[petsc-users] how do you calculate the relative tolerance

[Barry Smith]

  This is discussed in https://petsc.org/release/docs/manual/ksp/ <https://petsc.org/release/docs/manual/ksp/> you can see the exact formulas in the code at https://petsc.org/release/src/ksp/ksp/interface/iterativ.c.html#KSPConvergedDefault <https://petsc.org/release/src/ksp/ksp/interface/iterativ.c.html#KSPConvergedDefault>

   If one uses a nonzero initial guess then there are two choices for the initial residual definition, either the residual at the initial guess or the right hand side for the problem see KSPConvergedDefaultSetUIRNorm

  Barry


> On Jan 6, 2022, at 1:52 PM, Ning Li <ning.li at simtechnologyus.com> wrote:
> 
> Hello,
>  
> I am trying to compare PETSc and Lis libraries. 
> 
> In PETSc I could use absolute or relative tolerance. I want to know what is your definition of relative tolerance?
> 
> In Lis, they have 3 convergence conditions:
> <image001.png>
>  
> Thanks 
>  
> Ning Li

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