[petsc-users] Multigrid Preconditioned CG on cell-centered Poisson with DMStag

[Barry Smith]

Using 

PetscCall(MatNullSpaceCreate(PetscObjectComm((PetscObject)dmPressure), PETSC_TRUE, 0, nullptr, &matNullSpace));

should be fine (the other approach is equivalent).

Any change if you use -ksp_type gmres? 

Please send all the output from a sample run with -ksp_monitor_true_residual -ksp_view 







> On Oct 1, 2024, at 6:42 PM, Gautam Luhana <gkluhana at cs.ubc.ca> wrote:
> 
> I did think that I accounted for that but now I see that I am getting the same number of iterations in the pressure space with and without the call to MatSetNullspace so maybe I am not doing that correctly and that's where the problem is.
> 
> I tried it in the following way first:
> 
> PetscCall(MatNullSpaceCreate(PetscObjectComm((PetscObject)dmPressure), PETSC_TRUE, 0, nullptr, &matNullSpace));
> 
>       PetscCall(MatSetNullSpace(*A, matNullSpace));
> 
> and then also as:
> 
>       PetscCall(DMGetGlobalVector(dmPressure, &constantPressure));
>       PetscCall(VecSet(constantPressure, 1.0));
>       PetscCall(VecNorm(constantPressure, NORM_2, &nrm));
>       PetscCall(VecScale(constantPressure, 1.0 / nrm));
>       PetscCall(DMCreateGlobalVector(dmPressure, &basis));
>       PetscCall(VecCopy(constantPressure, basis));
> PetscCall(MatNullSpaceCreate(PetscObjectComm((PetscObject)dmPressure), PETSC_FALSE, 1, &basis, &matNullSpace));
>       PetscCall(MatSetNullSpace(*A, matNullSpace));
> 
> Is this the right way? are there other options that need to be set?
> 
> -Gautam
> 
> On 2024-10-01 12:47 p.m., Barry Smith wrote:
>> [CAUTION: Non-UBC Email]
>> 
>>    Are you handling the null space? Like with MatSetNullSpace()?
>> 
>>    Can you try src/ksp/ksp/tutorials/ex32.c it solves the same problem but with a simpler DMDA. I don't see any reason why using DMStag with the cell-centered values should be different.
>> 
>>    Barry
>> 
>> 
>>> On Oct 1, 2024, at 3:37 PM, Gautam Luhana <gkluhana at cs.ubc.ca> wrote:
>>> 
>>> Hello,
>>> 
>>> I am solving the stokes equations on the MAC grid using DMStag with an approximate Schur complement based preconditioner. The preconditioner needs a subdomain solve of the Poisson equation on both the velocity as well as the pressure space. I am using multigrid preconditioned CG for both the subdomain solves. While the subdomain solve on the velocity space converges in 4-5 iterations and is independent of mesh-size, the pressure solve is not mesh-size independent. Relevant information:
>>> 
>>> Pressure space operator has Neumann boundary conditions.
>>> 
>>> Coarse-grid operators are being generated using the Galerkin approach (A = P*Af*Pt) for both of the subdomains.
>>> 
>>> For the pressure space, the jacobi and sor smoothers were tried with several omega and damping parameter choices.
>>> 
>>> Symmetric smoothing (same number of pre- and post-smoothing iterations)
>>> 
>>> Has anyone tried solving the poisson operator on the cell-centered pressure space with DMStag? Or know what might be going wrong with the mg-cg convergence?
>>> 
>>> Thanks!
>>> 
>>> -Gautam
>>> 
>>>