HI Nicolas,

On Tue, Mar 27, 2018 at 3:33 AM, Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> wrote:

> Hi Bob,
> yes, you force me to rethink. The way mark and sweep GC works, cycles in
> object graphs do not matter.
> As long as the objects in the cycles are only pointed to weakly from
> roots,
>
> | value weak array2 |
> value := Array new.
> weak := WeakArray with: value.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 1. "Not yet reclaimed, thisContext
> points to it thru value variable"
> value := nil.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 0. "This time, it is reclaimed"
>
> "Example of cycle"
> value := Array new: 1.
> array2 := Array with: value.
> value at: 1 put: array2.
> weak := WeakArray with: value.
> array2 := nil.
> value := nil.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 0. "The value is reclaimed despite
> the cycle"
>
> It's then VW that behaves strangely: I did not find any pointer to the
> class nor its metaclass apart themselves
> (I took care to not add the metaclass to metaclass superclass subclasses,
> because those are not weak).
> This has obscured my mind.
> Sorry for the noise.
>

Is there difference between 32-bit and 64-bit VisualWorks?  Like Spur,
64-bit VW uses a class table, whereas 32-bit VW has class pointers in
object headers.  So if you see the behavior in one but not the other that
could point to a bug in tracing the class table (and its likely my bug).

2018-03-26 23:15 GMT+02:00 Bob Arning <arning315 at comcast.net>:
>
>> Other that the WeakValueDictionary, is there anything referencing either
>> the class or metaclass? If not, then they would be collected. Perhaps VW is
>> saving a reference elsewhere?
>>
>> On 3/26/18 3:07 PM, Nicolas Cellier wrote:
>>
>> Hi all,
>> while porting MatFileReader to Squeak, I encountered this questionable
>> behaviour:
>>
>> I create classes on the fly for holding Matlab struct().
>> I want those classes to be garbage collected when no more used.
>>
>> If I store the classes in a WeakArray, then this should normally not work:
>> I mean that the classes should not be reclaimed.
>> Why?
>> Because the metaclass points strongly to the class (via thisClass
>> variable).
>> And the metaclass can't be reclaimed either, because it still has an
>> instance.
>> That's what happens in Visualworks, classes put in a WeakArray are never
>> reclaimed.
>> Fortunately, there is some Ephemeron support in VW to work around this
>> (such loop are detected by ephemerons, and strong references by some object
>> ephemerely pointed to does not count).
>>
>> In Squeak, I used a WeakValueDictionary, and to my surprise, discovered
>> that the classes were reclaimed.
>> Is this a bug?
>>
>> Nicolas
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>
>
>


-- 
_,,,^..^,,,_
best, Eliot
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