While I as a big fan of imageSegment and proposed to mariano to work on
imageSegment2 (it was the original idea for his phd)
he convinced us that imagesegment were not worth their complexity.
So why do you want to have imageSegment?
Stef
On 20/10/14 03:01, Eliot Miranda wrote:
Hi All,
I want to check my understanding of reference semantics for image
segments as I'm close to completing the Spur implementation.
Specifically the question is whether objects reachable only through
weak pointers should be included in an image segment or not.
Remember that an image segment is created from the transitive closure
of an Array of root objects, the /segment roots/. i.e. we can think of
an image segment as a set of objects created by tracing the object
graph from the segment roots.
The segment always includes the segment roots. Except for the roots,
objects are excluded from the segment that are also reachable form the
roots of the system (the /system roots/, effectively the root
environment, Smalltalk, and the stack of the current process).
Consider a weak array in the transitive closure that is not reachable
from the system roots, and hence should be included in the segment.
Objects referenced from that weak array may be in one of three categories
- reachable from the system roots (and hence not to be included in the
segment)
- /not/ reachable form the system roots, but reachable from the
segment roots via strong pointers (and hence to be included in the
segment)
- /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
