dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Jun 02, 2011 at 04:49:23PM +0100, Garth N. Wells wrote:
>
>
> On 02/06/11 13:41, Anders Logg wrote:
> > Anyone using or interested in the ODE solvers should take a look
> > below.
> >
> > On Thu, Jun 02, 2011 at 02:17:17PM +0200, Benjamin Kehlet wrote:
> >> On 2 June 2011 14:02, Anders Logg <logg@xxxxxxxxx> wrote:
> >>> On Thu, Jun 02, 2011 at 01:10:01PM +0200, Benjamin Kehlet wrote:
> >>>> On 2 June 2011 11:51, Anders Logg <logg@xxxxxxxxx> wrote:
> >>>>> On Thu, Jun 02, 2011 at 10:46:29AM +0100, Garth N. Wells wrote:
> >>>>>>
> >>>>>>
> >>>>>> On 02/06/11 10:26, Anders Logg wrote:
> >>>>>>> On Thu, Jun 02, 2011 at 10:07:59AM +0100, Garth N. Wells wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On 01/06/11 23:46, Anders Logg wrote:
> >>>>>>>>> Have you checked that there is no performance penalty?
> >>>>>>>>
> >>>>>>>> I just have - evaluating a Legendgre polynomial 10k times at the same
> >>>>>>>> point is just noise with both methods (of the order 10^-5 - 10^-4 s).
> >>>>>>>
> >>>>>>> It may be noise for some applications, but not for others. I'm not
> >>>>>>> sure this is a bottle-neck for the ODE code (Benjamin will know) but
> >>>>>>> we need to evaluate Legendre polynomials of degree > 100 many times
> >>>>>>> and then it may not be noise.
> >>>>>>>
> >>>>>>
> >>>>>> For very high degree (e.g. 200) Boost is marginally faster.
> >>>>>
> >>>>> Sounds promising then.
> >>>>>
> >>>>>>>> The Boost code is slightly slower because it doesn't cache the values
> >>>>>>>> (which is nice not to do), but may be faster if the call is inlined.
> >>>>>>>> It's not possible to inline it at the moment because of clashes between
> >>>>>>>> tr1:tuple and boost::tuple (Boost bug, I suspect). Old and new are the
> >>>>>>>> same when evaluating at different points.
> >>>>>>>
> >>>>>>> Let's wait for Benjamin to comment.
> >>>>>>>
> >>>>>>
> >>>>>> The speed is about the same (with scope to improve the speed for Boost)
> >>>>>> for unique values. The caller should be responsible for caching, if
> >>>>>> desired, since it can lead to memory blow out.
> >>>>>>
> >>>>>> Legendre does not appear in the ode code. It only appears in the
> >>>>>> computation of quadrature schemes.
> >>>>>
> >>>>> True, but the quadrature schemes are used in the ode code.
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Garth
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>>> Garth
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>> Benjamin has
> >>>>>>>>> worked quite hard on optimizing some of the basic math routines (in
> >>>>>>>>> some cases by many many orders of magnitude).
> >>>>>>>>>
> >>>>>>>>> Benjamin, can you take a look that it still works?
> >>>>
> >>>> Yes, the performance seems to be about the same, but I'm unable to
> >>>> compile it with support for GMP.
> >>>>
> >>>> /usr/include/boost/math/special_functions/legendre.hpp:178:
> >>>> instantiated from ‘typename boost::math::tools::promote_args<RT,
> >>>> float, float, float, float, float>::type boost::math::legendre_p(int,
> >>>> int, T, const Policy&) [with T = __gmp_expr<__mpf_struct [1],
> >>>> __mpf_struct [1]>, Policy =
> >>>> boost::math::policies::policy<boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy,
> >>>> boost::math::policies::default_policy>]’
> >>>> /usr/include/boost/math/special_functions/legendre.hpp:185:
> >>>> instantiated from ‘typename boost::math::tools::promote_args<RT,
> >>>> float, float, float, float, float>::type boost::math::legendre_p(int,
> >>>> int, T) [with T = __gmp_expr<__mpf_struct [1], __mpf_struct [1]>]’
> >>>> /home/benjamik/fenics/dolfin-wells_gmp/dolfin/math/Legendre.cpp:42:
> >>>> instantiated from here
> >>>> /usr/include/boost/math/special_functions/legendre.hpp:167: error: no
> >>>> matching function for call to ‘pow(__gmp_expr<__mpf_struct [1],
> >>>> __gmp_binary_expr<long int, __gmp_expr<__mpf_struct [1],
> >>>> __gmp_binary_expr<__gmp_expr<__mpf_struct [1], __mpf_struct [1]>,
> >>>> __gmp_expr<__mpf_struct [1], __mpf_struct [1]>,
> >>>> __gmp_binary_multiplies> >, __gmp_binary_minus> >,
> >>>> __gmp_expr<__mpf_struct [1], __gmp_binary_expr<__gmp_expr<__mpf_struct
> >>>> [1], __mpf_struct [1]>, long int, __gmp_binary_divides> >)’
> >>>> /usr/include/bits/mathcalls.h:154: note: candidates are: double
> >>>> pow(double, double)
> >>>> /usr/include/c++/4.4/cmath:358: note:                 float
> >>>> std::pow(float, float)
> >>>> /usr/include/c++/4.4/cmath:362: note:                 long double
> >>>> std::pow(long double, long double)
> >>>> /usr/include/c++/4.4/cmath:369: note:                 double
> >>>> std::pow(double, int)
> >>>> /usr/include/c++/4.4/cmath:373: note:                 float std::pow(float, int)
> >>>> /usr/include/c++/4.4/cmath:377: note:                 long double
> >>>> std::pow(long double, int)
> >>>> [...]
> >>>>
> >>>> boost::math::legendre seems to rely on std::pow which is not
> >>>> templated, only implemented with the most common types.
> >>>
> >>> If it's not possible to make it work, we need to revert back.
> >>
> >> I don't know of any solution to this. This is the same problem that we
> >> discussed some months back (then related to Armadillo): Templated
> >> libraries which rely on non-templated  code (often old and implemented
> >> i c), so they only support the types which these underlying libraries
> >> can handle. I think the only solution here is a change in
> >> boost::math::Legendre.
> >>
> >> Of course another solution would be to split the ODE solver from
> >> Dolfin and let it continue as a separate project, and then import code
> >> from that when we are going to look at automation/generating code for
> >> time-dependent problems.
> >
> > Yes, perhaps it's time for that. Since it is going to be removed soon
> > (and replaced by code generation), the best option might be to remove
> > it before the release of 1.0.
> >
> > Are there any objections? Is anyone using the ODE solvers?
> >
>
> No objection, I think that it's a good idea.
>
> Once the ODE solvers are out, we can re-design the arbitrary precision
> interface.

Is there a need for high precision other than for the ODE solvers?
There might be a need but I don't think it's being used anywhere
except for in the ODE solvers.

--
Anders


> Garth
>
> > (They will make a comeback later in new form.)
> >
>