ffc team mailing list archive

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

On 25/04/11 22:08, Anders Logg wrote:
> On Mon, Apr 25, 2011 at 07:40:21PM -0000, Garth Wells wrote:
>> On 25/04/11 20:00, Johan Hake wrote:
>>> On Monday April 25 2011 11:26:36 Garth Wells wrote:
>>>> On 25/04/11 18:51, Anders Logg wrote:
>>>>> On Mon, Apr 25, 2011 at 05:11:41PM -0000, Garth Wells wrote:
>>>>>> On 25/04/11 17:53, Johan Hake wrote:
>>>>>>> On Monday April 25 2011 08:59:18 Garth Wells wrote:
>>>>>>>> On 25/04/11 16:47, Johan Hake wrote:
>>>>>>>>> Commenting out the cache is really not a fix. The problem is within
>>>>>>>>> dolfin. Isn't there another way to deal with this?
>>>>>>>>
>>>>>>>> It is a fix if the cache isn't needed.
>>>>>>>
>>>>>>> Sure.
>>>>>>>
>>>>>>>>> First: How much penalty are there with a disabled memory cache. Maybe
>>>>>>>>> the problem isn't that bad?
>>>>>>>>
>>>>>>>> I don't get the point of this cache. The way it is now, a form is only
>>>>>>>> preprocessed if it hasn't already been preprocessed, which seems ok to
>>>>>>>> me. The old code tried to avoid some preprocessing, but it was highly
>>>>>>>> dubious and I doubt that it was effective.
>>>>>>>
>>>>>>> I think the preprocessing stage actually do take some time. AFAIK the
>>>>>>> preproces stage essentially do two things. It creates a canonical
>>>>>>> version of the Form so two Forms that are the same, but constructed at
>>>>>>> different times are beeing treated equal wrt form generation. Then are
>>>>>>> DOLFIN specific guys extracted. I am not sure what takes the most
>>>>>>> time. We should probably profiel it... But if it is the latter we
>>>>>>> could consider putting another cache in place which is more robust wrt
>>>>>>> changing DOLFIN objects.
>>>>>>
>>>>>> It should be easy to avoid the overhead of preprocessing by keeping the
>>>>>> object in scope. If the object changes, the only robust way to make sure
>>>>>> that the form is the same as one in the cache is to compare all the
>>>>>> data. This requires preprocessing the form, which then defeats the
>>>>>> purpose of a cache. It may be possible to add a lightweight preprocess
>>>>>> to UFL, but I don't think that it's worth the effort or extra
>>>>>> complication.
>>>
>>> I think a light weight version might be the way to go. This is then stored in
>>> memory cache. If we are able to strip such a form for all DOLFIN specific
>>> things we would also prevent huge memory leaks with mesh beeing kept.
>>>
>>> Then we always grab DOLFIN specific data from the passed form instead of
>>> grabbing from the cache. Not sure how easy this will be to implement, but I
>>> think we need to explore it, as the DOLFIN specific part of the form really
>>> has nothing to do with the generated Form.
>>>
>>> Martin:
>>> Why is it important to have the _count in the repr of the form? I guess that
>>> is used in ufl algorithms? Would it be possible to include a second repr
>>> function, which did not include the count? This would then be used when the
>>> signature is checked for. We could then use that repr to generate a form which
>>> is stored in the memory cache. This would then be tripped for any DOLFIN
>>> specific objects. This should work as the _count attribute has nothing to do
>>> with what code gets generated, but it is essential for internal UFL
>>> algorithms, right?
>>>
>>>>> I'm not very happy with this change.
>>>>
>>>> The bright side is that slow and correct is a better starting point than
>>>> fast but wrong ;).
>>>>
>>>> An easy fix is to attach the preprocessed form to a Form object. This
>>>> would work robustly if we can make forms immutable once they've been
>>>> compiled. Is it possible to make a Python object immutable?
>>>
>>> We can probably overload all setattribtue methods which prohibits a user to
>>> write to these but it might not be possible to prohibit a user to change
>>> attributes on instances owned by the Form. I guess this is similare to the
>>> difficulties of preserving constness in C++, but I think it is even harder in
>>> Python.
>>>
>>
>> What if we have the FFC jit compiler return the preprocessed form, and
>> inside dolfin.Form simply do
>>
>>     class Form(cpp.Form):
>>         def __init__(self, form, . . .. )
>>         ....
>>
>>         (...., preprocessed_form) = jit(form, . . . . )
>>
>>         form = preprocessed_form
>>
>>         .....
>>
>> This way, form will have form_data, and the FFC jit function will know
>> not to call ufl.preprocess.
> 
> Here's another strange thing. In the JITObject class, we have two
> functions: __hash__ and signature. As far as I understand, the first
> is used to located objects (generated code/modules) in the Instant
> in-memory cache, while the second is used for the on-disk cache.
> 
>>From some simple tests I did now, it looks like the __hash__ function
> does not need to any significant speedup. The JIT benchmark runs just
> as fast if I call signature from within __hash__.
> 
> Furthermore, the __hash__ function must also be broken since it relies
> on calling id on the form.
> 
> Ideally, we should get Instant to handle the caching, both in-memory
> and on-disk, by providing two functions __hash__ (fast, for in-memory
> cache) and signature (slow, for on-disk cache).
> 
> Since __hash__ cannot call id, it must be able to attach a unique
> string to the form (perhaps based on an internal counter in FFC).
> My suggestion would be to add this to UFL, something like set_hash
> and hash (which would return None if set_hash has not been called).
> If Martin does not like that, we should be able to handle it on the
> DOLFIN side.
> 
> So in conclusion: no in-memory cache in FFC (handled by Instant) and
> FFC attaches a hash to incoming forms so that Instant may recognize
> them later.
> 

The code that I disabled was caching preprocessed forms, so I don't see
how this can be handled by Instant.

Garth

> Maybe even better: Instant checks whether an incoming object has a
> set_hash function and if so calls it so it can recognize objects it
> sees a second time.
> 
> I'm moving this discussion to the mailing list(s).
> 
> --
> Anders
> 
> _______________________________________________
> Mailing list: https://launchpad.net/~ffc
> Post to     : ffc@xxxxxxxxxxxxxxxxxxx
> Unsubscribe : https://launchpad.net/~ffc
> More help   : https://help.launchpad.net/ListHelp