[petsc-users] Strongly nonlinear equation solved within the framework of PETSc

[Jed Brown]

On Thu, Dec 29, 2011 at 08:10, Fatcharm <wolfshow at gmail.com> wrote:

> We can see that the SNES Function norm is extremely large. I think it
> is because that the initial value for the unknown function H(X,T) is
> quite small and there is  some terms like (1/H)(dH/dX) or
> (1/H^2)(dH/dX) in my equations.
>

That may cause ill-conditioning, but you could still scale the equations so
that the initial norm was of order 1. It shouldn't matter here though,
because most methods are unaffected by scaling.

Are you computing an analytic Jacobian or using finite differencing?


>
> For "Linear solve did not converge due to DIVERGED_DTOL iterations
> 3270", does this mean I should change the ksp_type?
>

It's important to solve the linear systems before worrying about
convergence rates for Newton methods. Try a direct solve on a small problem
first, then read this

http://scicomp.stackexchange.com/questions/513/why-is-my-iterative-linear-solver-not-converging

If you fix the linear solve issues, but SNES is still not converging, read

http://scicomp.stackexchange.com/questions/30/why-is-newtons-method-not-converging
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