Hi, Sreeram,
You can try this code. Since x, y are both MPI vectors, we just need to say we want to scatter x[0:N] to y[0:N]. The 12 index sets with your example on the 12 processes would be [0..8], [9..17], [18..26], [27..35], [], ..., []. Actually, you can do it arbitrarily, say, with 12 index sets [0..17], [18..35], .., []. PETSc will figure out how to do the communication.

  PetscInt   rstart, rend, N;

  IS         ix;

  VecScatter vscat;

  Vec        y;

  MPI_Comm   comm;

  VecType    type;

  PetscObjectGetComm((PetscObject)x, &comm);

  VecGetType(x, &type);

  VecGetSize(x, &N);

  VecGetOwnershipRange(x, &rstart, &rend);

  VecCreate(comm, &y);

  VecSetSizes(y, PETSC_DECIDE, N);

  VecSetType(y, type);

  ISCreateStride(PetscObjectComm((PetscObject)x), rend - rstart, rstart, 1, &ix);

  VecScatterCreate(x, ix, y, ix, &vscat);

--Junchao Zhang

On Wed, Oct 4, 2023 at 6:03 PM Sreeram R Venkat <srvenkat@utexas.edu> wrote:

Suppose I am running on 12 processors, and I have a vector "v" of size 36 partitioned over the first 4. v still uses the PETSC_COMM_WORLD, so it has a layout of (9, 9, 9, 9, 0, 0, ..., 0). Now, I would like to repartition it over all 12 processors, so that the layout becomes (3, 3, 3, ..., 3). I've been trying to use VecScatter to do this, but I'm not sure what IndexSets to use for the sender and receiver.

The result I am trying to achieve is this:

Assume the vector is v = <0, 1, 2, ..., 35>

Start Finish
Proc | Entries Proc | Entries
0 | 0,...,8 0 | 0, 1, 2
1 | 9,...,17 1 | 3, 4, 5
2 | 18,...,26 2 | 6, 7, 8
3 | 27,...,35 3 | 9, 10, 11
4 | None 4 | 12, 13, 14
5 | None 5 | 15, 16, 17
6 | None 6 | 18, 19, 20
7 | None 7 | 21, 22, 23
8 | None 8 | 24, 25, 26
9 | None 9 | 27, 28, 29
10 | None 10 | 30, 31, 32
11 | None 11 | 33, 34, 35

Appreciate any help you can provide on this.

Thanks,
Sreeram