[petsc-users] Getting a local vector with ghosts knowing ghost indices

[Matthew Knepley]

On Sun, Jan 29, 2017 at 3:45 AM, Praveen C <cpraveen at gmail.com> wrote:

> Dear all
>
> In my problem, I know the ghost vertices in each partition since I have
> used metis to partition my unstructured grid. I want to get a vector ul
> with ghosts, from a global vector ug.
>
> I will use ul to assemble my non-linear rhs vector.
>
> Is the following approach optimal for this purpose ?
>

This should work and be scalable

   Matt


> PetscInt nvar; // number of variables at each vertex
> PetscInt nvl; // number of local vertices in this partition
> PetscInt nvg; // number of ghost vertices for this partition
> PetscInt *vghost; // global index of ghost vertices for this partition
> PetscReal **u; // size u[nvl+nvg][nvar]
> Vec ul; // vector with ghosts
> Vec ug; // global vector without ghosts
>
>    VecCreate(PETSC_COMM_WORLD, &ug);
>    VecSetSizes(ug, nvar*nvl, PETSC_DECIDE);
>    VecSetFromOptions(ug);
>
>    VecCreateGhostBlockWithArray(PETSC_COMM_WORLD, nvar, nvar*nvl,
>                                        PETSC_DECIDE, nvg, vghost,
>                                        &u, ul);
>
> // Following would be inside RHSFunction
>    double **array;
>    VecGetArray2d(ug, nvl, nvar, 0, 0, &array);
>    for(unsigned int i=0; i<nvl; ++i)
>       for(unsigned int j=0; j<nvar; ++j)
>          u[i][j] = array[i][j];
>    VecRestoreArray2d(ug, nvl, nvar, 0, 0, &array);
>
>    // Fill ghost values
>    VecGhostUpdateBegin(ul, INSERT_VALUES, SCATTER_FORWARD);
>    VecGhostUpdateEnd  (ul, INSERT_VALUES, SCATTER_FORWARD);
>
> // now use u[][] to compute local part of rhs vector
>
> Thanks
> praveen
>



-- 
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
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