[petsc-users] [EXTERNAL] Re: What exactly is the GlobalToNatural PetscSF of DMPlex/DM?

[Ferrand, Jesus A.]

Matt:

Thank you for the reply.
The bulk of it makes a lot of sense.
Yes! That need to keep track of the original mesh numbers (AKA "Natural") is what I find pressing for my research group.
Awesome! I was separately keeping track of these numbers using a PetscSection that I was inputting into DMSetLocalSection() but of the coordinate DM, not the plex.
It is good to know the "correct" way to do it.

"What is repetitive? It should be able to be automated."

Absolutely as the intrinsic process is ubiquitous between mesh formats.
What I meant by "repetitive" is the information that is reused by different API calls (namely, global stratum sizes, and local point numbers corresponding to owned DAG points).
I need to define a struct to bookkeep this. It's not really an issue, rather a minor annoyance (for me).
I need the stratum sizes to offset DMPlex numbering cells in range [0,nCell) and vertices ranging in [nCell,nCell+nVert) to other mesh numberings where cells range from [1, nCell] and vertices range from [1, nVert]. In my experience, this information is needed at least three (3) times, during coordinate writes, during element connectivity writes, and during DMLabel writes for BC's and other labelled data.

This information I determine using a code snippet like this:
PetscCall(PetscObjectGetComm((PetscObject)plex,&mpiComm));
 PetscCallMPI(MPI_Comm_rank(mpiComm,&mpiRank));
 PetscCallMPI(MPI_Comm_size(mpiComm,&mpiCommSize));
 PetscCall(DMPlexCreatePointNumbering(plex,&GlobalNumberIS));
 PetscCall(ISGetIndices(GlobalNumberIS,&IdxPtr));
 PetscCall(DMPlexGetDepth(plex,&Depth));
 PetscCall(PetscMalloc3(//
   Depth,&LocalIdxPtrPtr,//Indices in the local stratum to owned points.
   Depth,&pOwnedPtr,//Number of points in the local stratum that are owned.
   Depth,&GlobalStratumSizePtr//Global stratum size.
 ));
 for(PetscInt jj = 0;jj < Depth;jj++){
   PetscCall(DMPlexGetDepthStratum(plex,jj,&pStart,&pEnd));
   pOwnedPtr[jj] = 0;
   for(PetscInt ii = pStart;ii < pEnd;ii++){
     if(IdxPtr[ii] >= 0) pOwnedPtr[jj]++;
   }
   PetscCallMPI(MPI_Allreduce(&pOwnedPtr[jj],&GlobalStratumSizePtr[jj],1,MPIU_INT,MPI_MAX,mpiComm));
   PetscCall(PetscMalloc1(pOwnedPtr[jj],&LocalIdxPtrPtr[jj]));
   kk = 0;
   for(PetscInt ii = pStart;ii < pEnd; ii++){
     if(IdxPtr[ii] >= 0){
       LocalIdxPtrPtr[jj][kk] = ii;
       kk++;
     }
   }
 }
 PetscCall(ISRestoreIndices(GlobalNumberIS,&IdxPtr));
 PetscCall(ISDestroy(&GlobalNumberIS));
________________________________
From: Matthew Knepley <knepley at gmail.com>
Sent: Thursday, July 11, 2024 8:32 PM
To: Ferrand, Jesus A. <FERRANJ2 at my.erau.edu>
Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: [EXTERNAL] Re: [petsc-users] What exactly is the GlobalToNatural PetscSF of DMPlex/DM?

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On Mon, Jul 8, 2024 at 10:28 PM Ferrand, Jesus A. <FERRANJ2 at my.erau.edu<mailto:FERRANJ2 at my.erau.edu>> wrote:
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Dear PETSc team:

Greetings.
I keep working on mesh I/O utilities using DMPlex.
Specifically for the output stage, I need a solid grasp on the global numbers and ideally how to set them into the DMPlex during an input operation and carrying the global numbers through API calls to DMPlexDistribute() or DMPlexMigrate() and hopefully also through some of the mesh adaption APIs. I was wondering if the GlobalToNatural PetscSF manages these global numbers. The next most useful object is the PointSF, but to me, it seems to only help establish DAG point ownership, not DAG point global indices.

This is a good question, and gets at a design point of Plex. I don't believe global numbers are the "right" way to talk about mesh points, or
even a very useful way to do it, for several reasons. Plex is designed to run just fine without any global numbers. It can, of course, produce
them on command, as many people remain committed to their existence.

Thus, the first idea is that global numbers should not be stored, since they can always be created on command very cheaply. It is much more
costly to write global numbers to disk, or pull them through memory, than compute them.

The second idea is that we use a combination of local numbers, namely (rank, point num) pairs, and PetscSF objects to establish sharing relations for parallel meshes. Global numbering is a particular traversal of a mesh, running over the locally owned parts of each mesh in local order. Thus an SF + a local order = a global order, and the local order is provided by the point numbering.

The third idea is that a "natural" order is just the global order in which a mesh is first fed to Plex. When I redistribute and reorder for good performance, I keep track of a PetscSF that can map the mesh back to the original order in which it was provided. I see this as an unneeded expense, but many many people want output written in the original order (mostly because processing tools are so poor). This management is what we mean by GlobalToNatural.

Otherwise, I have been working with the IS obtained from DMPlexGetPointNumbering() and manually determining global stratum sizes, offsets, and numbers by looking at the signs of the involuted index list that comes with that IS. It's working for now (I can monolithically write meshes to CGNS in parallel), but it is resulting in repetitive code that I will need for another mesh format that I want to support.

What is repetitive? It should be able to be automated.

  Thanks,

    Matt


Sincerely:

J.A. Ferrand

Embry-Riddle Aeronautical University - Daytona Beach - FL
Ph.D. Candidate, Aerospace Engineering

M.Sc. Aerospace Engineering

B.Sc. Aerospace Engineering

B.Sc. Computational Mathematics


Phone: (386)-843-1829

Email(s): ferranj2 at my.erau.edu<mailto:ferranj2 at my.erau.edu>

    jesus.ferrand at gmail.com<mailto:jesus.ferrand at gmail.com>


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