[petsc-users] Preconditioning in SNES

[Jed Brown]

On Tue, Nov 1, 2011 at 09:21, Matthew Knepley <knepley at gmail.com> wrote:

> It could be that:
>
>   a) Your system is really ill-conditioned, and the PC is not that good
>
>   b) Your system has a null space, and the initial guess had no component
> in it
>
>   c) Your Jacobian is not actually the Jacobian of the system, so the PC
> is not good
>
>   d) The FD approximation to the Jacobian action is not good
>
> When you have so many different possible problems, the right thing to do
> is simplify the system until you can narrow down the cause
>
>   a) Run in serial
>
>   b) Shrink the problem
>
>   c) Fully form the Jacobian using -snes_fd
>
>   d) Run with -pc_type lu
>

We keep a pretty comprehensive list here, I should have linked it:

http://www.mcs.anl.gov/petsc/petsc-as/documentation/faq.html#kspdiverged
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