Why dose max heap size change?

[=?GB2312?B?stzQ8bar?=]

Hi  Krystal,

Thanks for your explaination.

I understand it. Thanks a lot.

Best Regards.

caoxudong




2012/8/12 Krystal Mok <rednaxelafx at gmail.com>

> Hi Xudong,
>
> I believe the class you're referring to is
> java.lang.management.MemoryUsage. Quoting the docs [1]:
>
> A MemoryUsage object represents a snapshot of memory usage. Instances of
> the MemoryUsage class are usually constructed by methods that are used to
> obtain memory usage information about individual memory pool of the Java
> virtual machine or the heap or non-heap memory of the Java virtual machine
> as a whole.
>
> Which means MemoryUsage is not just used to represent the usage of the
> Java heap as a whole. -Xmx only locks the maximum size of the Java heap,
> but doesn't say anything about how the spaces within the Java heap should
> be arranged.
>
> Let's look at an example of a MemoryUsage object representing the usage of
> a memory pool.
> Try running JConsole on a HotSpot Server VM with default arguments. The
> collector used by default would be the Parallel collector.
> Go to the MBean tab, find the MBean of "PS Eden Space" from java.lang ->
> MemoryPool, and open its Usage property. Refresh the value a few times, and
> see if the max field changes.
> In my environment, it does change over time. That's because the Parallel
> collector uses an adaptive size policy, which could change the maximum size
> of the generations adaptively.
>
> Regards,
> Kris
>
> [1]:
> http://docs.oracle.com/javase/7/docs/api/java/lang/management/MemoryUsage.html
>
>
> On Sun, Aug 12, 2012 at 4:10 PM, ²ÜÐñ¶« <caoxudong818 at gmail.com> wrote:
>
>> Hi all,
>>
>> I am doing some monitor jobs for JVM with JMX, and has been stuck on a
>> question about max heap size.
>>
>> The javadoc of the field *max* of class *java.lang.management.HeapUsage*says,
>>
>>       "represents the maximum amount of memory (in bytes) that can be
>> used for memory management. Its value may be undefined.
>>
>>        *The maximum amount of memory may change over time if defined*. "
>>
>> So, my question is,
>>
>>        Why dose max heap size change, if I defined it with Xmx?
>>
>> Any response will be appreciated.
>>
>> Best Regards.
>>
>> caoxudong
>>
>>
>>
>> _______________________________________________
>> hotspot-gc-use mailing list
>> hotspot-gc-use at openjdk.java.net
>> http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20120813/9235b639/attachment.html