What is the intended difference between DMDA_BOUNDARY_GHOSTED and
DMDA_BOUNDARY_MIRROR?
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
