Matrix convergence.

[billy at dem.uminho.pt]

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.