[petsc-dev] DMDA_*PERIODIC and DMDA_XYZGHOSTED

[Ethan Coon]

On Tue, 2010-12-07 at 13:45 -0600, Barry Smith wrote:
> DMDA_XYZGHOSTED does not exist for 2d and 3d it was added, I'm guessing, as an experiment and was never in the initial design of DMDA. To fully support it one needs to go back tot he design of DMDA and see how to have it properly done and not just bolt it on.  Some people like to use these types of ghost nodes so I agree it is a useful thing to have but who is going to properly add it?
> 

At some point in the not-too-distant future I'll get frustrated enough
to look into this, but I don't have the time at the moment.  At first
glance it looks like:

- Ensure DMDA{X,Y,Z}Periodic() macros are used everywhere instead of
direct comparisons to dd->wrap (they aren't used everywhere currently).

- Define macros DMDA{X,Y,Z}Ghosted() to (in some places) replace
DMDA{X,Y,Z}Periodic() and then choosing the appropriate macro in the
right places.

- This probably doesn't merit a change in the DMDACreate* API (it would
affect a very large amount of user code).  The most obvious alternative
to an API change would be a larger, somewhat convoluted enum for the
PeriodicType (DMDA_XPERIODIC_YGHOSTED, DMDA_XYGHOSTED, etc) which could
at least be made backward compatible.
 
At least all of the functionality should be there already (since it's
needed in the periodic case)... it's just higher level code that would
need to change.

Ethan

> 
> On Dec 7, 2010, at 1:30 PM, Jed Brown wrote:
> 
> > On Tue, Dec 7, 2010 at 20:21, Ethan Coon <ecoon at lanl.gov> wrote:
> > 'd like a DA where there are ghost cells on every boundary, and some of
> > those ghost cells (but not all) are filled in with periodic values.
> > 
> > It would be useful to people doing explicit stuff if there was a way to get ghost nodes in the local vector without implying periodic communication (and weird coordinate management).
> > 
> > A related issue for purely explicit is to have a way to VecAXPY without needing to copy to and from a global vector.  (TSSSP has low-memory schemes, paying for an extra vector or two is actually significant in that context, and (less significant) I'm certain I can cook up a realistic benchmark where the memcpy costs more than 10%.)  I think I know how to implement this sharing transparently (more-or-less using VecNest) so we could make it non-default but be able to activate it at runtime.
> 
>    Why can you not use VecAXPY() on the local Vecs?
> 
>    Barry
> 
> 
> 
> > 
> > Jed
> 

-- 
-------------------------------------
Ethan Coon
Post-Doctoral Researcher
Mathematical Modeling and Analysis
Los Alamos National Laboratory
505-665-8289

http://www.ldeo.columbia.edu/~ecoon/
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