[petsc-users] questions about dmdacreaterestriction

[Smith, Barry F.]

> On May 7, 2018, at 8:11 PM, Xiangdong <epscodes at gmail.com> wrote:
> 
> Hi Barry,
> 
> Thanks for your message. When I use DMCreateInterpolation, I got the error message like "[0]PETSC ERROR: Invalid argument
> [0]PETSC ERROR: Coarsening factor in x must be 2".  The error will be gone if I change refine_x = 3 to refine_x=2. Why must the coarsening factor be a factor of 2?

   We've only coded interpolation for some simple situations. DMDA is not fully function to all kinds of grid operations, it just does a few basic things to demonstrate how to use the solvers.

   Barry

> 
> Thank you.
> 
> Xiangdong
> 
> Below is my test code:
> 
> int main(int argc, char **argv)
> {
>   PetscErrorCode ierr;
> 
>   PetscInitialize(&argc, &argv, PETSC_NULL, PETSC_NULL);
> 
>   int Nx=6, Ny=4, Nz=1;
>     
>   DM da;
>   
>   ierr = DMDACreate2d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE, DMDA_STENCIL_STAR,Nx,Ny,PETSC_DECIDE, PETSC_DECIDE,1,1,NULL,NULL,&da);CHKERRQ(ierr);
> 
>   int refine_x=3, refine_y=2, refine_z=1;
>   ierr = DMDASetRefinementFactor(da,refine_x,refine_y,refine_z);
>     
>   ierr = DMDASetInterpolationType(da,DMDA_Q0);
> 
>   ierr = DMSetFromOptions(da); CHKERRQ(ierr);
>   ierr = DMSetUp(da); CHKERRQ(ierr);
> 
>   DM dac;
>   ierr = DMCoarsen(da,NULL,&dac);
>  
>   Mat M;
>   Vec V;
>   
>   ierr = DMCreateInterpolation(dac,da,&M,&V);
>   
>   MatView(M,PETSC_VIEWER_STDOUT_WORLD);
> 
> 
>   ierr = PetscFinalize(); CHKERRQ(ierr);
>   PetscFunctionReturn(0);
> }
> 
> On Mon, May 7, 2018 at 6:16 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> 
> 
> 
> > On May 7, 2018, at 3:25 PM, Xiangdong <epscodes at gmail.com> wrote:
> > 
> > Hello everyone,
> > 
> > I have a 2D DMDA with Nx=6 and Ny=4. If I set refine_x=3 and refine_y=2, I can get a coarse DMDA by DMCoarsen(). However, when I tried to call the DMCreateRestriction between these two DMDAs, I got error about " No support for this operation for this object type. DMCreateRestriction not implemented for this type."
> > 
> > Can you help me get around this?
> 
>    Us DMCreateInterpolation() and then use MatMultTranspose() for example to apply the restriction.
> 
> > 
> > Another simple question, where can I find the explanation/meaning of DMDA_Q0 and DMDA_Q1?
> 
>     This is finite element terminology, Q0 means piecewise constant basis functions (and hence piecewise constant interpolation) while Q1 means linear interpolation.
> For Q0 you should think of the unknowns as living on the centers of each element while for Q1 the unknowns live on the vertices of the elements.
> 
>    Barry
> 
> > 
> > Below is my test example.
> > 
> > Thank you.
> > 
> > Best,
> > Xiangdong
> > 
> > #include <petsc.h>
> > 
> > int main(int argc, char **argv)
> > {
> >   PetscErrorCode ierr;
> > 
> >   PetscInitialize(&argc, &argv, PETSC_NULL, PETSC_NULL);
> > 
> >   int Nx=6, Ny=4, Nz=1;
> >     
> >   DM da;
> >   
> >   ierr = DMDACreate2d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE, DMDA_STENCIL_STAR,Nx,Ny,PETSC_DECIDE, PETSC_DECIDE,1,1,NULL,NULL,&da);CHKERRQ(ierr);
> > 
> >   int refine_x=3, refine_y=2, refine_z=1;
> >   ierr = DMDASetRefinementFactor(da,refine_x,refine_y,refine_z);
> >     
> >   ierr = DMDASetInterpolationType(da,DMDA_Q0);
> > 
> >   ierr = DMSetFromOptions(da); CHKERRQ(ierr);
> >   ierr = DMSetUp(da); CHKERRQ(ierr);
> > 
> >   DM dac;
> >   ierr = DMCoarsen(da,NULL,&dac);
> >  
> >   Mat M;
> >   Vec V;
> >   
> >   ierr = DMCreateRestriction(dac,da,&M);
> >   
> >   MatView(M,PETSC_VIEWER_STDOUT_WORLD);
> > 
> > 
> >   ierr = PetscFinalize(); CHKERRQ(ierr);
> >   PetscFunctionReturn(0);
> > }
> > 
> 
>