Increasing convergence rate

[jerome ho]

On Sat, Jan 24, 2009 at 1:53 AM, Jed Brown <jed at 59a2.org> wrote:
> The 20% result difference makes me very worried that the matrices are
> actually different.

> Are you still using BoomerAMG?  If your 1Mx1M matrix comes from a 2D
> problem you might be able to compare with a direct solve (-pc_type lu
> -pc_factor_mat_solver_package mumps) but if it's 3D, that would take
> way too much memory.  It's a good idea to make the problem as small as
> possible (like 100x100 or less) when dealing with issues of
> correctness.  It's really hard to make a preconditioner exactly the
> same in parallel, even parallel ILU (like Euclid with default options)
> is not exactly the same.   It's silly, but if you can't make the
> problem smaller, can't use a direct solver, and don't have an easy way
> to determine if the parallel matrix is the same as the serial one, try
> -pc_type redundant -pc_redundant_type hypre, the results (up to
> rounding error due to non-associativity) and number of iterations
> should be the same as in serial but the monitored residuals won't be
> exactly the same since they are computed differently.

Thanks for your advice. I finally managed to nail down the problem.
Earlier, on smaller test cases, the matrices on both serial and
parallel was verified to be the same.
I didn't thought it was the matrices. But when I tried the redundant
method I still got the 20% difference.

So, I recheck the matrix stamping again and there were a few elements
that I missed when distributed into more processors, which makes it
even harder to converge.
Now, both the serial and parallel results correlates and converges
within several iterations. Thanks again!

Jerome