[petsc-users] KSP: when to use initial residual norm (ksp_converged_use_initial_residual_norm)

[Daniel.Abele at dlr.de]

Hi,
we are solving a time dependent problem with a single KSP in every time step. We are debating which convergence criterion to use. Is there general guidance around when to use one of the norms with initial residual (ksp_converged_use_initial_residual_norm or ksp_converged_use_min_initial_residual_norm) over the default norm? If I understand the formulas correctly, the initial residual norm "norm(b - A * x0)" (maybe add preconditioning) means that if you have a very good initial guess (as is often the case in time dependent problems if you can use the result if the last time step as initial guess), the norm is much stricter than the default norm "norm(b)". Is this meant as a way to control error accumulation over time? Or does it have some other purpose?
Thanks and Regards,
Daniel
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