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

[Franck Houssen]

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. 

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(); 
} 

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