parallel solve question

[Matt Funk]

Hi Matt,

i don't understand how it can be the condition of the system.
After all, the matrix as well as the RHS vector is EXACTLY the same between 
the 2 runs. This is what puzzles me so much.

The only difference is whether i solve it using 1 core vs 4 cores.

Of course i could be missing something.


matt



On Wednesday 07 October 2009, Matthew Knepley wrote:
> 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


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