2010/1/13 Eliot Miranda <[email protected]>: > > > On Wed, Jan 13, 2010 at 12:45 PM, Igor Stasenko <[email protected]> wrote: >> >> 2010/1/13 Martin McClure <[email protected]>: >> > Eliot Miranda wrote: >> >> A *much* better way to implement this is to support immutability in the >> >> VM (I know, I know, but all the code is available in the Newspeak VM), >> >> mark objects one wants to mark as dirty as immutable, catch the >> >> NoModificationError exception, and proceed after having made the object >> >> mutable and marked it dirty. No creating hidden classes, no trying to >> >> get change class to work for compact classes, etc. Just a simple >> >> VM-implemented write barrier that can also be used for a host of other >> >> things (object-database mapping, debugging, immutable literals etc). >> > >> > Since object dirtying is at the core of the product I work with, and >> > I've worked extensively with both methods of implementing write >> > barriers... >> > >> > I very strongly agree with Eliot. *So* much nicer a way to do this. >> > >> >> It could be more efficient, in respect that you don't need to create >> shadow classes. >> But triggering exception leads to stack unwinding to find a handler, >> which inevitably leads to deoptimizing all contexts.. > > Uh, not so. In VW and Cog examining a context does _not_ deoptimize a > context. A context will be "deoptimized" (actually converted to a vanilla > heap context) only if you write to other than its stack contents or sender. > i.e. a context's frame is only discarded if > - one assigns to any of its instance variables other than sender > - the stack zone runs out of room for new frames and a stack page must be > vacated, causing all frames on that page to be converted to stack contexts > So exception handling does *not* usually involve converting contexts. It > would be very slow if it did. > Ah, cool.. So, even when debugging (and hence accessing context state) does not leads automatically to deoptimization?
>> >> and if stack depth is high >> (between point of writing attempt and hook, where magma will handle >> exception), this will be much slower >> than WriteBarrier implementation, described by Cris, >> which checking the value in-place, without the need of touching >> exception machinery. > > I disagree strongly. Martin's experience with Gemstone (and Gemstone's > experience) covers over twenty years and the VM-supported immutability > implementations (first in VisualAge IIRC) of Gemstone are far more > performant and less problematic than the code-rewriting implementations > similar to Magma. Martin really knows what he's talking about. I'm not saying anything against immutability, but capturing the object state change seem will be less efficient. By reading Cris description i imagine that WriteBarrier rewriting a code like: SomeClass>>foo: newValue foo := newValue to: SomeClass*>>foo: newValue | old | old := foo. foo := newValue. old == foo ifFalse: [ Magma markAsDirty: self ] now compare performance of evaluating: [[[[[[ self foo: 5 ]]]]]] with: [[[[[[ self foo: 5 ]]]]]] on: ImmutableException do: [:ex | Magma markAsDirty: ex receiver. ex receiver beMutable. ex resume ] where [[[[[]]]]]] is a call stack, which can be deeeeep. >> >> Just 2 cents. >> >> > Regards, >> > >> > -Martin >> > >> > >> > _______________________________________________ >> > Pharo-project mailing list >> > [email protected] >> > http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project >> > >> >> >> >> -- >> Best regards, >> Igor Stasenko AKA sig. >> >> _______________________________________________ >> Pharo-project mailing list >> [email protected] >> http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project > > _______________________________________________ > Pharo-project mailing list > [email protected] > http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project > -- Best regards, Igor Stasenko AKA sig. _______________________________________________ Pharo-project mailing list [email protected] http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project
