about ghosted vectors and there local representation

[Barry Smith]

   The ltog has to exactly match the way the ghosts are provided to 
VecCreateGhost(). Are they? Please send the exact example code.

    Barry


On Tue, 20 Jun 2006, Thomas Geenen wrote:

> Dear Petsc users,
>
> I think that I do not use the ghosted vector concept in the right way.
>
> I create a vector with
> ierr = VecCreateGhost(PETSC_COMM_WORLD, nc,
>                            		 PETSC_DECIDE, nghosts, ghosts, &x);
> The entries in ghost contain the global vector positions that I want to have
> available locally.
>
> I have a local representation so i apply (after having constructed the mapping
> of course)
> ierr = VecSetLocalToGlobalMapping(x,ltog);
> I now insert the values
> ierr = VecSetValuesLocal(x, nc, indx, x_values, INSERT_VALUES);
> VecAssemblyBegin(x);
> VecAssemblyEnd(x);
> I am now under the assumption that petsc divided the vector entries among
> processes according to his global representation of the vector.
>
> I solve my system of equations
> KSPSolve(ksp,b,x);
> afterwards i do
> ierr = KSPGetSolution(ksp, &x);
> ierr = VecGhostUpdateBegin(x,INSERT_VALUES,SCATTER_FORWARD);
> ierr = VecGhostUpdateEnd(x,INSERT_VALUES,SCATTER_FORWARD);
> ierr = VecGhostGetLocalForm(x, &lx);
>
> I now expect the vector to be in the local form. That means that the positions
> that I created as ghost points are filled with the solution corresponding to
> the global solution. I expect that they are inserted in the position in my
> local vector corresponding to there corresponding global position.
>
> apparently this is not what happens.
>
> a small example
> 2 cpu's
> each cpu contains 3 vector entries however there is an overlap of 1
> on cpu 1 the vector contains points (1, 2, 3)
> on cpu 2 the vector contains points (3, 4, 5)
>
> I make position 1 on cpu 2 a ghost points with nghost=1 ghosts(3)
> after solving and restoring I expect vector lx to contain the solution as (3,
> 4, 5)
>
>
> I hope I have explained my misunderstanding of the underlying concept clear
> enough
>
> thanks for your help
> Thomas Geenen
>
>