Question

Hi I have a program written in C++ in which one or two functions out of many are using PETSc.

When I’m calling these functions which would be the proper approach while using PETSc

1) a)change all MPI_COMM_WORLD to PETSC_COMM_WORLD for my entire program
   b)change MPI_Init to PetscInitialize

..

2) create a separate comm world for petsc and pass that on to the function

    MPI_Init(&argc, &argv);
    ..
    //some many line of code
    ..

    MPI_Comm_split(MPI_COMM_WORLD, rank, 0, &PETSC_COMM_WORLD);

    petsc_function(PETSC_COMM_WORLD,.....

    //some how reverting the comm_split here to continue as normal so all the non petsc functions work

..

3) or can i simply get by just using MPI_COMM_WORLD with the Petsc function
..

4) or is there another approach I am unaware of?

Answer 1

You need to call PetscInitialize first. You can replace the call to MPI_Init with it.

There is no need to set PETSC_COMM_WORLD unless you only want to use PETSc on a subset of the ranks. If you do not set PETSC_COMM_WORLD yourself, it will be set automatically by PetscInitialize to be a copy of MPI_COMM_WORLD.

Then, when you call PETSc functions you can use PETSC_COMM_WORLD. At the end of the program call PetscFinalize, which will call MPI_Finalize for you unless you called MPI_Init manually instead of using PetscInitialize alone.

There is no need to change MPI_COMM_WORLD to PETSC_COMM_WORLD in the rest of the program.