max heap size for compressed oops

[Andrew Dinn]

On 11/12/15 15:13, Matthias Wahl wrote:
> Thanks for your quick reply.
> 
> This makes complete sense.
> 
> It sheds some light on how compressed oops work.
> Unfortunately it does not answer my initial question.
  . . .
> Does anybody have a clue on why heaps > 30.5 GB may induce really worse
> performance characteristics? Be it G1 or CMS Garbage Collector.

Evidently it didn't make *complete* sense.

If you retry your experiment using both a 32 GB and a heap slightly
smaller than 32GB you will see that Andrew's answer really does address
your question.

Here's a 32GB heap:

[adinn at sputstik ~]$ java -XX:+UnlockDiagnosticVMOptions
-XX:+PrintCompressedOopsMode -Xmx30g -Xms30g -XX:+UseCompressedOops -version

Protected page at the reserved heap base: 0x00007fe0b7700000 / 524288 bytes

heap address: 0x00007fe0b7780000, size: 30802 MB, Compressed Oops with
base: 0x00007fe0b777f000

java version "1.7.0_75"
Java(TM) SE Runtime Environment (build 1.7.0_75-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.75-b04, mixed mode)

So, this heap is not zero based which means compressed oops are stored
and loaded in other objects as offsets. They need to be converted
to/from addresses by adding or subtracting the heap base.

Now, here's a 28GB heap

[adinn at sputstik ~]$ java -XX:+UnlockDiagnosticVMOptions
-XX:+PrintCompressedOopsMode -Xmx28g -Xms28g -XX:+UseCompressedOops -version

heap address: 0x00000000fad80000, size: 28754 MB, zero based Compressed Oops

java version "1.7.0_75"
Java(TM) SE Runtime Environment (build 1.7.0_75-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.75-b04, mixed mode)
[adinn at sputstik ~]$

This smaller heap is zero-based which means compressed oops are absolute
addresses, rather than offsets. So, they can be stored and loaded
into/from objects without the need to add or subtract a base address.

That is enough to make a *big* difference to performance.

regards,


Andrew Dinn
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Senior Principal Software Engineer
Red Hat UK Ltd
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