[Vm-dev] Re: [squeak-dev] Image Segment semantics and weakness

[Eliot Miranda]

Hi Andreas,

On Sun, Oct 19, 2014 at 6:23 PM, Andres Valloud <
avalloud at smalltalk.comcastbiz.net> wrote:

>
> At first glance this sounds ok to me... however, I'd feel better if GC
> cleaned up the recently loaded segment before anything in the segment can
> become active or receive a message or be visible elsewhere.  If that does
> not happen, there will be a window of opportunity for really weird stuff to
> occur.
>

Oh thats a very good point.  I new it was wise to ask :-)  I'll sleep on
this but at the moment I'm close to being convinced that there either needs
to be the equivalent of a GC immediately on loading, or the equivalent of a
GC on creating the segment, so as not to include category three objects in
the segment.  Damn, I suspect that's tricky to implement efficiently ;-)

For example, consider what happens if you have an ephemeron reachable from
> a weak array, the freshly loaded segment turns the weak array into a strong
> array (not a far fetched example --- this is done in practice for
> performance), and now you have an ephemeron that can go ahead and do random
> stuff in finalization.  But how would that ephemeron know it is no longer
> living in the image where it was instantiated?
>
> Assuming I am still understanding image segments correctly, the more I
> think about them, the more I start liking loadable modules from declarative
> specifications --- in that other world, such weird brain surgery stuff is
> impossible.  Nevertheless, I understand you may be in a situation where you
> have to support existing features.


Yes, image segments are used heavily in etoys (project saving and loading)
and Terf (exchanging behavior and initial state between replicas).   So
Spur does need to support image segments if it is to be easy to adopt.  And
that's always been an important criterion for Cog.

On 10/19/14 18:01 , Eliot Miranda wrote:
>
>> - /not/ reachable form the system roots, /not/ reachable from the
>> segment roots via strong pointers
>>
>> Should this last category be included or excluded from the segment?  I
>> think that it makes no difference, and excluding them is only an
>> optimization.  The argument is as follows.  Imagine that immediately
>> after loading the image segment there is a garbage collection.  That
>> garbage collection will collect all the objects in the last category as
>> they are only reachable from the weak arrays in the segment.  Hence we
>> are free to follow weak references as if they are strong when we create
>> the image segment, leaving it to subsequent events to reclaim those
>> objects.
>>
>> An analogous argument accounts for objects reachable from ephemerons.
>> Is my reasoning sound?
>>
>


-- 
best,
Eliot
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