Initial preview: JEP-149 Reduced Class instance size (original) (raw)

Martijn Verburg martijnverburg at gmail.com
Thu Apr 5 06🔞40 UTC 2012

Hi Hinkmond,

Is there a corpus of code you can look at in the embedded space to see what ratio of real-world apps use a lot of reflection? We have access to a corpus in the SE space (some Cambridge PhD students that work with us) and I know Brian/Joe et al use one internally at Oracle as well, would be an interesting comparison.

Cheers, Martijn

On 5 April 2012 09:07, Hinkmond Wong <hinkmond.wong at oracle.com> wrote:

Hi Brian,

One of the issues we have in the Java Embedded group (as David points out in his summary), is that while on paper the theoretical max savings seems great (as you point out also), in practice as David points out in his note, this might be a wash if there are a lot more reflection using classes vs. non-reflection using classes in "typical" real-world applications, not the low or zero reflection using class ratio that happens in the theoretical "best case". So, a question comes up if we should judge the merit of this change on the theoretical "best case" scenario, or should we judge it on real-world applicability to "typical" apps (such as a finite set of customer surveyed embedded apps that we feel represent a real-world scenario).

Thanks, Hinkmond On 4/4/12 8:28 PM, Brian Goetz wrote: Reducing the number of SoftReferences in ReflectionHelper also seems an attractive target for memory reduction. Rather than eight soft references (eight extra objects), maintaining a SoftRef to the entire RH, or at least to the part of the RH that is currently SR'ed if the two non-SR'ed fields can't be recomputed, would save you a whole pile of objects per class (and might also reduce pressure on GC, having 8x fewer SRs to process.)

Finally, you may be able save an extra field per Class by storing the ReflectionHelper in a ClassValue on Java SE 8, rather than a field. On 4/4/2012 10:50 PM, David Holmes wrote:

http://cr.openjdk.java.net/**dholmes/JEP-149/webrev/<http://cr.openjdk.java.net/dholmes/JEP-149/webrev/>

This is an early look at a proposed change to reduce the instance size of Java Class objects in the common case that reflection is not used. In SE 7 a java.lang.Class instance is 104 bytes on 32-bit systems. It consists of the 8-byte object header, 19 declared fields and 5 injected fields (fields added by the VM as-if they were declared in java.lang.Class but which do not appear in the Java source code). There are 10 reference fields associated with reflection caching that can be moved to a helper object with no impact on the VM or serialization protocols. Adding back a reference for the helper, that saves 9 references. Notionally this is 36 bytes on 32-bit but due to 8-byte alignment it only saves 32 bytes. That gives a size of 72 bytes - a reduction of 30%. This initial modification has been prototyped for initial performance measurements. Note that if reflection is used then the amount of memory used by the Class will increase by 8-bytes - that being the additional object header of the ReflectionHelper instance. So the net gain depends on the ratio of reflection using classes to non-reflection-using classes in an application. Please note that I've put this out just before I disappear on vacation for 10 days, so if you don't see any responses from me that is why. :) Thanks, David Holmes

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