[petsc-users] Multigrid as a preconditioner

[coco at dmi.unict.it]

Dear all,

I have chosen the multigrid as a preconditioner of a Richardson kps  
solver and I have set all the multigrid stuff (smoother,  
interpolation, restriction, coarse solver). When I perform two  
iterations of the Richardson ksp solver (with the option -ksp_max_it  
2), which means two cycles of the multigrid, I obtain the same result  
as in the case I perform only one iteraton.
I noticed that when the second iteration starts, the initial guess is  
zero, and then it provides the same solution as in the first  
iteration. Anyway, the norm residual printed by the -ksp_monitor  
option decreases.
How can I investigate the problem?
One more detail: the smoother is implemented by a user-provided  
routine UserSOR() which is registered as MATOP_SOR of a shell matrix,  
which is assigned to the ksp smoother. Even if the UserSOR() takes in  
input a MatSORType flag, I do not take care about it inside the routine.

Thank you.
Best regards,
Armando