EnumData space optimization in j.l.Class (JEP-146) (original) (raw)

Peter Levart peter.levart at gmail.com
Sat Dec 22 15:38:56 UTC 2012

At end...

On 12/18/2012 07:50 PM, Remi Forax wrote:

On 12/18/2012 10:18 AM, Peter Levart wrote:

On 12/17/2012 11:39 PM, Remi Forax wrote:

On 12/17/2012 11:14 PM, Peter Levart wrote:

On 12/17/2012 10:26 PM, Mandy Chung wrote:

On 12/17/12 7:36 AM, Peter Levart wrote:

Hi David and others,

Here's a patch that eliminates one of two fields in java.lang.Class, related to caching enum constants: http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149.enum/webrev.01/index.html

It does it by moving one field to a subclass of HashMap, which is referenced by a remaining field that serves two different purposes/stages of caching. Your observation of merging the enumConstants and enumConstantDirectory is a good one. I see that caching of enumConstantDirectory is important as it's used by EnumMap and EnumSet whose performance is critical (specified with constant time operations). I'm unsure about Class.getEnumConstants whether it's performance critical and worths the complexity of your proposed fix (the enumData field of two types). If a class has cached an enumConstantDirectory, Class.getEnumConstants can return a clone of its values(). Anyone knows how Class.getEnumConstants is commonly used and needs to be performant? I suspect it's more typical to obtain the list of enum constants statistically (calling Enum.values()) than reflectively. Hi Mandy, public Class.getEnumConstants() is a reflection mirror of SomeEnum.values(). It returns a defensive copy of the constants array. The primary place for Enum constants is in a private static final $VALUES field, generated by compiler in each Enum subclass. But that I think is not part of specification, so for internal usage (as far as I have managed to find out only in the constructors of EnumSet and EnumMap), the package-private Class.getEnumConstantsShared() is used which obtains a copy of the array by calling SomeEnum.values() and than caches is. The Class.enumConstantDirectory() on the other hand is an internal package-private method that returns a shared/cached Map<String, T>, which is used internally to implement SomeEnum.valueOf(String) and Enum.valueOf(Class, String) static methods. Both package-private methods must be fast. Regards, Peter for what it worth, I'm the guy behind the patch of bug 6276988 (it was before OpenJDK was setup BTW), http://bugs.sun.com/bugdatabase/viewbug.do?bugid=6276988 and for the little story, I need that patch because I was developing an Eclipse plugin that uses EnumSet to represent the possible completion values. So to answer to Mandy, this application needs really fast EnumSet creation thus really fast getEnumConstantShared() because the EnumSets was created as user types code. Hi Rémi, is 600M EnumSets / sec good enough for a fast typist? The eclipse plugin uses is a LR parser (a GLR exactly) which has the stupid property that it can change the whole parse tree if you just add one character. Also i'm maybe wrong but, if you test with several different 'enum classes', you should see a slowdown. Rémi You're right. I modified the test to use 5 different enum classes interchangeably in a loop:

https://raw.github.com/plevart/jdk8-tl/JEP-149.enum/test/src/test/EnumTest.java

The slowdown of EnumSet.noneOf() compared to original code was more apparent - about 50% when there's an array assigned to enumData field and more than 60% when there's already an EnumData object holding the array reference. MyEnum.valueOf(String) seems to be unaffected - more cycles spent elsewhere in the code-path to be significant.

When I changed the "if (enumData instanceof Enum[])" with "if (enumData != null)", the slowdown of EnumSet.noneOf() was less apparent - about 24% when there's an array assigned to enumData and about 44% when there's EnumData object. MyEnum.valueOf(String) shows the same performance.

So in order to affect the performance as little as possible, I devised a more direct approach without instanceof checks and casts. Here it is:

http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149.enum/webrev.02/index.html

With this change, EnumSet.noneOf() is only about 12% slower than original code and MyEnum.valueOf(String) performs the same. This code has a space overhead of an extra object with two fields, but it is expected that the number of enum classes compared to non-enum classes is small and the reduction of a field in the java.lang.Class pays off. Even though this is a single field and it might not shrink (not every but approx. half of all?) java.lang.Class object (s) on 32bit architectures due to alignment constraints, it presents an opportunity for shrinkage when other individual fields in java.lang.Class are similarly removed. One such opportunity presents the 2+1 remaining fields used for caching annotations.

I understand that contributions to JEP-149 are closed already, but I post this here for any future reference.

Regards, Peter

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