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

[Matthew Knepley]

On Fri, Dec 2, 2011 at 9:25 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 *Matthew Knepley
> *Sent:* Friday, December 02, 2011 4:01 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 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?****
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> GHOSTED adds extra space at the boundary so you can always use the same
> stencil, but you decide what goes in there. ****
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> Does this apply to both matrices and vectors, ie. Will the ghost points be
> considered part of my computational domain or not?
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The ghost nodes only exist in local vectors, not the global vectors for the
solver.

  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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