Review Request CR#7118743 : Alternative Hashing for String with Hash-based Maps

[Mike Duigou]

Dear OpenJDK CoreLibs community,

A significant enhancement to the Java SE hashing-based Map implementations in planned for inclusion in Java SE 7u6. All of the hashing based Map implementations: HashMap, Hashtable, LinkedHashMap, WeakHashMap and ConcurrentHashMap will now use an enhanced hashing algorithm for string keys when the capacity of the hash table has ever grown beyond 512 entries. The enhanced hashing implementation uses the murmur3 hashing algorithm[1] along with random hash seeds and index masks. These enhancements mitigate cases where colliding String hash values could result in a performance bottleneck.

In order to provide the greatest opportunity for developers to test compatibility with their applications this change will be incorporated into JDK7u6 build 12 and JDK8 build 39. Both builds are planned for release next week. ***For 7u6 build 12 only, the alternative hashing will be unconditionally enabled (always on).*** The threshold default will be reset to the intended release default (512) for 7u6 build 13. 

The quick promotion of this change into builds with limited opportunity for public review and the special behaviour for build 12 is intended to make it easier for developers to test their application compatibility. Feedback on the approach, implementation, compatibility and performance is eagerly sought and encouraged both before *and after* this change is incorporated into the OpenJDK repositories. 

A new system property, jdk.map.althashing.threshold, allows adjustment of the threshold for enabling the enhanced hashing algorithm. If changed from the default value of 512, the enhanced hashing will be invoked any time after map capacity exceeds the value of jdk.map.althashing.threshold. To completely disable the enhanced hashing (not recommended), set jdk.map.althashing.threshold to -1 or a very large number such as 2^31 -1 (Integer.MAX_VALUE).

The iteration order of keys, values and entries for hash-based maps where the new algorithm has been invoked will vary for each HashMap instance. While the Java SE Map documentation makes no promises that iteration order of items returned from Maps will be consistent, developers should check if their applications have incorrectly created a dependency on the iteration order of Map entries, keys or values.

Webrevs for the Java 7u6 and 8 changes are available for download at [2] and [3] for your review. There are some important differences between the Java 7 and 8 implementations of this enhancement. Most specifically in the Java 8 implementation alternative string hashing is always enabled--no threshold is used for enablement and alternative hashing cannot be disabled. (The Java 8 implementation completely ignores the jdk.map.althashing.threshold system property). The Java 8 implementation is also subject to additional refinement as Java 8 develops.

If you have any questions or concerns with this planned enhancement, please use the corelibs development mailing list, <mailto:core-libs-dev at openjdk.java.net>, or you may also respond privately to me if you prefer.

Thanks,

Mike

[1] Murmur3 : https://code.google.com/p/smhasher/wiki/MurmurHash3
[2] althashing "7" webrev : http://cr.openjdk.java.net/~mduigou/althashing7/8/webrev/
[3] althashing "8" webrev : http://cr.openjdk.java.net/~mduigou/althashing8/8/webrev/