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

[Smith, Barry F.]

> On Jan 3, 2018, at 10:59 AM, Franck Houssen <franck.houssen at inria.fr> wrote:
> 
> I need the exact opposite operation of an entry in the FAQ called "How do I collect all the values from a parallel PETSc vector into a vector on the zeroth processor?"

  You can use VecScatterCreateToZero() then do the scatter with scatter reverse.

> 
> Shall I use VecScatterCreateToZero "backward", or, use VecScatterCreate instead ? If yes, how ? (my understanding is that VecScatterCreate can take only parallel vector as input)

   This understanding is completely incorrect.

  Barry


> Not sure how to do this.
> 
> Its not clear what you want. Do you want a seq vector duplicated on all procs?
> 
> No. The seq vec should "live" only on the master proc. This seq master vec should be "converted" into a parallel vector. Not sure how to do that
> 
> 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/
>