> petsc-users-bounces at mcs.anl.gov] *On Behalf Of *Jed Brown
> *Sent:* Friday, December 02, 2011 1:32 PM
> *To:* PETSc users list
> *Subject:* Re: [petsc-users] newbie question on the parallel allocation
> of matrices****
>
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> On Fri, Dec 2, 2011 at 03:32, Treue, Frederik <frtr at risoe.dtu.dk> wrote:**
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>
> OK, but that example seems to assume that you wish to connect only one
> matrix (the Jacobian) to a DA – I wish to specify many and I think I found
> this done in ksp ex39, is that example doing anything deprecated or will
> that work for me, e.g. with the various basic mat routines (matmult,
> matAXPY etc.) in a multiprocessor setup?****
>
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> What do you mean by wanting many matrices? How do you want to use them? There
> is DMCreateMatrix() (misnamed DMGetMatrix() in petsc-3.2), which you can
> use as many times as you want.`****
>
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> And this was the one I needed. However I have another question: What does
> DMDA_BOUNDARY_GHOSTED do, compared to DMDA_BOUNDARY_PERIODIC? From
> experience I now know that the PERIODIC option automagically does the right
> thing when I’m defining matrices so I can simply specify the same stencil
> at all points. Does DMDA_BOUNDARY_GHOSTED do something similar? And if so,
> how is it controlled, ie. How do I specify if I’ve got Neumann or Dirichlet
> conditions, and what order extrapolation you want, and so forth? And if
> not, does it then ONLY make a difference if I’m working with more than on
> processor, ie. If everything is sequential, is DMDA_BOUNDARY_GHOSTED and
> DMDA_BOUNDARY_NONE equivalent?****
>

GHOSTED adds extra space at the boundary so you can always use the same
stencil, but you decide what goes in there. PERIODIC
fills that extra space with the values from the connected side.

  Matt

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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