Jed Brown five9a2 at gmail.com
Thu Jun 21 18:09:24 CDT 2012

I think that is fine, though having mirrored ghost points in the
local-to-global map would be more convenient for matrix assembly.
On Jun 20, 2012 2:51 PM, "Barry Smith" <bsmith at mcs.anl.gov> wrote:

>  My inclination is to remove the mirror enum and simply provide a utility
> function that updates the ghost points by mirroring in the local vector
> form that the user can call if they want mirroring?
>   Barry
> On Jun 20, 2012, at 5:41 PM, Barry Smith wrote:
> >
> >     What is the intended difference between DMDA_BOUNDARY_GHOSTED and
> >
> >     My understanding is that both cause ghost points around the physical
> boundary that can be indexed in the usual way (for example on the left side
> just indexing with -1 to get the first of the ghost points? Looking at the
> code but not testing it I assume that the correctly sized local vectors are
> created and the appropriate scatters are created that put things in the
> correct place despite these extra locations with calls to DMGlobalToLocal()?
> >
> >     My guess is that whoever decided to have both of these was thinking
> that with DMDA_BOUNDARY_GHOSTED the user would fill up the ghost points
> themselves however they liked with whatever values they want whenever they
> want?  Meanwhile with mirror PETSc would fill up the ghost points by
> copying the mirrored values from inside the real physical domain?  Thus
> ghosted is more general.
> >
> >   But when was it intended that the mirrored values be copied over?
> During the DMGlobalToLocal call or in a new call?  (Note that this copying
> requires no parallel communication)
> >
> >    Is there an example of using DMDA_BOUNDARY_GHOSTED where the ghosted
> values are set via mirroring? Should there be a utility to do that?
> >
> >    We have someone who needs this mirroring (due to Jed's advice) but
> appear not to have any code to make that trivial. How shall we fix this?
> >
> >   Thanks
> >
> >   Barry
> >
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