PETSc, by default, using iterative solvers which only solve the problem
within a certain accuracy. You can control this accuracy by using -ksp_rtol tol
where for example you could pick tol = 1.0e-12 You can also use a direct
solver
-pc_type lu Run with -ksp_monitor_true_residual -ksp_converged_reason to see
if the linear solver is solving to the requested accuracy.
Barry
On Oct 27, 2012, at 3:46 PM, Chung-Kan Huang <ckhuangf at gmail.com> wrote:
> Hi,
>
> I am solving a linear system A * X = B that sometime contains A_{I,I} != 0
> A_{I,J} = 0. where I!=J and B_I = 0. so I would expect X_i to be perfect zero
> not some tiny small value. The same linear system is solved by other solvers
> but they gave zero for aforementioned situation even the system is fair large
> to our applications. I have tried several configurations in PETSc hoping that
> problem can be solved but all attempts failed. I wonder if someone can give
> me some pointers that help to solve this issue. Thanks,
>
> Chung-Kan
>
>
>
> --
> Cheers
>
>
