Fwd: [concurrency-interest] ConcurrentHashMap footprint and contention improvements

Doug Lea dl at cs.oswego.edu
Wed Apr 13 00:13:02 UTC 2011

Members of this list are also invited to check this out.
I'm not sure if it will make Java7 schedule, but the
improvements seem worthwhile given the increased
use of ConcurrentHashMap inside the JDK (including
multiple tables per class loader in JDK7.)


-------- Original Message --------
Subject: [concurrency-interest] ConcurrentHashMap footprint and contention 
Date: Tue, 12 Apr 2011 20:07:17 -0400
From: Doug Lea <dl at cs.oswego.edu>
To: Concurrency-interest at cs.oswego.edu <Concurrency-interest at cs.oswego.edu>

For years, we've known that ConcurrentHashMaps have initial
footprints (over 1000 bytes using default constructor) that
are too big for casual use. And that the best way to address
this would be to use the Fences API to emulate "final field"
memory model guarantees in the presence of lazy initialization.
But we aren't releasing the Fences API. So I committed a version
that instead uses Unsafe calls to essentially the same effect
(reducing initial footprint to around 100 bytes, plus a few
percent savings for large populated tables). Also, this
version includes throughput improvements under contention
(mainly by interleaving locking with probes, to absorb cache misses),
which can double performance on big tables with many threads.
While conceptually straightforward, these lead to many
line-of-code changes.

The main price paid for these improvements is a greater reliance
of "volatile" vs "final" reads, which are essentially equivalent
in cost on most machines, but can be more costly on ARM and POWER.
Even on these though, the net effect should be positive.

It would be helpful if members of this list could help check
that this is so. The committed version is now
in java.util.concurrent sources (at
and can be run by getting jsr166.jar and using
"java -Xbootclasspath/p:jsr166.jar" with any java7 build
or binary (http://dlc.sun.com.edgesuite.net/jdk7/binaries/index.html).
Also, as an alternative, I temporarily placed an unpackaged
source version (with the class renamed "CHM")
at http://gee.cs.oswego.edu/dl/wwwtmp/CHM.java
You can compile and somehow run in any java6/7 JVM.

While working on these changes, I also contemplated other
more extensive redesigns, including Cliff Click's non-blocking
version (http://sourceforge.net/projects/high-scale-lib/)
which usually has better scalability with large numbers
of threads solely using get and put, but not otherwise
uniformly a better choice.


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