[petsc-users] interpolation function in multigrid

[coco at dmi.unict.it]

Dear all,

I am using the multigrid as preconditioner, and I got in a trouble  
with the interpolation function. In practice, I defined a user  
interpolation function:

PetscErrorCode UserMultAddInterpolation(Mat mat,Vec x,Vec v2, Vec y);

and I noticed that the vectors v2 and y are zero in input, while I am  
expected that they are the solution computed in the smooth down  
process at the same level. In detail, given the following source code  
of the petsc file mg.c :

ierr =  
KSPSolve(mglevels->smoothd,mglevels->b,mglevels->x);CHKERRQ(ierr);  /*  
pre-smooth */
[...]
     while (cycles--) {
       ierr = PCMGMCycle_Private(pc,mglevelsin-1,reason);CHKERRQ(ierr);
     }
[...]
     ierr =  
MatInterpolateAdd(mglevels->interpolate,mgc->x,mglevels->x,mglevels->x);CHKERRQ(ierr);

I observed that mglevels->x is a vector which contains the correct  
values after the KSPSolve calling for the pre-smooth step, but it is a  
zero vector when given in input to the MatInterpolateAdd routine.
I would like to debug the mg.c code, for instance accessing the single  
values of the vector mglevels->x and figuring out where they are  
zeroed out between the KSPSolve and MatInterpolateAdd callings. Would  
you have some suggestion for this debugging?

Thank you for the collaboration.
Best regards,
Armando