Fwd: Re: about ghosted vectors and there local representation

[Thomas Geenen]

nrows is the global number of rows
local2global is the local to global renumbering
nghosts are the number of ghosts
ghosts is the global row number i want to have local access to after the solve


ISLocalToGlobalMappingCreate(PETSC_COMM_WORLD, nrows, local2global, &ltog);
VecCreateGhost(PETSC_COMM_WORLD, nc, 
                            	PETSC_DECIDE, nghosts[mype], ghosts, &x);
VecSetLocalToGlobalMapping(x,ltog);
VecSetValuesLocal(x, nc, indx, x_values, INSERT_VALUES);
VecAssemblyBegin(x);
VecAssemblyEnd(x);

solve the system

ierr = KSPGetSolution(ksp, &x);
ierr = VecGhostUpdateBegin(x,INSERT_VALUES,SCATTER_FORWARD);
ierr = VecGhostUpdateEnd(x,INSERT_VALUES,SCATTER_FORWARD);
ierr = VecGhostGetLocalForm(x, &lx);

nrows is the global number of rows
local2global is the local to global renumbering
nghosts are the number of ghosts
ghosts is the global row number i want to have local access to after the solve


thomas

On Thursday 22 June 2006 09:20, you wrote:
> ----------  Forwarded Message  ----------
>
> Subject: Re: about ghosted vectors and there local representation
> Date: Tuesday 20 June 2006 20:39
> From: Barry Smith <bsmith at mcs.anl.gov>
> To: petsc-users at mcs.anl.gov
>
>    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
>
> -------------------------------------------------------