[Pharo-project] [Ann] Ephemerons for Cog

Eliot Miranda eliot.miranda at gmail.com
Tue May 24 05:28:04 CEST 2011

On Mon, May 23, 2011 at 5:47 PM, Alexandre Bergel
<alexandre.bergel at me.com>wrote:

> What is Ephemerons?

Smart associations that can be used to implement per-object finalization.
 An ephemeron holds onto some object via its key field in a quasi-weak
manner such that the garbage collector will only consider the key live if it
can be reached from the roots and not reachable by the ephemeron's other
inst vars.  If the only references to objects referred to by ephemeron keys
are through the other fields of ephemerons with otherwise unreachable keys
then the these objects are collectable.  The garbage collector works in two
phases.  First it traces all objects from the roots, topping the trace at
any ephemeron whose key is unreachable.  The second phase then loops until a
fixed point is reached; all ephemerons with unreachable keys are added to
the finalization queue and the objects reachable from the key and the
ephemeron's other fields are traced.  Any ephemerons with unreached keys are
added to the finalization queue.  This phase continues until no more
ephemerons are found.  Then, if the finalization queue is non-empty, the GC
signals the rest of the system that there are finalizable ephemerons.  The
image then takes over, drain ing the ephemerons in the queue and sending
e.g. mourn to each ephemeron which can then send finalize to its key and
remove itself from any dictionaries in which it exists, allowing it and its
key to be garbage-collected after finalization.

What's the rationale?  Often one wants to associate state with finalizable
objects, such that that state refers back to the finalizable obejcts.  The
classic examples are views in MVC.  RThe views are dependents of some model
and we may want want the model to be finalized when no object (including
dependents) refer to the model.  But we don't want references from the
dependents to keep the object alive.  But if the dependents are stored in
some global dictionary from model to sequence of dependents then the
reference from the global dictionary keeps both the model and the dependents
alive.  If we replace the association with a weak key association then the
references from the dependents back to the model keep the model alive.  But
with ephemerons there is an implicit switch, implemented by the GC, that
causes the system to exclude references from the transitive closure of the
dependents unless the model is reachable from the roots, excluding
references from ephemeron keys.

So ephemerons provide a mechanism for providing per-instance finalization
and property lists (dependents being a special case of property list).  They
allow the system to detect when an object is reachable from the roots,
excluding an arbitrary set of references (reachable from an ephemeron's
other fields) which do not count towards an object being considered live.
 They allow for finalization to be managed by the system, above the VM,
while the VM's job is just to determine reachability and liveness.

make sense?  clear as mud?


> Alexandre
> Le 23 mai 2011 à 20:43, Igor Stasenko <siguctua at gmail.com> a écrit :
> > I felt a bit hacky today and decided to implement Ephemerons.
> >
> > I tried to run it on both StackVM and Cog VMs,
> > it seems to work fine (at least all 4 tests, which my brain were
> > capable to produce,  are green. ;)
> >
> >
> > You can find the VM code on SqS/VMMaker in package:
> >  VMMaker-oscog-IgorStasenko.67
> >
> > Also, i created a separate issue on Cog issue tracker to track it.
> > http://code.google.com/p/cog/issues/detail?id=44
> >
> > The implementation may contain a bugs. So, it needs review and testing.
> > Now i would like all interested parties to give a feedback and help
> > integrating it into our production images.
> >
> > --
> > Best regards,
> > Igor Stasenko AKA sig.
> >
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