[petsc-users] Ghost communication

[Barry Smith]

  How are b and x obtained? They need to be optained via VecDuplicate() from petsc_vector, if you obtain them via a regular VecCreate() they won't be of ghosted style hence mess up your plan.

   Barry

Plus you need to do as Jed suggested.


On Jun 22, 2011, at 3:45 AM, Milan Mitrovic wrote:

> Hello everyone,
> 
> I'm trying to use petsc with ppm and here is what I have so far:
> 
> First I create parallel vectors with ghost locations using an existing
> array that already holds the data:
> 
>          CALL VecCreateGhostWithArray(PETSC_COMM_WORLD, &
>               Particles%Npart,                          &
>               PETSC_DECIDE,                             &
>               Particles%Mpart-Particles%Npart,         &
>               gi(Particles%Npart:Particles%Mpart),    &
>               wp, petsc_vector, info)
> 
> where gi is the global indexing, wp is the existing data array, Npart
> is the number of real, and Mpart the number of real+ghost particles.
> 
> Then, I create a matrix shell, and provide my own multiplication
> routine (this part works fine I think).
> 
> Inside the multiplication routine I do this:
> 
>          CALL VecGhostUpdateBegin(from,INSERT_VALUES,SCATTER_FORWARD,info)
>          CALL VecGhostUpdateEnd(from,INSERT_VALUES,SCATTER_FORWARD,info)
> 
>          ! get ghosted versions of vectors
>          CALL VecGhostGetLocalForm(from,xl,info)
>          CALL VecSet(to,0.0_MK,info)
>          ! get arrays from petsc vectors
>          CALL VecGetArray(xl,xx,xxi,info)
>          CALL VecGetArray(to,yy,yyi,info)
> 
> Then I modify the data, and do the following:
> 
>          CALL VecRestoreArray(xl,xx,xxi,info)
>          CALL VecRestoreArray(to,yy,yyi,info)
>          ! release local form of vectors
>          CALL VecGhostRestoreLocalForm(from,xl,info)
> 
>          CALL VecGhostUpdateBegin(to,ADD_VALUES,SCATTER_REVERSE,info)
>          CALL VecGhostUpdateEnd(to,ADD_VALUES,SCATTER_REVERSE,info)
> 
> And then I call KSP solve:
> 
>  call KSPCreate(PETSC_COMM_WORLD, ksp, info)
>  call KSPSetOperators(ksp, A, A, DIFFERENT_NONZERO_PATTERN, info)
>  call KSPSolve(ksp, b, x, info)
> 
> I noticed that the values of the ghost particles never change. The
> communication never seems to take place...
> 
> Can you give me some hints on what to try? Or what I do wrong?
> 
> Thanks!
> 
> Milan Mitrovic