parallel solve question

[Matthew Knepley]

This sounds like it has to do with the condition of your system, not any
parallel problem. Errors
in the solution can only be reduced to about (condition number) * (residual
norm).

  Matt

On Wed, Oct 7, 2009 at 5:26 PM, Matt Funk <mafunk at nmsu.edu> wrote:

> Hi,
>
>
> i have a problem for which i am not exaclty sure about what to do.
> I set up a simple 2D rectangular domain and decompose it into four equal
> boxes. I then build the petsc matrix based on this layout as well as the
> corresponsing RHS vector.
>
>
> I print out the matrix and RHS vector right before my KSPSolve call, and
> right after that call i print out the solution vector 'x'.
>
>
> I do this for 2 runs.
> 1) 1 processor
> 2) 4 processors.
>
>
> For both runs i do a difference (i.e. on the output files using diff) on
> all 3 quantities (the matrix, the RHS vector and the solution vector).
>
>
> The 'diff' command reports no difference between the files for the matrix
> and RHS vector.
>
>
> However, the soltution vector is different between the 2 runs. How
> different depends a little on what precond/solver combination i use and the
> tolerances.
>
>
> However, for example for BJacobi/GMRES with reltol=abstol=1e-12 the vector
> element with the maximum difference is on the order 1e-05. This is only
> after the first timestep. My problem has some nonlinearlity to it such that
> this will become a problem later on.
>
>
> The worst difference i have seen is if i use hypre's euclid. It was on the
> order of 1e-02.
>
>
>
> So my question is whether someone has an idea why this is happening (i
> suppose it is related to the parallel communication) and if there is way to
> fix it.
>
>
>
> thanks
> matt
>



-- 
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
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