How to identify individual object's location in JVM heap space

Lijie Xu csxulijie at gmail.com
Fri Aug 23 02:04:04 PDT 2013


Many thanks, I just want to say this is an amazing place.


On Fri, Aug 23, 2013 at 4:34 PM, 云达(Yunda) <yunda.mly at taobao.com> wrote:

> Xu,****
>
> ** **
>
> Actually the heap dump records the address of each object(The structure of
> heap dump is described in share/vm/services/heapDumper.cpp and object ID of
> an object is its address). So you can write a program to analyze the heap
> dump on your own to get the heap location of each object. ****
>
> ** **
>
> As for the address information generated by +PrintGCDetails, the three
> addresses of a generation are low boundary address, high address and high
> boundary address. High address is the current end address and high boundary
> address is the maximum address the generation can expand to, so the “total”
> of a generation is high address minus low boundary address. While the three
> addresses of a space are bottom address, top address and end address, and
> the size of a space is end address minus bottom address and the used size
> is top address minus bottom address. If there are four addresses of a
> space, the third one is used by the OffsetTable which I think you can just
> ignore.****
>
> ** **
>
> ** **
>
> Yunda****
>
> ** **
>
> *From:* hotspot-gc-dev-bounces at openjdk.java.net [mailto:
> hotspot-gc-dev-bounces at openjdk.java.net] *On Behalf Of *Lijie Xu
> *Sent:* Friday, August 23, 2013 9:46 AM
> *To:* hotspot-gc-dev at openjdk.java.net
> *Subject:* How to identify individual object's location in JVM heap space*
> ***
>
> ** **
>
> Hi, all. With the help of jmap or other tools, we can dump heap objects at
> a specific time. Then we can find out both the number and size of the
> individual object.****
>
> ** **
>
> At this point, I would like to know the heap location (Eden, S0/S1, or
> Old) of each object. But I cannot find any related information in the heap
> dump. How to identify? I really need this information in my project.****
>
> ** **
>
> I think there is a way to know the address of each object and address
> boundary of each space, since GC algorithms have methods to scan objects in
> a specific space.****
>
> ** **
>
> I also noticed that there are some A about each space. For example,****
>
> ** **
>
> Heap****
>
>  def new generation   total 153600K, used 152935K [0x14710000, 0x1edb0000,
> 0x1edb0000)****
>
>   eden space 136576K, 100% used [0x14710000, 0x1cc70000, 0x1cc70000)****
>
>   from space 17024K,  96% used [0x1dd10000, 0x1ed09f60, 0x1edb0000)****
>
>   to   space 17024K,   0% used [0x1cc70000, 0x1cc70000, 0x1dd10000)****
>
>  tenured generation   total 341376K, used 341220K [0x1edb0000, 0x33b10000,
> 0x33b10000)****
>
>    the space 341376K,  99% used [0x1edb0000, 0x33ae9160, 0x33ae9200,
> 0x33b10000)****
>
>  compacting perm gen  total 12288K, used 188K [0x33b10000, 0x34710000,
> 0x37b10000)****
>
>    the space 12288K,   1% used [0x33b10000, 0x33b3f2c0, 0x33b3f400,
> 0x34710000)****
>
>     ro space 10240K,  42% used [0x37b10000, 0x37f50348, 0x37f50400,
> 0x38510000)****
>
>     rw space 12288K,  54% used [0x38510000, 0x38b906f0, 0x38b90800,
> 0x39110000)****
>
> ** **
>
> What's the exact meaning of these multiple address?****
>
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