Hi,
there are still some "Yoda" style expressions in String, HashMap, maybe more...
there is still {@inheritDoc} in String.hash32(), which points to nowhere, since interface Hashable32
is not implemented.
-Ulf
Am 30.05.2012 01:05, schrieb Mike Duigou:
Another round of updates for Java 7 [1] and Java 8 [2] implementations. This
revision incorporates Remi's suggestions and some feedback from Doug Lea
regarding applying the per-instance seed to the result of String.hash32()
[1] althashing "7" webrev :
http://cr.openjdk.java.net/~mduigou/althashing7/10/webrev/
[2] althashing "8" webrev :
http://cr.openjdk.java.net/~mduigou/althashing8/10/webrev/
Barring any emergencies this will be integrated to both Java 7 and Java 8 on
Wednesday May 30th, 2012.
Thanks to all who provided feedback!
Mike
On May 25 2012, at 10:38 , Rémi Forax wrote:
On 05/24/2012 09:34 PM, Mike Duigou wrote:
Hello All;
I have updated the webrevs for alternative hashing for String with feedback
from Remi, Doug, Ulf and internal reviewers.
Additional feedback is welcome.
Mike
[1] althashing "7" webrev :
http://cr.openjdk.java.net/~mduigou/althashing7/9/webrev/
[2] althashing "8" webrev :
http://cr.openjdk.java.net/~mduigou/althashing8/9/webrev/
Hello Mike,
just two small issues.
In WeakHashMap.hash(), the null check should be done at callsite,
It's not even needed at the callsite as it always follows a maskNull call.
the call to hash32() should be directly returned like in HashMap.
Fixed.
Is Hashable32 is used anymore ?
It's the "source" of the hash32 method that String now implements and hopefully
more later. This bit is sort of unfinished in this patch but I would like to leave it in
for now.
Mike
Rémi
On May 22 2012, at 22:16 , Mike Duigou wrote:
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@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/
