[petsc-users] setting correct tolerances for MG smoother CG at the finest level

[Barry Smith]

Send the output using -ksp_view 

Normally one uses a fixed number of iterations of smoothing  on level with multigrid rather than a tolerance, but yes PETSc should respect such a tolerance.

Barry


> On Sep 26, 2025, at 12:49 PM, Moral Sanchez, Elena <Elena.Moral.Sanchez at ipp.mpg.de> wrote:
> 
> Hi, 
> I am using multigrid (multiplicative) as a preconditioner with a V-cycle of two levels. At each level, I am setting CG as the smoother with certain tolerance.
> 
> What I observe is that in the finest level the CG continues iterating after the residual norm reaches the tolerance (atol) and it only stops when reaching the maximum number of iterations at that level. At the coarsest level this does not occur and the CG stops when the tolerance is reached.
> 
> I double-checked that the smoother at the finest level has the right tolerance. And I am using a Monitor function to track the residual.
> 
> Do you know how to make the smoother at the finest level stop when reaching the tolerance?
> 
> Cheers,
> Elena.

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