Matrix convergence.

[Matthew Knepley]

billy at dem.uminho.pt writes:

  Its definitely possible. Usually you need to know something about
your system in order to precondition and solve it effectively. For instance,
is it the discretization of an elliptic operator?

   Matt

> Hi,
>
>
> I have been testing my parallel implementation with a 2D case. I have used the
> following grids with 2 and 4 processes: 4x4, 10x10, 16x16 and 18x18.
>
> It works for the first three (max. number of iterations to converge aprox. 70)
> but when I increase to 18x18 the matrix does not converge. I have increased the
> maximum number of iterations and I have used GMRES, BiCGSTAB with
> preconditioners ASM, JACOBI, etc.
>
> With KSPType = GMRES and PCType = ASM:
> Number of iterations: 500 Residual: +1.420471E-01
>
> With KSPType = GMRES and PCType = JACOBI:
> Number of iterations: 500 Residual: +2.351882E-02
>
> With KSPType = BCGS and PCType = ASM:
> Number of iterations: 136 Residual: +3.911446E+04
>
> With KSPType = BCGS and PCType = NULL:
> Number of iterations: 157 Residual: +3.254728E+03
>
> Is it normal that the convergence detiorates so much with such a slight increase
> in the dimension of the matrix? Is there any optimization parameter that I may
> be missing?
>
>
> Billy.
>
>
>

-- 
"Failure has a thousand explanations. Success doesn't need one" -- Sir Alec Guiness