[petsc-users] [KSP] Different behaviour between

[Norihiro Watanabe]

Matthew, Thank you for your quick response. I followed your suggestion and
confirmed that the matrices are identical (I checked the norm1 of a matrix
resulted from MatAXPY()).

The next step is to check whether PETSc objects are correctly reset before
reassembling equations. A question I have is, do I have to reset or
recreate a KSP object? The coefficient matrix, RHS and solution vectors are
all set zero by calling MatZeroEntries() and VecSet().


Best,
Nori



On Mon, Jan 6, 2014 at 7:36 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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
>



-- 
Norihiro Watanabe
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