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

[Dawid Weiss]

Thanks Kris. This is very useful.

Dawid

On Wed, Mar 14, 2012 at 3:45 PM, Krystal Mok <rednaxelafx at gmail.com> wrote:
> Hi,
>
> Just in case you'd care, the same MXBean could be used to detect compressed
> references on JRockit, too. It's probably available starting from JRockit
> R28.
>
> Instead of "UseCompressedOops", use "CompressedRefs" as the VM option name
> on JRockit.
>
> Don't know how to extract this information for J9 without another whole
> bunch of hackeries...well, you could try this, on a "best-effort" basis for
> platform detection:
> IBM J9's VM version string contains the compressed reference information.
> Example:
>
> $ export JAVA_OPTS='-Xcompressedrefs'
> $ groovysh
> Groovy Shell (1.7.7, JVM: 1.7.0)
> Type 'help' or '\h' for help.
> ----------------------------------------------------------------------------------------------------------------------------
> groovy:000> System.getProperty 'java.vm.info'
> ===> JRE 1.7.0 Linux amd64-64 Compressed References 20110810_88604 (JIT
> enabled, AOT enabled)
> J9VM - R26_Java726_GA_20110810_1208_B88592
> JIT  - r11_20110810_20466
> GC   - R26_Java726_GA_20110810_1208_B88592_CMPRSS
> J9CL - 20110810_88604
> groovy:000> quit
>
> So grepping for "Compressed References" in the "java.vm.info" system
> property gives you the clue.
>
> - Kris
>
>
> On Wed, Mar 14, 2012 at 9:06 PM, Uwe Schindler <uschindler at apache.org>
> wrote:
>>
>> 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
>> >
>> >
>>
>>
>
>