dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

mspieg wrote:
> On a related thread,
>    I think there is a bug in the Newton solvers (in 0.8.1) in that they 
> now don't   zero the correction dx before each linear solve.  For direct 
> solvers this isn't a problem, however for krylov methods (in particular 
> PETScKrylovSolver), this ends up passing the last correction as the 
> initial guess for the next iteration.  Eventually, when the residual 
> becomes sufficiently small, this throws a divergence_tolerance error in 
> Petsc. (To see this, just change the solver in demo/nls/nonlinearpoisson 
> to something like gmres/ilu and use Petsc as the back end).

Thanks for the report. I fixed this in the development version (zeroing 
dx), but eventually I'll set it up to zero dx only after the first 
iteration. Fixing this exposed another problem. The convergence criteria 
for the Newton solver were more stringent that for the Krylov solver. To 
make explicit what's going on with the linear solver, I've changed the 
demo to use the NewtonSolver interface which requires passing the linear 
solver during construction. This way one can fine tune the linear 
solver, set tolerances, etc.

Garth

> Let me know if I'm being incoherent as usual.
> cheers
> marc
>  
>
>
> On Sep 27, 2008, at 4:00 PM, Garth N. Wells wrote:
>
> > Nuno David Lopes wrote:
> > > Well, in my stokes/uzawa codes I get much worst times, more 
> > > iterations and the 
> > > global error diverges. (But I've to think a little more about it, 
> > > even in the 
> > > first iteration i get different times.) So they don't work anymore. :(
> > >
> > > So I tested with convection-difusion and please take a look at the 
> > > solutions.   
> > > Even with the LU solver.
> >
> > This should be fixed now.
> >
> > Garth
> >
> >
> > > On Friday 26 September 2008, Garth N. Wells wrote:
> > > > Garth N. Wells wrote:
> > > > > Nuno David Lopes wrote:
> > > > > > Is there a simple way of setting an initial guess for an Iterative
> > > > > > LinearSolver?
> > > > > > In Umfpack and PETSc the default initial guess is the zero vector 
> > > > > > right?
> > > > > At the moment, yes (note the UMFPACK is an LU solver, so an initial
> > > > > guess doesn't do anything).
> > > > >
> > > > > It's very simple, and I've been meaning to add an option for using an
> > > > > initial guess. It's also useful for Newton solvers. I'll add something
> > > > > in the next few days.
> > > > I've added this, although it's untested. Let me know if it works ok.
> > > >
> > > > Garth
> > > >
> > > > > Garth
> > > > >
> > > > > > For time dependent problems it would  be great if one could use 
> > > > > > the last
> > > > > > step solutions as the initial guess.
> > > > > > I was thinking of something like
> > > > > > -----------------------------------------
> > > > > > solver.solve(A,x_(n+1),x_n,b)
> > > > > > -----------------------------------------
> > > > > > where x_n is the initial guess for the iterative solver.
> > > > > >
> > > > > > Also another doubt, (i'm guessing  that is my math ignorance only)
> > > > > > is it usual that  PETSc::gmres with hypre::amg preconditioner doesn't
> > > > > > work. (After reading that it is so powerfull...)
> > > > > > In a Stokes 566000^2  subsystem it simply blows up with all of 
> > > > > > the  RAM
> > > > > > memory (16Gb).
> > > > > > (I tried it with simpler problems but with bigger systems   and it
> > > > > > worked perfectly, converging in <10 iterations.)
> > > > > >
> > > > > > Sorry for some disturbance in  the troubled-hard-development times you
> > > > > > are having and thanks again.
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> >
> >
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> ----------------------------------------------------
> Marc Spiegelman
> Lamont-Doherty Earth Observatory
> Dept. of Applied Physics/Applied Math
> Columbia University
> http://www.ldeo.columbia.edu/~mspieg
> tel: 845 704 2323 (SkypeIn)
> ----------------------------------------------------