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From "Jack Krupansky" <j...@basetechnology.com>
Subject Re: [JENKINS] Lucene-Solr-trunk-Linux-Java8-64 - Build # 15 - Failure!
Date Sat, 14 Jul 2012 12:08:12 GMT
"... tests doing Map.toString() and compare against a hardcoded String 
(which is of course wrong for HashMaps or HashSets because order is 
undefined)."

Map.toString sounds like a great operation to forbid in Solr/Lucene, 
especially in tests. Although Map.toString is still helpful for 
debug/logging, what would really be helpful are two things: 1) display the 
map sorted/ordered by key, and 2) compare two maps for equality (build a map 
to compare against rather than using a presumed toString value.) And, an 
"assertMapEquals" method as well. Maybe even a "assertMapKeys" method that 
simply verifies that the keys of a map are "equal" to a list of keys (set 
equality but not order.)

-- Jack Krupansky

-----Original Message----- 
From: Uwe Schindler
Sent: Saturday, July 14, 2012 5:51 AM
To: yonik@lucidimagination.com ; dev@lucene.apache.org
Subject: RE: [JENKINS] Lucene-Solr-trunk-Linux-Java8-64 - Build # 15 - 
Failure!

> Or it could just be that something in Java8 causes orderings of some
container (like a Map) to be different.

That's definitely the case, also (partly in Java 7 for
HashSet/HashMaps/ConcurrentHashMap > 512 entries). They have now a better
has algorithm enabled by default. I already fixed some Solr tests doing
Map.toString() and compare against a hardcoded String (which is of course
wrong for HashMaps or HashSets because order is undefined).

After reading the mail below we were already thinking about setting the
threshold in Java 7 randomly :-)

Here the Mail on OpenJDK list:

---------- Forwarded message ----------
From: Mike Duigou <mike.duigou@oracle.com>
Date: Sat, Jul 14, 2012 at 1:09 AM
Subject: Update on alternative hashing for String keys with hash-based Maps
To: core-libs-dev Libs <core-libs-dev@openjdk.java.net>,
jdk7u-dev@openjdk.java.net


Hello all;

About a month ago a significant change was made to the Java SE 7 and 8 hash
based Map implementations. This change was previously proposed and discussed
on this list[1]. The change affects all of the hashing based Map
implementations (HashMap, Hashtable, WeakHashMap, LinkedHashMap,
ConcurrentHashMap), the derived Set implementations (HashSet, LinkedHashSet,
etc.) and other classes which depend upon these classes (Properties,
UIDefaults). The change to hash based Maps improves performance when a large
number of key hash code collisions are encountered. This is accomplished by
altering the handling of String keys to use a different (better) hash
function.

As initially proposed, the alternative hashing behaviour was planned to
apply to all hash based Maps larger than 512 elements. Smaller maps would
continue to use the existing hashing approach.
ConcurrentHashMap, because reasons related to the complexity of it's
implementation, would always use the improved approach regardless of map
capacity. Providing capacity based triggering of the alternative hashing is
intended to improve compatibility by ensuring that the order in which
elements are iterated does not change. Specifically, at less than the
threshold capacity Map elements will be iterated in the same order as today.
At or above the threshold, the iteration order will be different than the
current order. Testing and evaluation of existing Java applications has
shown us that some applications have explicit or implicit dependence upon
the order that keys, values or entries will be iterated. The vast majority
of iteration order dependent cases involve small maps--once a map contains
hundreds of elements generally incorrect assumptions about iteration order
will have already been found and resolved. We believed that triggering the
alternative hashing behaviour at 512 element capacity would protect
iteration order in cases which depended upon the existing iteration order.
Java SE 7 and Java SE 8 have different policies. Java SE 8 is intended to
always use alternative hashing of String keys regardless of the capacity of
the Map.

After  integration a number of cases of iteration order dependence were
encountered within the OpenJDK code itself, in tests and in user code. Some
of these faults were easily diagnosed and correct. Some other cases, because
iteration order is not consistent from run-to-run under alternative hashing,
proved difficult to isolate and resolve.

Following this testing and and consultation with Java licensees and
developers it was decided disable the alternative hashing behaviour for Java
SE 7.  To ensure the greatest degree of compatibility with existing
applications it seems best to only enable alternative hashing by explicit
control. In Java SE 7u6 it will be necessary to set the system property
jdk.map.althashing.threshold in order to enable alternative hashing. It is
also still possible that the feature may be enabled by default in future
Java SE 7 releases but this will only happen if further testing indicates
compatibility can be reasonably assured.

Because Java 8 is unreleased and we still wish to shake out iteration order
dependencies alternative hashing remains enabled in Java SE 8.
Alternative hashing is very likely to be enabled by default in Java SE 8.
Doug Lea has been investigating further improvements to handling of key hash
code collisions and his design is extremely likely to be the basis for all
Java SE 8 hash based Map implementations.[2]

Developers and deployers are strongly encouraged to test their applications
by enabling the alternative String key hashing feature in Java SE 7u6 or
later and/or testing with Java SE 8 builds. BE WARNED:
it will probably not be possible to disable alternative hashing in Java SE
8. Applications MUST remove dependencies upon iteration order before they
can be deployed with Java SE 8.

Thanks,

Mike

TL;DR:
- Java SE 7 and 8 both now support alternative hashing for String keys with
hash based Maps
- Alternative hashing improves performance when many String key hash codes
collide
- Alternative hashing impacts key, value and element iteration order
- Alternative hashing is currently DISABLED by default for Java SE 7
- Future Java SE 7 releases may enable alternative hashing for "large"
(>512 capacity) maps
- Developers can enable the eature in Java SE 7 for testing and deployment
with a system property
- Alternative hashing is ENABLED for all maps in Java SE 8
- It will probably not be possible to disable alternative hashing in Java SE
8
- Hash map key, value and element iteration order WILL be different and
unpredictable in Java SE 8
- Different implementation approaches are still being investigated for Java
SE 8 and remain subject to change

[1]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2012-May/010238.html
[2]
http://cs.oswego.edu/pipermail/concurrency-interest/2012-June/009505.html

-----
Uwe Schindler
H.-H.-Meier-Allee 63, D-28213 Bremen
http://www.thetaphi.de
eMail: uwe@thetaphi.de


> -----Original Message-----
> From: yseeley@gmail.com [mailto:yseeley@gmail.com] On Behalf Of Yonik
> Seeley
> Sent: Saturday, July 14, 2012 12:43 PM
> To: Uwe Schindler
> Cc: dev@lucene.apache.org
> Subject: Re: [JENKINS] Lucene-Solr-trunk-Linux-Java8-64 - Build # 15 -
Failure!
>
> On Sat, Jul 14, 2012 at 6:37 AM, Uwe Schindler <uwe@thetaphi.de> wrote:
> > Thanks for taking care. Interestingly this test was mostly failing in
> > Java 8 EA builds. Strange, maybe there is something different with Java
8!
>
> Yeah, there still could be something else wrong with the test or elevate
> component itself - I just fixed the most obvious top-level bug.
> Or it could just be that something in Java8 causes orderings of some
container
> (like a Map) to be different.
>
> -Yonik
> http://lucidimagination.com


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