ffc team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Monday April 25 2011 14:44:23 Garth N. Wells wrote:
> On 25/04/11 22:29, Johan Hake wrote:
> > I am working on a simple solution, where we store everything in the
> > original ufl form.
> > 
> > I might have something soon.
> 
> OK.
> 
> I played around with something like this, but had some issues when the
> form changes (i.e. a += . . . ) after being preprocessed.

Oups! I guess my "fix" wont get this either... Maybe we need to destroy the 
form_data object whenever += ... are called?

Johan

> Garth
> 
> > Johan
> > 
> > On Monday April 25 2011 14:26:18 Garth N. Wells wrote:
> >> 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
> >>> 
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