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

[Franck Houssen]

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?" 

----- Mail original -----

> De: "Matthew Knepley" <knepley at gmail.com>
> À: "Franck Houssen" <franck.houssen at inria.fr>
> Cc: "For users of the development version of PETSc" <petsc-dev at mcs.anl.gov>
> Envoyé: Mercredi 3 Janvier 2018 17:51:02
> Objet: Re: [petsc-dev] How do I collect all the values from a sequential
> vector on the zeroth processor into a parallel PETSc vector ?

> 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?

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/
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