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

[Uwe Schindler]

Hi,

 

I tried this code as platform (means JVM independent) detection for
compressed Oops:

 

boolean compressedOops = false;

if (JRE_IS_64BIT) try {

  final Class<?> beanClazz =
Class.forName("com.sun.management.HotSpotDiagnosticMXBean");

  final Object hotSpotBean = ManagementFactory.newPlatformMXBeanProxy(

    ManagementFactory.getPlatformMBeanServer(),

    "com.sun.management:type=HotSpotDiagnostic",

    beanClazz

  );

  final Object vmOption = hotSpotBean.getClass().getMethod("getVMOption",
String.class)

    .invoke(hotSpotBean, "UseCompressedOops");

  compressedOops = Boolean.valueOf(

    vmOption.getClass().getMethod("getValue")

    .invoke(vmOption).toString()

  ).booleanValue();

} catch (Exception e) {

  // if anything fails before, we assume that oops are not compressed

  compressedOops = false;

}

 

If anybody finds this useful, here it is,

 

-----

Uwe Schindler

uschindler at apache.org 

Apache Lucene PMC Member / Committer

Bremen, Germany

http://lucene.apache.org/

 

From: hotspot-compiler-dev-bounces at openjdk.java.net
[mailto:hotspot-compiler-dev-bounces at openjdk.java.net] On Behalf Of Krystal
Mok
Sent: Wednesday, March 14, 2012 10:42 AM
To: Dawid Weiss
Cc: hotspot compiler
Subject: Re: How to detect if the VM is running with compact refs from
within the VM (no agent)?

 

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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