improve error message for failed Krylov solver

[Nico Schlömer]

When a PETSc Krylov solver hasn't converged for one reason or another, the error message currently reads something like

*** Error: Unable to solve linear system using PETSc Krylov solver.
*** Reason: Solution failed to converge in 1000 iterations (PETSc reason DIVERGED_ITS, norm 1.165166e-18).
*** Where: This error was encountered inside PETScKrylovSolver.cpp.

It isn't quite clear what the "norm" here is.

I suggest replacing this by

*** Reason: Solution failed to converge in 1000 iterations (PETSc reason DIVERGED_ITS, ||r|| = 1.165166e-18, ||r||/||b||=<whatever value>).

[Jan Blechta (blechta)]

Moreover it is probably norm of preconditioned rhs to be clear.

[Nico Schlömer (nschloe)]

We should probably make that "||r||_M" (or whatever symbol you use to denote the preconditioner) as Jan suggests.

[Garth Wells (garth-wells)]

On 28 March 2013 17:18, Nico Schlömer <email address hidden> wrote:
> We should probably make that "||r||_M" (or whatever symbol you use to
> denote the preconditioner) as Jan suggests.
>

It's not necessarily the preconditioned residual. We have a user
parameter to set the residual type. For complicated problems, I
normally use the true residual.

Garth

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