[petsc-users] Convergence is different with different processors

[Barry Smith]

  This is completely possible. The convergence of iterative methods is a complicated business. Even something seemingly simple like block Jacobi whose convergence depends not only on the number of blocks, but the "shapes" of the blocks and what part of the matrix is "discarded" in the preconditioner depending on the number and "shape" of the blocks. Decomposing the domain into 4 and 8 pieces results in different "shapes" of the blocks and different parts of the matrix being "discarded" in the preconditioner.

   Barry

On Feb 16, 2013, at 11:36 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

> 
> 
> I use the two commands in the same project. The first is divergent while the second is convergent.
> nohup mpiexec -n 4 ./ex4f -ksp_type bcgs -pc_type bjacobi -ksp_rtol 1.0e-5 -ksp_converged_reason >out.txt &
> nohup mpiexec -n 8 ./ex4f -ksp_type bcgs -pc_type bjacobi -ksp_rtol 1.0e-5 -ksp_converged_reason >out.txt &
> So what is the reason?
> Thanks.                    Jim
> 
> 
> 
> >> 2013-02-07 13:16:51，"Matthew Knepley" <knepley at gmail.com> 写道：
> >> On Thu, Feb 7, 2013 at 12:11 AM, w_ang_temp <w_ang_temp at 163.com> wrote:
> >> Hello,
> >>     I use the same project, but I find that when different number of processors is choosed,
> >> the convergence is different. For example, when the processors are 4, it is divergent; when
> >> the processors are 8, it is convergent.
> >>     So what is the reason?
> 
> >It is likely that your preconditioner changed.
> 
>   > Matt
>  
> >>     Thanks.
> >>                                    Jim.
> 
> 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
> 
>