dolfin team mailing list archive

[mspieg <mspieg@xxxxxxxxxxxxxxxxx>]

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).

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.
> > > > _______________________________________________
> > > > DOLFIN-dev mailing list
> > > > DOLFIN-dev@xxxxxxxxxx
> > > > http://www.fenics.org/mailman/listinfo/dolfin-dev
>
>
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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)
----------------------------------------------------