[petsc-dev] How do I collect all the values from a sequential vector on the zeroth processor into a parallel PETSc vector ?

[Matthew Knepley]

On Wed, Jan 3, 2018 at 11:47 AM, Franck Houssen <franck.houssen at inria.fr>
wrote:

> How do I collect all the values from a sequential vector on the zeroth
> processor into a parallel PETSc vector ?
> Shall I use VecScatterCreateToZero "backward", or, use VecScatterCreate
> instead ? If yes, how ? (my understanding is that VecScatterCreate can take
> only parallel vector as input)
> Not sure how to do this.
>

Its not clear what you want. Do you want a seq vector duplicated on all
procs? Do you want it split up? The short
answer is, use the appropriate scatter.

   Matt


>
> Franck
>
> In this example, the final VecView of the parallel globVec vector should
> be [-1., -2.]
>
> >> mpirun -n 2 ./vecScatterGatherRoot.exe
> Vec Object: 2 MPI processes
>   type: mpi
> Process [0]
> 1.
> Process [1]
> 2.
> Vec Object: 1 MPI processes
>   type: seq
> 1.
> 2.
> Vec Object: 1 MPI processes
>   type: seq
> -1.
> -2.
> [1]PETSC ERROR: ------------------------------
> ------------------------------------------
> [1]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
> probably memory access out of range
>
> >> more vecScatterGatherRoot.cpp
> // How do I collect all the values from a sequential vector on the zeroth
> processor into a parallel PETSc vector ?
> //
> // ~> g++ -o vecScatterGatherRoot.exe vecScatterGatherRoot.cpp -lpetsc
> -lm; mpirun -n X vecScatterGatherRoot.exe
>
> #include "petsc.h"
>
> int main(int argc,char **argv) {
>   PetscInitialize(&argc, &argv, NULL, NULL);
>   int size = 0; MPI_Comm_size(MPI_COMM_WORLD, &size);
>   int rank = 0; MPI_Comm_rank(MPI_COMM_WORLD, &rank);
>
>   PetscInt globSize = size;
>   Vec globVec; VecCreateMPI(PETSC_COMM_WORLD, 1, globSize, &globVec);
>   VecSetValue(globVec, rank, (PetscScalar) (1.+rank), INSERT_VALUES);
>   VecAssemblyBegin(globVec); VecAssemblyEnd(globVec);
>   VecView(globVec, PETSC_VIEWER_STDOUT_WORLD);
> PetscViewerFlush(PETSC_VIEWER_STDOUT_WORLD);
>
>   // Collect all the values from a parallel PETSc vector into a vector on
> the zeroth processor.
>
>   Vec locVec = NULL;
>   if (rank == 0) {
>     PetscInt locSize = globSize;
>     VecCreateSeq(PETSC_COMM_SELF, locSize, &locVec); VecSet(locVec, -1.);
>   }
>   VecScatter scatCtx; VecScatterCreateToZero(globVec, &scatCtx, &locVec);
>   VecScatterBegin(scatCtx, globVec, locVec, INSERT_VALUES,
> SCATTER_FORWARD);
>   VecScatterEnd  (scatCtx, globVec, locVec, INSERT_VALUES,
> SCATTER_FORWARD);
>
>   // Modify sequential vector on the zeroth processor.
>
>   if (rank == 0) {
>     VecView(locVec, PETSC_VIEWER_STDOUT_SELF);
> PetscViewerFlush(PETSC_VIEWER_STDOUT_SELF);
>     VecScale(locVec, -1.);
>     VecView(locVec, PETSC_VIEWER_STDOUT_SELF);
> PetscViewerFlush(PETSC_VIEWER_STDOUT_SELF);
>   }
>   MPI_Barrier(MPI_COMM_WORLD);
>
>   // How do I collect all the values from a sequential vector on the
> zeroth processor into a parallel PETSc vector ?
>
>   VecSet(globVec, 0.);
>   VecScatterBegin(scatCtx, locVec, globVec, ADD_VALUES, SCATTER_REVERSE);
>   VecScatterEnd  (scatCtx, locVec, globVec, ADD_VALUES, SCATTER_REVERSE);
>   VecView(globVec, PETSC_VIEWER_STDOUT_WORLD);
> PetscViewerFlush(PETSC_VIEWER_STDOUT_WORLD);
>
>   PetscFinalize();
> }
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20180103/956cc145/attachment.html>