On Nov 29, 2012, at 3:59 PM, John Peterson <peter...@cfdlab.ae.utexas.edu> wrote:
> On Wed, Nov 28, 2012 at 2:08 PM, Barry Smith <bsm...@mcs.anl.gov> wrote: >> >> On Nov 28, 2012, at 3:00 PM, Michael Povolotskyi <mpovo...@purdue.edu> wrote: >> >>> Thank you. >>> Looks like also others treated this parameter erroneously as the absolute >>> tolerance: >>> >>> For example, the Libmesh library: >>> http://libmesh.sourceforge.net/doxygen/classlibMesh_1_1PetscNonlinearSolver.php#a1d5ed777465c1f08785bc321c84c7c71 >>> >>> 00486 // Set the tolerances for the non-linear solver. >>> 00487 ierr = SNESSetTolerances(_snes, this->absolute_residual_tolerance, >>> this->relative_residual_tolerance, >>> 00488 this->absolute_step_tolerance, >>> this->max_nonlinear_iterations, this->max_function_evaluations); >>> >>> Am I right that they have a problem? >> >> Based on the name of their variable yes it does appear to be wrong. The >> reason it is a relative tolerance (and not absolute) is that one is >> computing >> >> x = delta x + x >> >> and once delta x is sufficiently smaller than x, delta x + x == x (in >> floating point) so we don't want to iterate past the point where x is not >> being changed. > > So, if I'm interpreting everything correctly, it would be more correct > for the third argument to be > > this->relative_step_tolerance > > instead of > > this->absolute_step_tolerance? Yes. > > We could change this, but it might break backwards compatibility: some > users may not currently use relative_step_tolerance at all, etc. This tolerance is really a property of how many digits one has in PetscScalar so no really should really be setting it explicitly except under exceptional circumstances. Barry > > -- > John > > ------------------------------------------------------------------------------ > Keep yourself connected to Go Parallel: > VERIFY Test and improve your parallel project with help from experts > and peers. http://goparallel.sourceforge.net > _______________________________________________ > Libmesh-devel mailing list > Libmesh-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/libmesh-devel ------------------------------------------------------------------------------ Keep yourself connected to Go Parallel: VERIFY Test and improve your parallel project with help from experts and peers. http://goparallel.sourceforge.net _______________________________________________ Libmesh-devel mailing list Libmesh-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmesh-devel
