ffc team mailing list archive

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

On 12 September 2011 23:24, Marie E. Rognes <meg@xxxxxxxxx> wrote:
> On 09/12/11 21:56, Kristian Ølgaard 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.
>>
>>>> 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.
>>
>> As you have probably found out, the generated code was simply a mirror
>> of what is going on in FIAT (translated to C++).
>
> Yep.
>
>> Perhaps there are more places where we can simplify the generated code?
>>
>
> Probably, did you have anything particular in mind?

Not really, just wondered if you found something while poking around.

> One thing we could do to reduce code size
> would be to move the evaluation of the modal(?) basis functions
> outside of the switch and just do the vector-vector product inside.

Yes, I thought about reusing code which is identical for dofs, but as
I don't really know how those values are being generated I can't tell
which parts of the code are guaranteed to be identical _always_.
So, I would have to implement some ad hoc look up/mapping of values
unless you can explain what is going on :)
And of course we still need to consider mixed elements etc.

> Also, I think it would significantly speed up evaluate_basis_all,
> if we just did the evaluation of the modal basis functions once,
> and then the vector-vector product 'local_dimension'-times.

> Actually, I plan on doing that unless anyone protests vehemently.
> The reduction in generated code from the one should more or less
> counteract the increase in generated code from the other.

The code for the evaluate_*_all functions was just a simple way of
getting something working fast, I don't use it (others might), so feel
free to improve the code.

Kristian

>> 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.
>>
>
> Sound neat to me.
>
> --
> Marie
>
>> Kristian
>>
>>> --
>>> Marie
>>>
>>>
>>>
>>>
>>>
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>
>