DOLFIN

funny stopping criteria for Krylov solvers

Asked by Nico Schlömer

I just got the following output from a Krylov solve in Dolfin

*** Error: Unable to solve linear system using PETSc Krylov solver.
*** Reason: Solution failed to converge in 1000 iterations (PETSc reason DIVERGED_ITS, norm 1.165166e-18).
*** Where: This error was encountered inside PETScKrylovSolver.cpp.

This was with

    solve(a2 == L2, p,
          solver_parameters={'linear_solver': 'iterative',
                             'symmetric': False,
                             'preconditioner': 'ilu',
                             'krylov_solver': {'relative_tolerance': 1.0e-8,
                                               'absolute_tolerance': 0.0,
                                               'maximum_iterations': 1000,
                                               'monitor_convergence': verbose}
                            })

I suppose we're dealing with an inconsistency in scaling here.
Can someone point me to where the stopping criteria are set?

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Gerd Wachsmuth (gerd-wachsmuth) said : 		#1

Your absolute tolerance 0.0 would likely never be reached. Try a large number if you want to ignore the absolute tolerance.

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Nico Schlömer (nschloe) said : 		#2

According to <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPDefaultConverged.html#KSPDefaultConverged>, the default stopping criterion is

 rnorm < MAX (rtol * rnorm_0, abstol),

meaning that setting abstol to 0.0 will effectively remove it.

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Jan Blechta (blechta) said : 		#3

DIVERGED_ITS means (required more than its to reach convergence). Try increasing maximum_iterations or improve preconditiong.

> Can someone point me to where the stopping criteria are set?
dolfin/la/PETScKrylovSolver.cpp

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Nico Schlömer (nschloe) said : 		#4

The issue is not that I wouldn't get the solver to converge when increasing max_iter, it's that it says it hasn't converged when the residual norm really reached a whooping 1.165166e-18.

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Nico Schlömer (nschloe) said : 		#5

Okay, I think I see now what the issue is:

What the error message spits out is ||r||, the stopping criterion is ||r||/||b||. I'll suggest improving the message in a bug.