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. 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@lists.launchpad.net Unsubscribe : https://launchpad.net/~ffc More help : https://help.launchpad.net/ListHelp
