How to detect if the VM is running with compact refs from within the VM (no agent)?

[Krystal Mok]

Hi Dawid,

For product releases of Oracle/Sun JDK:

The option -XX:+UseCompressedOops became available from around JDK6u14 [1].
Before 6u23, the option has to be explicitly set on the command line or
from a config file (.hotspotrc) in order to use compressed oops.
>From 6u23 and on, the ergonomics in HotSpot VM will automatically enable
UseCompressedOops when the whole GC heap is less than around 32GB and G1GC
is not in use. 7017008 [2] removed the G1GC limitation, and was delivered
in JDK7 (but not in any of the released JDK6 versions).

- Kris

[1]:  http://www.oracle.com/technetwork/java/javase/6u14-137039.html
[2]:  http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7017008

On Wed, Mar 14, 2012 at 4:44 PM, Dawid Weiss <dawid.weiss at gmail.com> wrote:

> Thanks Kris!
>
> I thought about parsing command-line but I wasn't sure about the
> runtime -- can it switch to compressed OOPS on its own (or rather: is
> there a heuristic to switch to compressed OOPS or is it always full
> refs in 64 bit mode unless an explicit command line option's been
> given)?
>
> Dawid
>
> > You could use HotSpotDiagnosticMXBean via JMX to get VM options. An
> example
> > here [1].
> >
> > If you're trying to get this information from the command line, use:
> >   jinfo -flag UseCompressedOops <pid>
> >
> > HTH,
> > - Kris
> >
> > [1]:  https://gist.github.com/1333043
> >
> >
> > On Wed, Mar 14, 2012 at 4:30 PM, Dawid Weiss <dawid.weiss at gmail.com>
> wrote:
> >>
> >> Hi everyone,
> >>
> >> This has been asked on Apache Lucene JIRA and I admit I don't know the
> >> answer:
> >>
> >> Is there a way to detect the reference size (which boils down to
> >> compact vs. full reference on 64 bit machines) from within Java?
> >>
> >> I assume the VM starts without an agent so Instrumentation is not
> >> available. I'd appreciate a comment if somebody knows right away (even
> >> if it's a: "no"), otherwise I'll go through the sources and try to
> >> figure it out on my own.
> >>
> >> Thanks,
> >> Dawid
> >
> >
>
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