[petsc-users] Question on reverse scatters from VecScatterCreateToAll

[Dave May]

On Fri 5. Jun 2020 at 13:52, Fabian Jakub <
Fabian.Jakub at physik.uni-muenchen.de> wrote:

> Dear Petsc list,
>
> I have a question regarding reverse vec-scatters:
>
> I have a shared memory solver that I want to use on a distributed DMDA and
> average its results.
>
> The shared mem solver needs some of the global state.
>
> So I want to create a full copy of a global vec on each master rank of a
> machine, compute some result
>
> and average the results back into the global vec.
>
>
> For example: the parallel layout for a global vector is
>
> 3 ranks in comm_world, each with Ndof = 6, so global size is 18
>
> Then I create a mpi sub communicator with MPI_COMM_TYPE_SHARED
>
> Then I create a local vec on each rank, with sizes 9 on each rank_0 on the
> sub comm, size 0 otherwise
>
>       if ( sub_id == 0 ) then
>         call VecGetSize(gvec, Nlocal, ierr); call CHKERR(ierr)
>       else
>         Nlocal = 0
>       endif
>       call VecCreateSeq(PETSC_COMM_SELF, Nlocal, lvec, ierr)
>
> This yields for example (first 2 ranks in a shared mem subcomm, 3rd rank
> on another machine):
>
> Global rank_id  |   sub_rank_id   |   local_size_gvec |  local_size_lvec
>
> 0                       |  0                    |
> 6                        | 18
>
> 1                       |  1                    |
> 6                        | 0
>
> 2                       |  0                    |
> 6                        | 18
>
>
> To copy the global vec on each shared mem master, I do:
>
> ISCreateStride(PETSC_COMM_SELF, Nlocal, 0, 1, is, ierr)
>
> VecScatterCreate(gvec, is, lvec, is, ctx, ierr)
>
> VecScatterBegin(ctx, gvec, lvec, INSERT_VALUES, SCATTER_FORWARD, ierr)
>
> VecScatterEnd  (ctx, gvec, lvec, INSERT_VALUES, SCATTER_FORWARD, ierr)
>
>
> That works fine.
>
> Then I want to do the reverse, i.e. add all values from the local vec to
> the global vec on comm_world to generate the average of the results.
>
> I tried:
>
> VecSet(gvec, zero, ierr)
>
> VecScatterBegin(ctx, gvec, lvec, ADD_VALUES, SCATTER_REVERSE, ierr)
>
>
> I was hoping to get the sum of svec in the global vec, so that gvec /
> comm_size(sub_comm_id==0) gives the mean.
>
> However, I get the following error:
>
>
> Nonconforming object sizes
>
> Vector wrong size 18 for scatter 6 (scatter reverse and vector to != ctx
> from size)
>
>
> Going with the same approach with VecScatterCreateToAll
> <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecScatterCreateToAll.html#VecScatterCreateToAll>
> leads to the same issue.
>
> Do you have suggestions on how I could/should achieve it?
>
>

You have to flip the lvec, gvec args in reverse mode (as per the man pages
states under Notes)

Thanks
Dave



> Many thanks!
>
> Fabian
>
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