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> On 07 Jun 2016, at 11:32, Guille Polito <[email protected]> wrote:
>
> Hi All,
>
> Since this morning, in Pharo #60065, Ephemeron support is in the image. Most
> of the changes are infrastructural, so far transparent for the users. It is
> important to notice that even while the support is there, it is not enabled
> by default. Also, this required changes in the virtual machine that are not
> yet distributed everywhere. For the ones that would like more detail, I
> invite you to read the following :)
>
> * On the infrastructure side
> - There is support to create Ephemeric classes and load them from monticello
> - There is a new finalization mechanism (by default disabled) that will
> process Ephemerons using a finalization queue. This will avoid scanning
> collections in look for weak objects to finalize ,as it happens now with the
> WeakDependent mechanism in WeakArray.
> - System-Finalization features two new classes *Ephemeron* and
> *EphemeronRegistry*. For the ones that want more details on Ephemerons, you
> can read the associated paper [1], or the class comment of Ephemeron:
>
> I represent ephemeric key-value objects. Ephemerons are key-value objects
> (subclasses of Association) with special semantics during garbage collection.
> My special behavior can resumed as follows:
>
> - The garbage collection will iterate my instances only if the key is not
> referenced strongly by another object.
> - Then, if no strong references to the key are found, then the values of this
> ephemeron are hold weakly.
> - Otherwise, the values are hold strongly.
>
> In this implementation, an Ephemeron can hold more than one value, which are
> all treated in the same manner. This ephemeron instance knows its container,
> which allows the ephemeron to remove itself from a container (such as a
> Dictionary) upon finalization.
>
> !! Example usages
>
> In general terms, do not use myself directly. Use instead an Ephemeric
> container like EphemeronRegistry. An Ephemeron registry will guarantee the
> collection of keys and values of the object inside the Ephemeron.
>
> Otherwise, if you want to use it, you can create an Ephemeron as any
> association:
>
> ephemeron := Ephemeron key: aKey value: aValue.
> ephemeron container: aContainer.
>
> !! Ephemeron Finalization
>
> When an ephemeron's key is hold strongly just by the ephemeron itself, the
> Ephemeron will be mourned (finalized). That means that the VM will:
> - put the Ephemeron in the mourning queue waiting for the image to take care
> of mourning
> - make the Ephemeron non ephemeric. That is, the ephemeron instance cannot be
> reused.
>
> On the image side, the finalization process will send the message #mourn to
> an Ephemeron. #mourn will #finalize the Ephemeron's key, and remove the
> Ephemeron from it's container to allow its collection during a subsequent
> garbage collection.
>
> !! More Documentation
>
> You can read the associated paper to understand better the semantics of
> ephemerons:
>
> [1]Ephemerons: A New Finalization Mechanism. Barry Hayes. OOPSLA '97
>
>
> - WARNING: to be able to use ephemerons, you need to use the *latestVm*
> that has several fixes for making ephemerons work, and you need to enable
> ephemerons on the image side by evaluating:
>
> Smalltalk supportsQueueingFinalization: true.
>
> - With latest vm and ephemerons enabled, tests should be green, otherwise
> they are skipped
>
> <PharoScreenshot.1.png>
>
> * From the user point of view:
>
> - The Weak registries were not yet migrated to the new finalization
> mechanism.
> - We expect nothing will change from the user point of view. Just less
> memory leaks.
>
> * Next steps (in order)
> 1) Bless the latest vm as stable
> 2) Enable queueing finalization by default
> 3) Replace Weak Registry by Ephemeron Registry.
>
>
> Informed by: Guille
