dolfin team mailing list archive

[mspieg <mspieg@xxxxxxxxxxxxxxxxx>]

On Aug 17, 2009, at 8:39 AM, Kent Andre wrote:

> On ma., 2009-08-17 at 13:14 +0100, Garth N. Wells wrote:
> > Johan Hake wrote:
> > > On Monday 17 August 2009 14:03:55 Garth N. Wells wrote:
> > > > Johan Hake wrote:
> > > > > On Monday 17 August 2009 13:33:21 Garth N. Wells wrote:
> > > > > > Please send any comments on how a SubFunction should work as  
> > > > > > I'm going
> > > > > > to work on it soon (the Cahn-Hilliard demo runs in parallel 
> > > > > > (!) ;) with
> > > > > > the exception of the extraction of sub-Functions for output). I'm
> > > > > > inlined to change a sub function such that for
> > > > > >
> > > > > >    Function u0 = u[0]
> > > > > >
> > > > > > the Function u0 will only point the vector belonging to u  
> > > > > > rather than
> > > > > > creating a new vector. The advantages are:
> > > > > >
> > > > > > - No need to copy a vector
> > > > > > - DofMap will not require 'is_view'
> > > > > > - Possible to do things like
> > > > > >     u0  = 0.0;
> > > > > >     u1  += v;
> > > > > >     u0.interpolate();
> > > > > What would Function::vector() for a SubFunction then return?  
> > > > > The original
> > > > > full Vector?
> > > > Yes.
> > >
> > > Ok.
> > >
> > > > > It would be cool to add a view of the original Vector that only
> > > > > represents the values of the dofs in the SubFunction, without  
> > > > > coping
> > > > > data. I fiddled around with this when adding slicing for the  
> > > > > PyDOLFIN la
> > > > > interface, but realized that it would be too difficult.
> > > > This is more or less what I plan to do, although internally. A user
> > > > wouldn't see the vector, but operations like interpolate would only
> > > > involve part of the vector.
> > >
> > > I see. Other direct Vector operations would then operate directly  
> > > on the
> > > shared Vector, like get, set, aso.
> > >
> > > > We could add a class like
> > > >
> > > >    VectorView(GenericVector& x, DofMap& dof),
> > > >
> > > > which could derive from GenericVector, to provide views. It isn't a
> > > > priority for me though.
> > >
> > > Yes, I also thought along these lines, however I did not think of  
> > > doing it
> > > using a DofMap, which really is the natural thing. I will also  
> > > not have
> > > possibility to priorities it for now.
> >
> > On second thought,
> >
> >    VectorView(GenericVector& x, std::vector<uint> map& map)
> >
> > would be better. The DofMap could produce the map,
> >
> >    std::vector<uint> map = dofmap.view();
> >
> > Garth
>
> This would be great! This is sort of the reason why I implemented  
> block
> matrices in the first place, but this is far more general and there
> is no need to mess with the block structure during assembly.

Hi All,
   just adding my me-too this suggestion (the Index Set (IS) in  
PETSc, provides similar functionality which is very useful)

In particular, I find that the new version of SubFunction  a bit  
misleading when it returns the entire vector rather than the sub  
vector corresponding to just the subfunction.
(I'm whining because it's  breaking some of my previous code, but  
that's less important).

Consider the following from a Stokes problem

Function w;  //full velocity-pressure solution
Function u = w[0];
Function p = w[1];
Function ux = u[0];
Function uy = u[1];

if you plot or write to a file  u,p,ux, or uy you get what you expect  
just the portion of the solution that is the subfunction,

but if you request something like
ux.vector().max()
you'll get the unexpected result of the maximum of the entire vector  
i.e.

ux.vector().max() == uy.vector().max() == p.vector().max()

which in this case will be the maximum pressure.  This really threw  
me (particularly since I was trying to calculate the maximum vector  
magnitude and getting very strange solutions)

I very much like the idea of not having all these copies floating  
around, but it seems like a subfunction should still retain its  
original consistent behavior of being restricted to just a portion of  
the original function.

Just my 2 cents...you guys do all the hard work
cheers
marc



I'm running into some bugs with the new SubFunction design returning  
the full vector in a PETSc Vector,

I have code
> Kent
>
> _______________________________________________
> DOLFIN-dev mailing list
> DOLFIN-dev@xxxxxxxxxx
> http://www.fenics.org/mailman/listinfo/dolfin-dev

----------------------------------------------------
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)
----------------------------------------------------