About the location of data structure and its inner objects

Thomas Schatzl thomas.schatzl at oracle.com
Wed Aug 28 13:32:28 UTC 2013


On Wed, 2013-08-28 at 16:42 +0800, Krystal Mok wrote:
> Hi Lijie,
> On Wed, Aug 28, 2013 at 10:46 AM, Lijie Xu <csxulijie at gmail.com>
> wrote:
>         Thank Tao, I think I have a graph of memory layout now. I have
>         another five questions:

>         1) Can old space's max size be adjusted dynamically by GC
>         while the JVM is running?
> Yes if you're using UseParallelGC / UseParallelOldGC and
> UseAdaptiveSizePolicy is on (it is on by default). Other GCs in
> HotSpot don't implement adaptive size policy yet.

G1 too: In G1 the old gen is simply what is left over from the young gen
mostly sized by pause time and total available memory.

>         3) Whether MinorGC can copy an object directly into Old if the
>         Survior hasn't enough space to hold it currently?
> Yes, it could. That would be called a "premature promotion" /
> "survivor space overflow".
> It is even possible to directly allocate a new object in the old
> generation. Assuming you can read Chinese, refer to this thread for an
> example: http://hllvm.group.iteye.com/group/topic/38293 

This typically affects very large objects (large depending on the
collector, e.g. too large to fit into the young generation). They are
allocated directly into the old generation.


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