UseCompressedOops switch with more than 32gb heaps...

Vladimir Kozlov vladimir.kozlov at
Thu May 27 18:56:00 UTC 2010

Very soon. Watch for next changes coming into hotspot (jdk7).
I don't know when it will appear in the 6 update release.

"6916623: Align object to 16 bytes to use Compressed Oops with java heap up to 64Gb"
"6954029: Improve implicit null check generation with COOP"

The first bug is closed so you may not see it but I will open it when I am done with changes.


atulksh wrote:
> Hi Vladimir,
> So what you are saying is, in a future Hotspot release, we will have the
> ability to specify a heap greater than 32Gb for a VM and still use the
> useCompressedOops switch. The VM will use compressed oops references, while
> still allowing the VM overall to address more than 32Gb of heap.
> What is the timeframe of such a feature (even if it is vague and not precise
> it is fine) are we looking at - is it coming soon (less than a year)?
> Thanks for your quick response.
> Atul
> Vladimir Kozlov-6 wrote:
>> I am currently working on this. You have to increase java objects
>> alignment
>> (from current 8 bytes) to be able to use compressed pointers (which are 32
>> bit)
>> which large heaps.
>> Thanks,
>> Vladimir
>> atulksh wrote:
>>> Hi,
>>> We are currently running our application servers with 26Gb of heap and
>>> have
>>> the UseCompressedOops enabled for memory savings. We are intending to
>>> boost
>>> the memory on our servers more, in which event, we would be bumping the
>>> heap
>>> above 32Gb. I know that the UseCompressedOops switch works only if you
>>> have
>>> a heap of 32Gb or less.
>>> Is there anything in the Hotspot VM roadmap where there may be support of
>>> heaps larger than 32Gb, and still using the UseCompressedOops switch for
>>> memory optimization (32 bit object pointers within the heap)? Or is this
>>> question irrelevent - meaning there is no possibility to have such a
>>> support
>>> on the 64 bit machines?
>>> Thank you for your response in advance.
>>> Atul

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