[petsc-users] Scattering a vector to/from a subset of processors

[Junchao Zhang]

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 at 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
>
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