ffc team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

On 12 September 2011 20:56, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
> On 12 September 2011 21:36, Marie E. Rognes <meg@xxxxxxxxx> wrote:
>> On 09/12/11 20:00, Marie E. Rognes wrote:
>>>
>>> On 09/12/11 19:54, Garth N. Wells wrote:
>>>>
>>>> On 12 September 2011 18:49, Marie E. Rognes<meg@xxxxxxxxx>  wrote:
>>>>>
>>>>> On 09/12/11 19:40, Garth N. Wells wrote:
>>>>>>
>>>>>> Which compiler options did you use when evaluating the speed up?
>>>>>>
>>>>> Tested Extrapolation.h with vanilla dolfin (which is dominated by
>>>>> evaluate_basis calls). No additional compiler options set.
>>>>>
>>>>> What are the default compiler options?
>>>>>
>>>> '-g' for plain JIT, which is dead slow.  You should test with at least:
>>>>
>>>>     parameters["form_compiler"]["cpp_optimize"] = True
>>>>
>>>> in the Python code. This will use '-O2'.
>
> Isn't this limited in a way? Would it be a problem to let users do:
>
> parameters["form_compiler"]["cpp_optimize"] = '-O2 -funroll-loops'
> parameters["form_compiler"]["cpp_optimize"] = '-O3'
>
> and then perhaps let
>
> parameters["form_compiler"]["cpp_optimize"] = True
>
> default to '-O2' as we do now?
> Just a thought.
>

It's possible to set the flags,

    parameters["form_compiler"]["cpp_optimize"] = True
    parameters["form_compiler"]["cpp_optimize_flags"] = '-O3'

>>>
>>> Ok, thanks -- I'll take a closer look.
>>>
>>
>> Take a look at the attached results in old_evaluate_basis.txt (results with
>> "old" FFC),
>> and new_evaluate_basis.txt (results with "new" FFC) from running the
>> attached
>> test_evaluate_basis.py.
>>
>> Acceptable?
>
> Looks good, and the generated code is much nicer now. :)
> It could have been fun to see the impact of the '-O2 -funroll-loops'
> option on the old code, but then you'll have to switch to C++. Anyway,
> I'm quite sure that the old code will never perform as well as the new
> code even with this option.
>

I did some simple tests, and the new code outperformed -funroll-loops.
Looking at the old code, it could probably have been optimised quite a
bit.

Garth

> As you have probably found out, the generated code was simply a mirror
> of what is going on in FIAT (translated to C++).
> Perhaps there are more places where we can simplify the generated code?
>
> Another thing in relation to improving the evaluate_basis* functions
> that I have thought about is if it's really necessary to support
> derivatives of arbitrary order. If we only generate code for the first
> derivative by default (and support arbitrary derivatives by a command
> line argument) the code will be a lot simpler (easier on C++ compiler)
> and much faster irrespective of which gcc optimisation is being used.
>
> Kristian
>
>> --
>> Marie
>>
>>
>>
>>
>>
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>



-- 
Garth N. Wells
Department of Engineering, University of Cambridge
http://www.eng.cam.ac.uk/~gnw20