[petsc-users] KSPBuildSolution

[Jed Brown]

On Wed, Feb 16, 2011 at 17:31, Juha Jäykkä <juhaj at iki.fi> wrote:

> Please let me double-check there has not been a misunderstanding here: the
> problems I describe occur with the PETSc built-in FD Jacobian
> approximation,
> not my own. Now, I realise this will be a less-than-optimal approximation,
> but
> I fail to see how there could be a programming mistake, when I am using
> SNESDefaultComputeJacobianColor and not my hand-written Jacobian.
>

> I do get the same symptoms with the hand-written one, too. That's why I
> wanted
> to check with the PETSc built in FD version.
>

If your system is poorly scaled or genuinely ill-conditioned, the FD
Jacobian could be bad. Sometimes it helps to use a more robust method of
determining the differencing parameter: -mat_fd_type ds (when using
coloring) or -mat_mffd_type ds (when using -snes_mf_operator). You can also
try solving the linear system to higher tolerance and looking at the true
residual to be sure the linear system really is solved accurately. What sort
of problem are you solving?
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