[concurrency-interest] RFC -- Java7 java.util.concurrent plans

Jeremy Manson jeremy.manson at gmail.com
Sat Dec 13 17:22:29 EST 2008

Actually, I had a performance problem with those dummy instances about
a week ago.  I ended up figuring out a way to call into it less,


On Tue, Dec 9, 2008 at 6:52 PM, Kevin Bourrillion <kevinb at google.com> wrote:
> On Tue, Dec 9, 2008 at 6:31 PM, Bob Lee <crazybob at crazybob.org> wrote:
>>> I'm curious as to what design aspects make ConcurrentReferenceHashMap
>>> superior to Google's ReferenceMap.  It looks to have implemented its own
>>> hash table, versus delegating to a backing ConcurrentMap, but I don't
>>> immediately understand why this is preferable.
>> ReferenceMap must create an object every time you call get() and it must
>> repeatedly lock/unlock in some situations whereas ConcurrentReferenceHashMap
>> can just grab the lock once and get the job done. I haven't looked at the
>> semantics of CRHM in awhile, but we have plenty of time to hammer those out.
> A ReferenceMap that lives outside java.util, without access to java.util
> internals, is forced to make this choice (either reimplement its own entire
> data structure, or discard a dummy instance on every query like ours does).
> We chose one way and Jason chose the other, but I'd still like to think that
> neither horrible fate must be suffered for the JDK code.
> For the record, Google's ReferenceMap is phenomenally well-tested and has
> been in production in Google apps for more than three years.  The
> performance impact of these discarded dummy instances has not been a serious
> problem in practice (I still think it's unsuitable for JDK code, but also
> unnecessary as explained above).  It's currently being used in 167 distinct
> projects.  Through this we've worked out a lot of fiendish bugs.  The JDK is
> welcome to as much or as little of its distilled wisdom as you please!
> --
> Kevin Bourrillion @ Google
> internal:  go/javalibraries
> google-collections.googlecode.com
> google-guice.googlecode.com
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