[petsc-users] norm of KSPBuildResidual does not match norm computed from KSPBuildSolution

[Moral Sanchez, Elena]

Hi,
I am running CG with a Jacobi preconditioner. I have a monitor function that prints the residual and saves the solution at every iteration. To get the solution at every iteration, I am using the function KSPBuildSolution. I am setting the KSP norm as UNPRECONDITIONED.

The convergence test is consistent with the 2-norm of KSPBuildResidual. However this norm does not match the 2-norm of the residual computed explicitly from the solution (obtained with KSPBuildSolution). It also does not match the preconditioned norm. What norm is it computing?

When the CG is not preconditioned, the norm of KSPBuildResidual and the norm of the residual computed from the solution match, as I expected.

This is KSPView():

    KSP Object: 1 MPI process
      type: cg
        variant HERMITIAN
      maximum iterations=100, nonzero initial guess
      tolerances: relative=1e-08, absolute=1e-08, divergence=10000.
      left preconditioning
      using UNPRECONDITIONED norm type for convergence test
    PC Object: 1 MPI process
      type: jacobi
        type DIAGONAL
      linear system matrix = precond matrix:
      Mat Object: 1 MPI process
        type: nest
        rows=524, cols=524
          Matrix object:
        type=nest, rows=3, cols=3
        MatNest structure:
        (0,0) : type=mpiaij, rows=176, cols=176
        (0,1) : NULL
        (0,2) : NULL
        (1,0) : NULL
        (1,1) : type=mpiaij, rows=172, cols=172
        (1,2) : NULL
        (2,0) : NULL
        (2,1) : NULL
        (2,2) : type=mpiaij, rows=176, cols=176

Cheers,
Elena Moral Sánchez

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20251103/4e395726/attachment.html>