ufl team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Apr 25, 2011 at 10:56:25PM +0100, Garth N. Wells wrote:
>
>
> On 25/04/11 22:48, Anders Logg wrote:
> > On Mon, Apr 25, 2011 at 10:41:58PM +0100, Garth N. Wells wrote:
> >>
> >>
> >> On 25/04/11 22:33, Anders Logg wrote:
> >>> On Mon, Apr 25, 2011 at 10:26:18PM +0100, 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.
> >>>
> >>> The point would be that one could check that "hash" of the form (some
> >>> unique string) instead of computing the signature which involves
> >>> preprocessing the form.
> >>>
> >>
> >> How would the hash be computed? To check if the mesh has changed, my
> >> limited understanding is that the entire object would have to be
> >> serialised, and then a hash computed. How expensive is that?
> >>
> >> The issue that I ran into was not related to signatures. It was related
> >> to the non-UFL data that is attached to arguments.
> >
> > The hash would be unique to each form. It could just be a counter
> > value and the counter would be increased inside Instant for each
> > object it gets as input.
>
> But how does Instant know if a form is new? I also don't see why Instant
> should need to know if the mesh associated with a form has changed, but
> is for the rest the same. Wouldn't Instant need to be DOLFIN-aware?

The hash() function would play the same role as the id() function
before with the difference that we can't get the same id for a new
form as for an old form that's gone out of scope.

Instant should not need to know anything it just does this:

   check if object has a set_hash() function
   if so, calls hash() to get the hash value
     checks the cache for that hash value
   if not, assign unique value by calling set_hash on the object

We would need to make sure from the DOLFIN side that when we change a
Form, we also change the hash value (for example by setting it to
None) which would trigger the Instant disk cache.

--
Anders