dolfin team mailing list archive

[Benjamin Kehlet <benjamik@xxxxxxxxx>]

On 2 June 2011 16:31, Garth N. Wells <gnw20@xxxxxxxxx> 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.
>>>>>
>>>>> --
>>>>> Anders
>>>>>
>>>>>
>>>>>> 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.
>
>> (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.

Agree, but I guess you don't need to support high precision?

>
> Garth
>
>> Benjamin
>>
>>>
>>> Garth
>>>
>>>
>>>> Benjamin
>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>> _______________________________________________
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>>>>>
>>>>> _______________________________________________
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>>>>>
>>>
>>>
>
>