[Pharo-project] Working with weak announcements...

Igor Stasenko siguctua at gmail.com
Tue Feb 15 17:46:59 CET 2011


On 15 February 2011 10:00, Stéphane Ducasse <Stephane.Ducasse at inria.fr> wrote:
> Igor
>
> now the problem is how much time do you expect it may take to introduce ephemerons?
> Else having well documented patterns is also important.


I think it is relatively easy to modify VM , since it touching only GC
related stuff.
And also there is already an implementation somewhere made by Ian..
which can be used as reference.

>
> Stef
>
>
> On Feb 14, 2011, at 10:33 PM, Igor Stasenko wrote:
>
>> On 14 February 2011 21:52, Esteban Lorenzano <estebanlm at gmail.com> wrote:
>>> Hi,
>>> I'm working with weak announcements, trying to make it work, and I have a problem in #on:do: protocol (or #when:do:)
>>> I try to explain:
>>>
>>> This method receives a block, not an object/selector, so I can't create a WeakMessageSend which is the appropriate message to handle in other cases.
>>> Well, the real question is... how can I produce a "Weak BlockClosure reference" who can die if receiver dies?
>>> I tried some hacks (really ugly hacks, btw), but fail completely.
>>> Any idea?
>>>
>>
>> Don't even try doing it until we will have ephemerons support at VM side :)
>>
>> I explained this few days ago to Stephane, and also i mentioned about
>> this maybe half of year ago when i took a look at announcements.
>>
>> With ephemerons you can have:
>> - reference a block strongly only if subscriber , which held weakly is alive.
>>
>> Once subscribed dies, you start referencing block weakly.
>>
>> In terms on GC it means, that during mark phase, you are visiting a
>> references which reachable through block closure only if your
>> subscriber are already marked as 'live'.
>>
>> So, this means that you can do weak subscriptions like following:
>>
>>
>> self weakOn: Foo do: [:announcement | self bar ].
>>
>> A 'self' is a subscriber. But if you create a subscription from a pair of:
>> <subscriber> and <block>
>> without using ephemerons you should make sure that reference to
>> subscriber are not reachable through given block.
>> Otherwise, a weak reference become strong one, because you referencing
>> a block strongly and therefore subscriber too,
>> since block referencing subscriber through one of its vars.
>>
>> So, without ephemerons you have to invent some workarounds by
>> rewriting above code with something like:
>>
>>
>> self weakOn: Foo do: [:announcement :myself |
>> myself bar
>> ].
>>
>> in the above, a block no longer referencing receiver (a subsriber),
>> and instead it will be passed as extra argument
>> to the block.
>>
>> ( except that still, you will have a problem, if closure keeps a
>> reference to home context, which in own turn referencing subscriber
>> through context's receiver.. so without emphemerons you should be
>> really careful)
>>
>> And of course, i think that the usefulness of Announcements is quite
>> limited without proper weak subscription support,
>> because weak subscriptions makes things a lot more easier: you don't
>> have to explicitly unsubscribe some object, once you no longer using
>> it ,
>> which as to me means that about 99% of all subscriptions in system
>> will be weak ones , simply to prevent potential problems with hanging
>> references, and because you don't have to worry about unsubscribing.
>>
>> And of course, without weak-block subscriptions, the ease of use of
>> subscription mechanism is a bit limited.
>> So, instead of writing simple and cool:
>>
>> self weakOn: Foo do: [:announcement | self bar ].
>>
>>
>> you must do more elaborate (and IMO less elegant)
>>
>> self weakOn: Foo send: #handleFooAnnouncement: .
>>
>> and then in #handleFooAnnouncement: you making 'self bar'
>>
>> P.S. the #weakOn: ... is not part of Announcements protocol , it is
>> just an example of 'meta-message' to reveal an intent to create a weak
>> subscription to some announcer by using receiver as a subscriber.
>>
>>
>>> best,
>>> Esteban
>>>
>>
>>
>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>>
>
>
>
>



-- 
Best regards,
Igor Stasenko AKA sig.




More information about the Pharo-project mailing list