Initial preview: JEP-149 Reduced Class instance size

[Hinkmond Wong]

On 4/4/2012 11:18 PM, Martijn Verburg wrote:
> 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.
>

Hi Martin,

There is no corpus of sample code for Embedded that I know of.  Maybe 
Bob Vandette or David Holmes knows of one for Java SE desktop/server 
also?  But, we've discussed in our team, possibilities for more 
embedded-appropriate real-world apps and are in process of obtaining 
those now from different embedded developers.  If anyone out there has 
any suggestions, please send them in.  Some that we've considered 
include various OSGi implementations for embedded and Jetty.

Can you send a pointer to the SE corpus of code that you know of (the 
one you mentioned that the Cambridge PhD students worked on)?  Or, is 
that Brian Goetz and Joe Darcy that you mean who have used it before at 
Oracle?


Thanks,
Hinkmond

>
> On 5 April 2012 09:07, Hinkmond Wong <hinkmond.wong at oracle.com 
> <mailto: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/%7Edholmes/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
>
>
>
>     -- 
>     Oracle <http://www.oracle.com>
>
>     Hinkmond Wong | Consulting Member of Technical Staff
>     Phone: _+1 408.276.7618 <tel:%2B1%20408.276.7618>_ | Fax: _+1
>     408.276.7674 <tel:%2B1%20408.276.7674>_
>     Oracle Java Embedded
>     4210 Network Ci., M/S USCA22-rm2364 | Santa Clara, CA 95054
>     Green Oracle <http://www.oracle.com/commitment> Oracle is
>     committed to developing practices and products that help protect
>     the environment
>
>


-- 
Oracle <http://www.oracle.com>

Hinkmond Wong | Consulting Member of Technical Staff
Phone: _+1 408.276.7618_ | Fax: _+1 408.276.7674_
Oracle Java Embedded
4210 Network Ci., M/S USCA22-rm2364 | Santa Clara, CA 95054
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to 
developing practices and products that help protect the environment