[concurrency-interest] Looking for a data struture

Aaron Grunthal aaron.grunthal at infinite-source.de
Mon Dec 22 09:24:39 EST 2014

On 22.12.2014 03:35, Luke Sandberg wrote:
> I have come across a few situations where i am looking for a
> datastructure and i feel like i keep coming up short.
> The situation is analogous to the 'waiters' list in
> java.util.concurrent.FutureTask.
> Basically, I need to be able to publish a non-null object reference into
> a data structure where
> * order doesn't matter (insertion order would be nice, but i don't care
> that much)
> * add and remove identity semantics
> * concurrent iteration (weakly consistent is fine)
> * add/remove are non-blocking
> CLQ is an obvious choice but removal is O(n), another choice would a
> simple synchronized identity hash set which is fine but the lock + high
> entry overhead is a deal breaker.
> An AtomicReferenceArray would be super easy, but i can't put a bound on
> the number of items.

You say that it's super easy, but how would you handle finding a free 
slot for insertion in constant or logarithmic time? Since you mention 
that order doesn't matter that seems to suggest it doesn't behave in a 
stack-like manner.

> Also, it would be fine for the code that adds items, to store additional
> state (an index, a 'Node' reference), in order to facilitate removal.
> The best thing i have seen from looking around appears to be something
> like what FutureTask does to implement 'awaitDone', but even there
> removeWaitier() is O(n).  that seems like a pretty good compromise when
> lists are short, but what would be a better solution when lists are long?
> Just to motivate this a little bit, the two cases I am looking at in
> particular are:
> * maintaining a set of threads associated with a server 'request'
> (threads enter/exit when they start executing tasks associated with the
> request)
> * maintaining a set of futures to be cancelled (futures are removed when
> they complete to avoid pinning their referents).
> Any pointers or ideas?
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