[petsc-users] [KSP] Different behaviour between

[Matthew Knepley]

On Mon, Jan 6, 2014 at 12:06 PM, Norihiro Watanabe <norihiro.w at gmail.com>wrote:

> Hi,
>
> I got a strange problem with PETSc 3.4 that linear equations assembled in
> our FEM codes cannot be solved within 5000 iterations, whereas linear
> equations loaded from binary files can be solved with only 24 iterations
> using ksp/examples/tutorials/ex10.c. The binary files were created by the
> FEM codes using MatView() and VecView() right before calling KSPSolve().
> Linear solver types are -ksp_type bcgs -pc_type bjacobi. I set the same
> tolerance to both programs.
>
> As you can see from attached log files, both programs calculate the same
> "true resid norm" at the beginning but different "preconditioned resid
> norm". Does it mean both programs are actually solving the same problem but
> somewhat with different preconditoner? It would be so helpful if you have
> any clue for this problem.
>

Yes, that would be the likely conclusion. It appears that the parallel
partition is the same, that the number of nonzeros in both
matrices is the same, and that the solvers are identical. Thus I would make
sure that the matrices are indeed identical. For example,
you can MatLoad the matrix you used in ex10, and MatAXPY() to get the
difference. It is a common error to forget to MatZeroEntries()
before reassembling the FEM operator which can produce the convergence
degredation you see.

  Thanks,

      Matt


> just tell you background of this: Currently I'm facing a convergence
> problem in linear solvers for solving transient heat transport problems
> using FEM. At early time steps, PETSc converges quickly (<20 iterations).
> Later, iteration numbers increase as time steps increase (>5000 after 19
> time steps). I'm in the middle of checking where is a problem for the slow
> convergence. Because I don't get  such slow convergence with other linear
> solvers (BiCGSTAB+Jacobi), I suspect the FEM  codes are missing some PETSc
> functions or options to be used. As I wrote above, if I use ex10.c with the
> binary files, the convergence problem is solved, which means something
> going wrong in the FEM codes.
>
>
> Thank you in advance,
> Nori
>
>


-- 
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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