[petsc-users] Very small right hand side and KSP_DIVERGED_DTOL

[Matthew Knepley]

On Fri, Mar 24, 2017 at 8:56 AM, alexandre this <alexandre.this at gmail.com>
wrote:

> Dear all,
>
> I'm in the configuration where I need to solve a linear system Ax = b
> where b is very small although not equal to zero.
>
> It appears that, in this configuration, the initial residual of the first
> iteration of the solver is completely out of reach and the
> KSP_DIVERGED_DTOL is raised.
>

For any convergence question, we need the output from

  -ksp_monitor_true_residual -ksp_converged_reason -ksp_view


> In particular, when using the "-ksp_monitor_true_residual flag", I get the
> following :
> 0 KSP preconditioned resid norm 2.907111674781e+00 true resid norm
> 1.955854211540e+02 ||r(i)||/||b|| 1.765818923254e+09
>

We need to see the error, not just the first line.

You can always raise the tolerance, but divergence means that the norm is
increasing.

  Matt


> What is the best course of action in this case ?
>
> Best,
> Alexandre
>
>
> --
> Alexandre This
>
> Doctorant CIFRE (Philips Healthcare - INRIA) : Fusion Image / Modèles
> Numériques pour la quantification de la sévérité de la régurgitation m
> itrale
>
> Master 2 Recherche Mathématiques, Vision, Apprentissage - ENS Cachan
> Ingénieur spécialisation Santé&Technologie - ECE Paris
> Bachelor of Science : Informatics - Aalborg University, Danemark
>
> tel : 06.32.57.12.44
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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