[jboss-user] [JBoss Microcontainer Development] New message: "Re: Profiling the dependency project"

[David Lloyd]

User development,

A new message was posted in the thread "Profiling the dependency project":

http://community.jboss.org/message/527291#527291

Author  : David Lloyd
Profile : http://community.jboss.org/people/david.lloyd@jboss.com

Message:
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> alesj wrote:
>  
> 
> > Since installCallbacks and uninstallCallbacks are ConcurrentHashMaps, I think the read lock is unnecessary here in AbstractController:
> Since I think callbacks are already used in some explicitly locked code (via Lock),
> what about if we leave the locks and just change the callbacks to plain HashMap?
>  
> Like discussed on the call, what is faster in a really-rare-concurrent env?
> * lock + plain hash collection
> * concurrent hash collection
>  
> Jason, DML?
 
Well, if you're already locking, concurrent hash maps will generally give you general throughput in the contended case; if there's only one contender then there's only one lock acquisition so it's a wash, on paper.  In reality, a CHM is actually a bit more complex, and it's also a really big structure (even if it's empty), consisting of a ReentrantLock for each lock stripe, each of which includes a bunch of stuff, so you get another kind of savings by using a simple synchronized map.  In fact I'm hesitant to ever use CHM just because of its excessive size, especially if you're creating a lot of them (if you ever do a memory profile of AS, you can see that CHM and its components tend to bubble to the top of the list).  So for low contention situations I'd always recommend a plain synchronized map.
 
The best solution would of course be to eliminate the lock - if you're already protected, you could use a plain hashmap.  Otherwise consider if copy-on-write is suitable for you - Jason's FastCopyHashMap is good for that kind of thing.

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