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

[Barry Smith]

On Dec 29, 2011, at 8:21 AM, Jed Brown wrote:

> 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,


    This is done by running with the additional option -pc_type lu 


> 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