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. Garth > It would work with Instant since Instant calls the __hash__ function > to see whether the form is in the memory cache. Only after the __hash__ > function has been called does it call the signature function (if I > remember correctly). > > -- > 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
