dolfin team mailing list archive

[Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx>]

On 23 March 2011 22:05, Peter Brune <prbrune@xxxxxxxxx> wrote:
> I saw this thread and had already started running the tests I had for this
> with the latest FFC to see if anything had changed recently.  I never made
> isoparametry through UFL public because I could never get the efficiency to
> be reasonable.  The situation appears to have improved a little bit from a
> year ago, but not much.  With the latest FFC, it's about 10x slower (in 2D)
> to assemble Poisson with P1 for the test/trial space and P2 for the
> coordinates than the same problem with affine coordinates.  It's even worse
> if you include things like Piola-transformed surface normals into the mix.
> When I was working on this I only assembled a very small fraction of the
> elements (around a cylinder in a flow) with the parametric Jacobian, so it
> worked OK for small problems.
>
> I believe that it would do much, much better if the optimizing quadrature
> compiler in FFC supported fractions.  This is necessary because you need to
> apply the inverse isoparametric Jacobian, which includes 1 / |J|, to any
> basis function derivatives in the form.

It does support fractions, but maybe I don't quite follow your
objection. It's difficult to give pointers when the code in question
is secret....

Kristian

> For reference, here are the assembly times for the iso(super, I suppose is
> more accurate)-parametric, optimized, and non-optimized Poisson problems in
> 2D on a 256x256 square (with the parametric coordinates just being the
> regular coordinates passed through).
>
> isoparametric assembly      |        3.2017      3.2017     1
> optimized assembly          |       0.34515     0.34515     1
> regular assembly            |       0.36524     0.36524     1
>
> I got really deep in FFC trying to make this work with no success, but this
> was before the rewrite.  I'd be willing to declare victory on this one and
> submit my code if someone else were willing to make it fast enough to use.
> There's also the issue of how exactly to extend the interface to support
> this in an elegant fashion.  Right now I just call a function that takes a
> form and a parametric Jacobian, runs a Transformer on the form, and spits
> out a new form.
>
> - Peter
>
>
> On Wed, Mar 23, 2011 at 3:30 PM, Anders Logg <logg@xxxxxxxxx> wrote:
>>
>> Peter Brune claims to have solved this by a small addition to the form
>> language that automatically expresses the curved elements as a mapping
>> and expands appropriately (and invisible to the user) those mappings
>> to yield a form that may then be assembled. The higher order geometry
>> is then expressed as a vector-field on the mesh.
>>
>> Perhaps Peter can be pushed to polish up on his code and submit it.
>>
>> --
>> Anders
>>
>>
>> On Wed, Mar 23, 2011 at 07:46:57PM +0000, Garth N. Wells wrote:
>> > We haven't really looked at this. It was discussed a while back, but no
>> > one has committed much time to it. We struggled to settle on an
>> > appropriate abstraction to push on with.
>> >
>> > Garth
>> >
>> > On 23/03/11 18:40, Douglas Arnold wrote:
>> > > What is the status of curved (e.g., isoparametric) elements in dolfin?
>> > > I gather they are not implemented in the main branch.   Has anyone
>> > > done anything with this can be used?  Is there any example code?
>> > > (For example, if you want to
>> > > solve the Poisson problem in a disc and get better than 2nd order
>> > > convergence, you need to do better than polygonal approximation of
>> > > the disc.)
>> > >
>> > >  -- Doug
>> > >
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>
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