[petsc-users] [KSP] Different behaviour between

[Norihiro Watanabe]

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.

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
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