[concurrency-interest] ConcurrentHashMapV8

Doug Lea dl at cs.oswego.edu
Thu Sep 8 07:45:13 EDT 2011


Among some further ongoing improvements and extensions (mainly
to support parallel aggregate operations), I'm contemplating
(nearly) ignoring "loadFactor" constructor arguments for
ConcurrentHashMap (in "V8" form for now, but ultimately
targeting revised j.u.c version). The main motivation
is that in concurrent hash maps  (of any form) collisions
and contention are interdependent, so anyone using a
higher-than-default load factor because they think it will
save space is also unexpectedly getting poorer multithreaded
performance. Given how hard it was to give clear answers to
questions on this list about exactly what blocks when based on
probability distributions with a parameter based on load factor,
I'm now thinking that is best to internally always use
the default, and use the optional constructor parameter
only as an initial sizing hint (larger loadFactor ->
smaller initial capacity).  This will enable simpler
characterization of expected performance.

Scanning for uses of ConcurrentHashMap constructors
on google code search, it seems that hardly anyone ever
overrides the default anyway, so this probably doesn't
matter much either way. But if you have ever done so,
and have a good reason, could you let me know? Thanks!

-Doug




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