[petsc-users] SNES question

[Barry Smith]

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

On Oct 3, 2011, at 12:38 PM, Milan Mitrovic wrote:

> Hello everyone,
> 
> I have a problem trying to use snes. I'm using it in a simulation to
> compute an implicit time stepping scheme.
> 
> I'm trying to validate my implementation by testing everything on the
> heat equation. When I use ksp with the analytic jacobian I get great
> results, but when I try to switch back to snes I notice that when I
> try to increase the resolution there is a lower limit on dt below
> which snes won't converge... It works fine with ksp, but snes fails
> with LS_FAILURE.
> 
> I also wrote an fd approximation and tested the jacobian and the two
> are equal up to machine precision. And the fact that it works with ksp
> tells me that it is correct...
> 
> What could be the problem with snes so that it fails to solve a linear
> equation with a very small dt?
> 
> Thanks in advance,
> Milan