Thanks for the reply. > "What I want to do is to exchange data (probably just MPI_Reduce)" which confuses me, because halo exchange is a point-to-point exchange and not a reduction. Can you clarify? PetscSFReduceBegin/End seems to be the function that do reduction for PetscSF object. What I intended to mention was either reduction or exchange, not specifically intended "reduction".
As a follow-up question: Assuming that the code has its own local solution arrays (not Petsc type), and if the plex's DAG indices belong to the halo region are the only information that I want to know (not the detailed section description, such as degree of freedom on vertex, cells, etc.). I have another PetscSection for printing out my solution. Also if I can convert that DAG indices into my local cell/vertex index, can I just use the PetscSF object created from DMGetPointSF(), instead of "creating PetscSection + DMGetSectionSF()"? In other words, can I use the PetscSF object declared from DMGetPointSF() for the halo communication? Thanks, Mike The PetscSF that is created automatically is the "point sf" ( > https://petsc.org/main/docs/manualpages/DM/DMGetPointSF/): it says which > mesh points (cells, faces, edges and vertices) are duplicates of others. > > In a finite volume application we typically want to assign degrees of > freedom just to cells: some applications may only have one degree of > freedom, others may have multiple. > > You encode where you want degrees of freedom in a PetscSection and set > that as the section for the DM in DMSetLocalSection() ( > https://petsc.org/release/docs/manualpages/DM/DMSetLocalSection.html) > > (A c example of these steps that sets degrees of freedom for *vertices* > instead of cells is `src/dm/impls/plex/tutorials/ex7.c`) > > After that you can call DMGetSectionSF() ( > https://petsc.org/main/docs/manualpages/DM/DMGetSectionSF/) to the the > PetscSF that you want for halo exchange: the one for your solution > variables. > > After that, the only calls you typically need in a finite volume code is > PetscSFBcastBegin() to start a halo exchange and PetscSFBcastEnd() to > complete it. > > You say > > > What I want to do is to exchange data (probably just MPI_Reduce) > > which confuses me, because halo exchange is a point-to-point exchange and > not a reduction. Can you clarify? > > > > On Fri, May 20, 2022 at 8:35 PM Mike Michell <mi.mike1...@gmail.com> > wrote: > >> Dear PETSc developer team, >> >> Hi, I am using DMPlex for a finite-volume code and trying to understand >> the usage of PetscSF. What is a typical procedure for doing halo data >> exchange at parallel boundary using PetscSF object on DMPlex? Is there any >> example that I can refer to usage of PetscSF with distributed DMPlex? >> >> Assuming to use the attached mock-up code and mesh, if I give >> "-dm_distribute_overlap 1 -over_dm_view" to run the code, I can see a >> PetscSF object is already created, although I have not called >> "PetscSFCreate" in the code. How can I import & use that PetscSF already >> created by the code to do the halo data exchange? >> >> What I want to do is to exchange data (probably just MPI_Reduce) in a >> parallel boundary region using PetscSF and its functions. I might need to >> have an overlapping layer or not. >> >> Thanks, >> Mike >> >
