On 25/04/11 23:19, Anders Logg wrote: > On Mon, Apr 25, 2011 at 03:14:45PM -0700, Johan Hake wrote: >> On Monday April 25 2011 15:04:43 Anders Logg wrote: >>> 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. >> >> Sounds complicated... > > I think it sounds very easy. Everything we need is there: Instant > already has memory and disk cache. We just need to provide the proper > input. > >> Now the preprocessed form is stored in the original form. This will never >> change. Whenever a form does not go out of scope the preprocessed form will >> live. >> >> Also Martin made it impossible to change a form without returning a new >> instance. This prevents any changing of the original form while keeping a >> preprocesses form attached to it. >> >> If a form has a preprocessed form that will be used for code generation. The >> preprocessed form will be used in instants memory cache. The preprocessed >> form >> has nothing to do the any DOLFIN objects that comes with the original form, >> such as mesh, expressions and such. >> >> Anything I have missed? > > What about the __hash__ function in jitobject.py? It still calls > id(). Isn't that a problem? >
Yes, but we're trying to tackle one issue at a time. First avoiding necessary calls to preprocess. Note that there will be common usage cases in which a call to preprocess in needed, but for which the compiled from can be extract from the Instant cache. Garth > -- > 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
