[petsc-users] DMPlex: Mapping cells before and after partitioning

[Bisht, Gautam]

> On Jan 9, 2020, at 2:38 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "'Bisht, Gautam' via tdycores-dev" <tdycores-dev at googlegroups.com> writes:
> 
>> Hi Matt,
>> 
>> Here is the situation that I’m running into and hoping you could provide some guidance. I create a mesh using DMPlexCreateBoxMesh() in which cells are ordered such that cells with increasing x coordinates are first, followed by cells with increasing y-corodinate and so forth. Next, I call DMPlexDistribute() that rearranges the cells after partitioning. How can I map the cell order after partitioning to cell order before partitioning? (In PFLOTRAN, we call this type of mapping as Ghosted—to—Application or Ghosted—to—Natural).
> 
> Do you need to rely on the element number, or would coordinates (of a
> centroid?) be sufficient for your purposes?  

I do need to rely on the element number.  In my case, I have a mapping file that remaps data from one grid onto another grid. Though I’m currently creating a hexahedron mesh, in the future I would be reading in an unstructured grid from a file for which I cannot rely on coordinates.


> I think DMLabel can be used
> to track that numbering, but I also think it's brittle and probably best
> avoided.

I will look into DMLabel.

> 
> Note that you should be able to get columns to appear on the same
> process after partitioning by setting large weights for the vertical
> coupling.  Alternatively, we could partition a 2D mesh and extrude it
> (see DMPlexExtrude).
> 

Thanks,
-Gautam

PS: I also asked the same question on petsc-users mailing list, so I’m including it in this email thread.


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