[petsc-users] MatSetValues dropping non-local entries

[Matthew Knepley]

On Wed, Aug 24, 2016 at 4:45 PM, Randall Mackie <rlmackie862 at gmail.com>
wrote:

> Well, I only need this particular matrix to multiply a vector (ordering
> based on the DMDA grid), so I don’t need to do any ghost communication
> (like residual calculations). I just need to be able to set a few non-local
> entries. Is there no way to do that without increasing the stencil width of
> the DMDA?
>

You could fall back to MatSetValuesStencil() instead, which does not use
the mapping.

   Matt


> Randy
>
>
> > On Aug 24, 2016, at 2:39 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >
> >> On Aug 24, 2016, at 4:27 PM, Randall Mackie <rlmackie862 at gmail.com>
> wrote:
> >>
> >> I’ve run into a situation where MatSetValues seems to be dropping
> non-local entries. Most of the entries that are set are local, but a few
> are possibly non-local, and are only maximum a few grid points off the
> local part of the grid.
> >>
> >> Specifically, I get the local to global mapping, and the indices like
> so:
> >>
> >> call DMGetLocalToGlobalMapping(da,ltogm,ierr)
> >> call ISLocalToGlobalMappingGetIndices(ltogm,ltog,idltog,ierr)
> >>
> >> then set the row using ltog(idltog + row +1) etc
> >>
> >> When run on 1 process, everything worked fine, but for > 1 I was not
> getting the right result (I know what the right answer should be for a
> simple case).
> >>
> >> I found that when I increased the stencil width on the DA (in the call
> to DACreate3d) that was used in the DMGetLocalToGlobalMapping to be large
> enough that the non-local points would be in the ghost region, everything
> was fine even for > 1 process.
> >>
> >>
> >> So, in conclusion, it seems like if I use a local to global mapping
> from DMGetLocalToGlobalMapping, then MatSetValues will drop any non-local
> entries that are not included in the ghost region.
> >>
> >>
> >> Is this the correct behavior and if so, is there another way to set
> these non-local values so they don’t get dropped?
> >
> >  Yes, this is the expected behavior. Note also that DMDA only allocates
> space in the matrix for these locations and if it did stick in your "extra"
> locations it would be very very slow because it would have to reallocate
> the matrix data structures.
> >
> >  Why would you want to put in matrix entries that are not represented in
> the ghosting? The whole point of the ghosting is to indicate what values
> need to be communicated so you putting additional values in that do not fit
> the ghosting does not match the paradigm.
> >
> >  Barry
> >
> >>
> >>
> >> Thanks,
> >>
> >> Randy
> >
>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20160824/f78b291f/attachment.html>