[petsc-users] Troubles with VecScatter routines: inverting vectors values

[Alice Raeli]

Hi All,

I had the necessity to read data on another node of a Vector Petsc. 
I followed the structure showed in the tutorial of VecScatterCreateToAll.
I call the function below 2 times with 2 different vectors vec1 and vec2.

void Scatter(Vec vec, PetscScalar &value, vector<int> NeighboursIndex, int NeighNumber){
    
    Vec V_SEQ;
    VecScatter ctx;
    PetscScalar *vecArray;
    
    VecScatterCreateToAll(vec,&ctx,&V_SEQ);
    VecScatterBegin(ctx,vec,V_SEQ,INSERT_VALUES,SCATTER_FORWARD);
    VecScatterEnd(ctx,vec,V_SEQ,INSERT_VALUES,SCATTER_FORWARD);
    ierr = VecGetArray(V_SEQ,&vecArray);
    for(int i=0; i<NeighNumber;i++){
        PetscInt index=NeighboursIndex[i];
        value = vecArray[index];

        if(NeighboursIndex[0]==869 && (index==363 || index == 364 || index ==365) ){
            cout << endl << index << "  inside function " << value;
        }

    }
    //
    ierr = VecRestoreArray(V_SEQ,&vecArray);
    VecScatterDestroy(&ctx);
    VecDestroy(&V_SEQ);
    return; 
}

When I run my code with one node i have a result coherent but if I use 2 nodes the vectors are inverted in values.
I used for a test a vector of 1 and a vector of 0 and I concentrated the output on a point that communicates with the other node.

Can you explain me what is effectively wrong when I call the vecscatter routines? I can’t see a reason coherent with a simple inversion of values from 2 vectors. 

Example of output
1 node:
first call vec1
364  inside function 87.797
363  inside function 89.0416
365  inside function 86.4618
second call vec2
364  inside function 0.0268704
363  inside function 0.0322394
365  inside function 0.0199141

2 nodes:
first call vec1
364  inside function 0.0268704
363  inside function 0.0322394
365  inside function 0.0199141
second call vec2
364  inside function 87.797
363  inside function 89.0416
365  inside function 86.4618

Many Thanks
Alice Raeli
Institut de Mathématiques de Bordeaux (IMB)

alice.raeli at math.u-bordeaux1.fr <mailto:alice.raeli at math.u-bordeaux1.fr>

alice.raeli at u-bordeaux.fr <mailto:alice.raeli at u-bordeaux.fr>
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