dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Jun 02, 2011 at 03:31:31PM +0100, Garth N. Wells wrote:
>
>
> On 02/06/11 15:21, Benjamin Kehlet wrote:
> > On 2 June 2011 14:59, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
> >>
> >>
> >> On 02/06/11 12:10, 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.
> >>>
> >>
> >> Looks like some tweaks are required to work with GMP:
> >>
> >> http://www.boost.org/doc/libs/1_43_0/libs/math/doc/sf_and_dist/html/math_toolkit/using_udt/use_mpfr.html
> >
> > That's not a bad solution, but it requires changing the
> > multi-precision type from mpf (provided by GMP) to mpfr (which is a
> > library that extends the floating point functionality in GMP). For
> > floating-point arithmetic MPFR is much better than pure GMP. I think
> > CGAL depends on MPFR, so it wouldn't even introduce new dependencies.
> > The problem is that MPFR doesn't ship with a C++-wrapper (as opposed
> > to GMP). Although several independent wrappers exists, none of them
> > are avalilable in Debian/Ubuntu through apt. The one Boost requires is
> > not updated since 2008 (MPFR has gone from version 2.3 to 3.0.1 since
> > then).
> >
>
> I've just plonked a copy of gmpfrxx in the DOLFIN dirs to test - it's
> licensed under GPL.

That won't work. We need LGPL.

--
Anders


> > (Another option would be to take the same approach as Boost ourself:
> > Implement the few functions that are required (pow() plus possibly a
> > few more) and place it in the global namespace before including
> > boost::mat::legendre), but GMP does not provide pow() when the
> > exponent is a floating point number, so this is not straight forward
> > without switching to MPFR).
> >
> > So I guess the question is whether we want to switch to MPFR now, to
> > get rid of the few lines of code in Legendre.cpp (which performs
> > reasonably well), when the code is likely to be thrown out pretty soon
> > anyway. I vote for "no", but I have no problems with moving the entire
> > ODE solvers to a separate project, then adding it back (without
> > supporting extended precision) later in the form of code generation
> > for time dependent problems.
> >
>
> There are a few issues here - even if the ODE code is moved out, I think
> that we should retain the polynomial and quadrature code in DOLFIN.
>
> Garth
>
> > Benjamin
> >
> >>
> >> Garth
> >>
> >>
> >>> Benjamin
> >>>
> >>>>>>>>
> >>>>>>>
> >>>>>>> _______________________________________________
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> >>>>
> >>>> _______________________________________________
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> >>>>
> >>
> >>
>