# Matrix convergence.

billy at dem.uminho.pt billy at dem.uminho.pt
Thu Mar 2 16:17:02 CST 2006

```Yes, it is elliptic.

Quoting Matthew Knepley <knepley at mcs.anl.gov>:

> 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
>
>

```