<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">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.<div><br><div><div>On Jun 27, 2013, at 4:08 AM, Thomas DE-SOZA <<a href="mailto:thomas.de-soza@edf.fr">thomas.de-soza@edf.fr</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
<br><font size="2" face="sans-serif">Dear PETSc users,</font>
<br>
<br><font size="2" face="sans-serif">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.</font>
<br><font size="2" face="sans-serif">Though we're quite happy with this new
feature, we've encountered a strange behaviour when using KSPGCR in conjunction
with the PCML preconditioner. </font>
<br><font size="2" face="sans-serif">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 :</font>
<br>
<br><font size="2" face="sans-serif">1st solve</font>
<br><font size="2" face="sans-serif">723 KSP unpreconditioned resid norm
2.911385065051e-02 true resid norm 2.911385065051e-02 ||r(i)||/||b|| 9.979426047969e-09</font>
<br>
<br><font size="2" face="sans-serif">2nd solve</font>
<br><font size="2" face="sans-serif">787 KSP unpreconditioned resid norm
2.896035212670e-02 true resid norm 2.896035212670e-02 ||r(i)||/||b|| 9.926810982197e-09</font>
<br>
<br><font size="2" face="sans-serif">3rd solve</font>
<br><font size="2" face="sans-serif">721 KSP unpreconditioned resid norm
2.913123687343e-02 true resid norm 2.913123687343e-02 ||r(i)||/||b|| 9.985385566274e-09</font>
<br>
<br>
<br><font size="2" face="sans-serif">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) ?</font>
<br><font size="2" face="sans-serif">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)).</font>
<br>
<br><font size="2" face="sans-serif">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).</font>
<br>
<br><font size="2" face="sans-serif">Thanks for any hints,</font>
<br><font size="2" face="sans-serif">Thomas</font><div><br class="webkit-block-placeholder"></div><p><br>
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