Trace object type in GC

[Krystal Mok]

Hi Nicolas,

If you're interested in the objects collected on a type-granularity, then
the following VM arguments gets close:

manageable(bool, PrintClassHistogramBeforeFullGC, false,          \
        "Print a class histogram before any major stop-world GC") \
                                                                  \
manageable(bool, PrintClassHistogramAfterFullGC, false,           \
        "Print a class histogram after any major stop-world GC")  \

What they do is basically the same as running jmap -histo before or after a
full GC, printing the histogram of objects grouped by class. Comparing the
numbers before and after a full GC will give you the idea of what's
collected in that full GC.

You can turn these flags on by specifying them in the VM command line
arguments, e.g. -XX:+PrintClassHistogramBeforeFullGC
Or you could change their values while the VM is running, e.g. jinfo -flag
+PrintClassHistogramBeforeFullGC <pid>

(There's no equivalent flag for minor GCs in HotSpot.)

If you're looking for information on a object-granularity, then it's
something HotSpot VM doesn't support. GC implementations seldom track such
information.

Hope it helps,
- Kris

On Mon, Jun 11, 2012 at 11:36 PM, RICHARD Nicolas
<nicolas.richard at atos.net>wrote:

>  Dear hotspot developper,
> I currently develop applications in Java.
>
> I am looking for a way to trace objects freed by the Garbage Collector.
> For the time being, I've only found options to get statistics about GC
> (execution time, average amount of freed memory, etc) but no one about the
> type of object handled by the GC.
>
> Is there any option that I could use or is it possible to develop a JVM TI
> agent to get this information ?
>
> Best regards
> Nicolas RICHARD
>
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