[petsc-users] Unreproducible number of iterations for consecutives solves (GCR + ML)

[Mark F. Adams]

If this happens in serial and does not happen with other solvers then there is probably a non-deterministic random seed (RHS for the eigen estimator...).  I could imagine the documentation not going into that much detail, but I would not imagine that ML would be non-determinstic by default ...  It could be a floating point issues also.

On Jun 27, 2013, at 4:08 AM, Thomas DE-SOZA <thomas.de-soza at edf.fr> wrote:

> 
> Dear PETSc users, 
> 
> We've been using the Krylov solvers in PETSc for a while in our software package for implicit structural mechanics and recently added algebraic multigrid preconditioning through the ML and Hypre libraries provided as part of PETSc. 
> Though we're quite happy with this new feature, we've encountered a strange behaviour when using KSPGCR in conjunction with the PCML preconditioner. 
> Indeed, identical and  consecutives solves inside the same run do not display the same number of Krylov iterations even in sequential and we were wondering why : 
> 
> 1st solve 
> 723 KSP unpreconditioned resid norm 2.911385065051e-02 true resid norm 2.911385065051e-02 ||r(i)||/||b|| 9.979426047969e-09 
> 
> 2nd solve 
> 787 KSP unpreconditioned resid norm 2.896035212670e-02 true resid norm 2.896035212670e-02 ||r(i)||/||b|| 9.926810982197e-09 
> 
> 3rd solve 
> 721 KSP unpreconditioned resid norm 2.913123687343e-02 true resid norm 2.913123687343e-02 ||r(i)||/||b|| 9.985385566274e-09 
> 
> 
> Would you say this case requires a great number of iterations and therefore reproductibility is not ensured (indeed it is ill-conditioned and is not very suited for algebraic multigrid) ? 
> Or is there a seed somewhere in ML that would explain this ? I searched the ML manual for that and couln't find any. We're using the uncoupled coarsening scheme in ML (call PetscOptionsSetValue('-pc_ml_CoarsenScheme', 'Uncoupled', ierr)). 
> 
> Final notes : several consecutive runs of the program do display identical behaviour (that is the 1st solve always require 723 iterations, the 2nd 787, etc). Moreover other cases that are well-conditioned require the same number number of iterations for each consecutive solve (though the converged residual differs a bit). 
> 
> Thanks for any hints, 
> Thomas
> 
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