[concurrency-interest] Looking for a data struture

Oleksandr Otenko oleksandr.otenko at oracle.com
Tue Dec 30 10:22:52 EST 2014

Do you know a non-concurrent structure that you are looking for?

(eg you don't like O(n) for removal of arbitrary items - but is it 
possible to have /all/ operations O(1), even in a non-concurrent case?)


On 22/12/2014 02:35, 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.
> 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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