java process memory usage is higher than Xmx

[YU ZHANG]

I did a study on G1 vs ParallelgGC native memory footprint.  The source 
for G1 using more memory includes:
mtGC: mainly for RS related data structure.
internal: internal for tracking
Thread: g1 has more internal threads and thread related data structures

In Yao's previous email
"It might use off-heap memory internally but the problem is, similar 
machine running under Java 6 with CMS do not have this problem and the 
real memory usage is very close to Xmx."

I am not quite familiar with CMS, does CMS need to keep a similar RS 
kinda data structure?

Thanks,
Jenny

On 1/6/2014 1:43 PM, Thomas Schatzl wrote:
> Hi,
>
> On Mon, 2014-01-06 at 12:03 -0800, yao wrote:
>> Hi All,
>>
>> I have a java process (HBase region server process ) running under
>> Java 7 (1.7.0_40-b43) with G1 enabled. Both Xms and Xmx are the same.
>> After running process for a few hours, I see the actual memory used by
>> the process is about 10 percent higher than given Xmx. Has anyone
>> experienced the similar when use Java 7 or G1 ? Is there useful tools
>> to diagnose the cause ?
>>
>>
>> I've tried jmap but the output doesn't say anything about high memory
>> usage. FYI, the java process use a large heap (90GB), but the actual
>> memory usage ($ top) is about 99GB.
>>
> Possibly remembered set size.
>
> Can you enable -XX:+UnlockDiagnosticVMOptions and -XX:
> +G1SummarizeRSetStats?
>
> The line
>
>    Total heap region rem set sizes = 5256086K.  Max = 8640K.
>
> gives you a good idea about remembered set size memory usage.
>
> I copied above line from one of your responses to the "G1 GC clean up
> time is too long" thread, and it seems the remembered set takes ~5GB
> there.
>
> Hth,
> Thomas
>
>
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