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 <srven...@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 >
