parallel solve question
Matt Funk
mafunk at nmsu.edu
Wed Oct 7 17:26:23 CDT 2009
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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