[petsc-users] DMMG with PBC

[Matthew Knepley]

On Fri, Apr 15, 2011 at 4:28 AM, khalid ashraf <khalid_eee at yahoo.com> wrote:

> Hi,
> I am running src/ksp/ksp/examples/tutorials/ex22.c
> I matched the output of single processor and multiprocessor results and it
> works fine.
> But I want to use a periodic boundary condition. I make the following
> changes in the main function and this works fine with this change as well:
>
>   ierr = DMMGCreate(PETSC_COMM_WORLD,1,PETSC_NULL,&dmmg);CHKERRQ(ierr);
>   ierr =
> DACreate3d(PETSC_COMM_WORLD,DA_XYZPERIODIC,DA_STENCIL_STAR,10,10,2,PETSC_DECIDE,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,0,&da);CHKERRQ(ierr);
>
>
> However, when I comment out these following lines since I am using a PBC,
> then the result of 1 proc and multi-proc are not the same. They vary within
> 5 decimal points and the difference increases with increasing number of
> processors.
>

The periodic operator has a null space. You must put that in the solver
DMMGSetNullSpace(), so that it is projected out at each step.

   Matt


> /*      if (i==0 || j==0 || k==0 || i==mx-1 || j==my-1 || k==mz-1){
>           v[0] = 2.0*(HxHydHz + HxHzdHy + HyHzdHx);
>           ierr =
> MatSetValuesStencil(B,1,&row,1,&row,v,INSERT_VALUES);CHKERRQ(ierr);
>         } else */
>
> Could you please tell me what is going wrong here.
>
> Thanks.
>



-- 
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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