dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Sep 25, 2008 at 06:03:59PM +0100, Garth N. Wells wrote:
> 
> 
> Anders Logg wrote:
> > On Thu, Sep 25, 2008 at 05:20:53PM +0200, Anders Logg wrote:
> >> On Thu, Sep 25, 2008 at 04:18:23PM +0100, Garth N. Wells wrote:
> >>>
> >>> Anders Logg wrote:
> >>>> On Thu, Sep 25, 2008 at 03:53:01PM +0100, Garth N. Wells wrote:
> >>>>> Anders Logg wrote:
> >>>>>> On Thu, Sep 25, 2008 at 03:11:27PM +0100, Garth N. Wells wrote:
> >>>>>>> Anders Logg wrote:
> >>>>>>>> On Thu, Sep 25, 2008 at 12:41:33PM +0100, 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.
> >>>>>>>>>
> >>>>>>>>> Garth
> >>>>>>>> It would be natural to let the x argument always be the initial
> >>>>>>>> guess. I thought we already did this.
> >>>>>>>>
> >>>>>>>> Would it be enough to make sure that Vector::init() does not reset the
> >>>>>>>> values to zero?
> >>>>>>>>
> >>>>>>> Yes.
> >>>>>>>
> >>>>>>> The danger is if someone sends an uninitialised vector to the solver.
> >>>>>>>
> >>>>>>> Garth
> >>>>>> We can just put something like this in the init() functions:
> >>>>>>
> >>>>>>   if (x && size() == N)
> >>>>>>     return;
> >>>>>>
> >>>>>> I think we had this a while back but at some point VecZeroEntries was
> >>>>>> inserted.
> >>>>>>
> >>>>> I think that we should have an option whether or not to use an initial 
> >>>>> guess. The default can be to use the guess (as in your above code extract).
> >>>>>
> >>>>> Garth
> >>>> I think it would work nicely if the solver calls x.init() and that
> >>>> will initialize x to a zero vector only if it has not already been
> >>>> initialized. 
> >>> Sounds a bit dangerous to me that Vector::init() sometimes zeroes a the 
> >>> vector, and sometimes it doesn't. Should we get rid of Vector::init() 
> >>> and use Vector::resize() and Vector::zero() instead? No ambiguity then.
> >>>
> >>> Garth
> >> That sounds good.
> > 
> > On second thought, we shouldn't mess with the init() functions unless
> > we want to change the meaning of init() for all the classes deriving
> > from GenericTensor, including also matrices for all backends. Also,
> > resize() sometimes means that existing values will be preserved when
> > the size is increased.
> >
> 
> I've already started on it. There are a lot of places in the code where 
> we only need to resize and not zero, so I like using resize() and 
> zero(). We even had places with
> 
>     x.init(N);
>     x.zero();
> 
> There is no ambiguity now with resize and zero.
>
> 
> > So the best solution is to do something like
> > 
> >   if (x.size() != N)
> >     x.init();
> > 
> 
> This is what I've done.

Do you mean that you've added a function named resize() to the vector
classes that does exactly this:

  if (x.size() != N)
    x.init();

?

-- 
Anders

Attachment: signature.asc
Description: Digital signature