# Matrix convergence.

billy at dem.uminho.pt billy at dem.uminho.pt
Fri Mar 3 10:41:47 CST 2006

Hi,

I tried to solve the 18x18 matrix with laspack and the results are the following:

Solver: GMRES, Preconditioning: NULL, Iterations: 500, Residual: 6.16E-2
Solver: GMRES, Preconditioning: SSOR, Iterations: 500, Residual: 6.57E-2
Solver: GMRES, Preconditioning: ILU, ERROR

Solver: BiCGSTAB, Preconditioning: NULL, Iterations: 101, Residual: 8.88E-7
Solver: BiCGSTAB, Preconditioning: SSOR, Iterations: 36, Residual: 9.87E-7
Solver: BiCGSTAB, Preconditioning: ILU, ERROR

Billy.

Quoting Barry Smith <bsmith at mcs.anl.gov>:

>
>    Billy,
>
>     Most likely there is something wrong with the matrix being
> generated. I would first look at the matrix entries and make
> sure they make sense. Then I would solve the linear systems
> with a direct solver -pc_type lu (on one processor and make
> sure the solutions are reasonable and your simulation has
>
>    It is possible the parallel matrix generation is wrong,
>
>    Barry
>
>
> On Thu, 2 Mar 2006, billy at dem.uminho.pt wrote:
>
> >
> > 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.
> >
> >
>
>