On Jun 21, 2012, at 6:09 PM, Jed Brown wrote: > I think that is fine, though having mirrored ghost points in the > local-to-global map would be more convenient for matrix assembly.
Hmm, what do you mean by this? What should we do? Barry > > 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 > > 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 > > >
