On 27 July 2012 20:05, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>
>
> On Fri, Jul 27, 2012 at 11:16 AM, Nicolas Cellier
> <nicolas.cellier.aka.nice at gmail.com> wrote:
>>
>> Ah OK, I was not concentrated enough when I read Eliot post :)
>>
>> Eliot, this is not a bug, and normal code would statistically create
>> other objects in between...
>> But isn't there a risk  that tools like Fuel populate a small
>> collection of simple objects (say an IdentitySet of Symbol), sharing
>> same identityHash for each pair?
>> It would noticeably increase the number of collisions well before the 4096
>> wall.
>
>
> Yes, but in practice no one's complained.  ANyway, look at both of these:
>
> Array new: 512 streamContents: [ :stream |
>         1 to: 4096 do: [ :e | stream nextPut: Object new identityHash ] ]
> "repeats every two objects"
>
> Array new: 512 streamContents: [ :stream |
>         1 to: 4096 do: [ :e | stream nextPut: Array new identityHash ] ]
> "repeats every 4 objects"
>
> I'll change the scaling so that the last one does not repeat.  So lets leave
> the test unchanged and put up with the failures until new VMs are in use.

In which case let's make it an expected failure, rather, and I've made
a note on the Squeak 4.4 page to un-expect it as part of prepping for
4.5.

frank

>> Nicolas
>>
>> 2012/7/27 Bert Freudenberg <bert at freudenbergs.de>:
>> >
>> > On 27.07.2012, at 05:18, Nicolas Cellier wrote:
>> >
>> >> 2012/7/27 Frank Shearar <frank.shearar at gmail.com>:
>> >>> On 26 July 2012 22:35, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>> >>>>
>> >>>>
>> >>>> On Thu, Jul 26, 2012 at 3:59 AM, Levente Uzonyi <leves at elte.hu>
>> >>>> wrote:
>> >>>>>
>> >>>>> On Thu, 26 Jul 2012, Frank Shearar wrote:
>> >>>>>
>> >>>>>> In a freshly updated trunk we have 3184 out of 3157 tests passing.
>> >>>>>> We
>> >>>>>> have 16 expected failures, and 11 failures, the latter being:
>> >>>>>> * BecomeTest>>#testBecomeIdentityHash
>> >>>>>
>> >>>>>
>> >>>>> This is failing due to a VM bug. There's a fix for it somewhere, but
>> >>>>> it
>> >>>>> seems like it's not integrated into Cog yet. Explore this to see
>> >>>>> that two
>> >>>>> consecutive objects share the same identityHash:
>> >>>>>
>> >>>>> Array new: 10 streamContents: [ :stream |
>> >>>>>        1 to: 10 do: [ :e | stream nextPut: Object new identityHash ]
>> >>>>> ]
>> >>>>
>> >>>>
>> >>>> IMO this isn't a bug.  The identity hash changes at least every other
>> >>>> object.  Hashes don't have to be unique.  But they do have to be
>> >>>> well-distributed.  With 12 bits of identityHash Cog does fine basing
>> >>>> its
>> >>>> identityHash on the allocation pointer.  The above will wrap around
>> >>>> after
>> >>>> 8192 allocations, and provide 4096 distinct hashes (the maximum
>> >>>> available).
>> >>>> So the test needs rewriting to be more statistical.  The rationale
>> >>>> for this
>> >>>> is to speed up allocation.  Instead of a read-modify-write cycle to
>> >>>> turn the
>> >>>> crank of a pseudo-random generator there's a masking of the
>> >>>> allocation
>> >>>> pointer, which has to be read anyway to allocate an object.  BTW, the
>> >>>> *right* way to implement this is to lazily allocate hashes, but for
>> >>>> that
>> >>>> there needs to be a flag (e.g. an identityHash of 0) to mark an
>> >>>> object as
>> >>>> not yet having a hash but existing Squeak images (because of the old
>> >>>> definition) use 0 as a valid hash, so lazy hashes requires either a
>> >>>> header
>> >>>> bit (not enough of those) or an image change (which is my plan, as
>> >>>> part of
>> >>>> the new object representation).
>> >>>
>> >>> If it's not a bug, let's nuke the test. We need to get to a position
>> >>> where we have a green light.
>> >>>
>> >>> frank
>> >>>
>> >>
>> >> If I understood Eliot correctly, it would suffice to keep a pointer
>> >> alive to the created objects...
>> >>
>> >> preserveObjectsFromGarbageCollection := IdentitySet new.
>> >> Array new: 10 streamContents: [ :stream |
>> >>         1 to: 10 do: [ :e | stream nextPut:
>> >> (preserveObjectsFromGarbageCollection add: Object new) identityHash ]
>> >> ]
>> >>
>> >> Nicolas
>> >
>> >
>> > Makes no difference. GC does not happen after each allocation. Here's
>> > one that would work with Cog because each allocation is larger:
>> >
>> > (1 to: 10) collect: [ :e | (Array new: 4) identityHash ]
>> >
>> > But as Eliot said the test is somewhat meaningless in its current form.
>> >
>> > - Bert -
>> >
>> >
>> >
>>
>
>
>
> --
> best,
> Eliot
>
>
>
>