G1gc compaction algorithm

[Yu Zhang]

Martin,

Humongous objects are treated specially.  They are not collected in 
young gc.

By definition, if the object size > 1/2 region size, it is humongous 
object.  If the object size is just a little over 1/2 region size, then 
the fragmentation could be high.

One trick we can do is to increase region size (-XX:G1HeapRegionSize) so 
that we do not see much humongous allocation.  Do you know the size of 
the humongous objects? If you use -XX:+PrintAdaptiveSizePolicy, you can 
see that information.

The humongous objects are handled better in latest jdk8, and more 
improvement under development.

Thanks,
Jenny

On 7/15/2014 8:45 PM, Martin Makundi wrote:
> Hi!
>
> Humongous allocation fails when there is not enough contiguous free 
> space in memory.
>
> How does g1gc compaction algorithm work, does it compact only within a 
> region or does it attempt to compact in a whole-heap-defragmentation 
> way? The latter would reduce the probability of "humongous allocation 
> failure". And if this is possible, is there a parameter/s that can be 
> changed to tune this?
>
> **
> Martin
>
>
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