RFR(s): 6764713: Enlarge the age field in object headers to allow a higher MaxTenuringThreshold

Karen Kinnear karen.kinnear at oracle.com
Fri Feb 13 14:42:06 UTC 2015

Seconded. For the hash code, talk to Coleen and ask her who did the work in core libs recently.

In addition, those bits are fiercely sought after - we have other things we would like to do with any available bits and I am
not convinced this is more valuable. We just resisted using one for the jdk9 frozen arrays although we might want one to mark
immutable objects or value types (yes, today those don't have an identity hash but I am not yet convinced that we won't have
a design where we need to distinguish reference objects from value types underneath a common object header for jdk10).

So - hmmm. 


On Feb 12, 2015, at 9:54 PM, David Holmes wrote:

> Hi Tom,
> If you are potentially messing with the (identity) hash of all Java objects in the 32-bit case then this needs a broader discussion eg on core-libs-dev (cc'd) as this will impact end-user code the most!
> The rest seems okay but I'm still mulling over it. :)
> Thanks,
> David H.
> On 13/02/2015 6:14 AM, Tom Benson wrote:
>> Hi,
>> I need reviewers and a commit sponsor for changes for bug 6764713, which
>> will increase the size of the age field in an object header from 4 bits
>> to 5. This will allow a maximum MaxTenuringThreshold of 31, though the
>> default will remain at the current value of 15.
>> This includes the same change to the 32-bit version, which would close
>> bug 6719225 as well.  In that case, the hash field in the header is
>> affected, losing one bit (25 bits -> 24), so I have asked for review
>> from hotspot-runtime-dev as well as gc-dev.
>> Webrev: http://cr.openjdk.java.net/~jprovino/6764713/webrev.00
>> JBS bug: https://bugs.openjdk.java.net/browse/JDK-6764713
>> Testing:  JPRT and reference server performance tests
>> Notes:
>> Contrary to what earlier notes in the JBS entry said, this does not
>> require stronger alignment for the JavaThread structure for when biased
>> locking stores that pointer in the header.   The JavaThread* was already
>> being aligned 1 power of 2 more strongly than it needed to be, so there
>> was an unused bit that could be stolen.
>> In the 32-bit version, it does require taking one bit from the hash
>> field, which goes from 25 to 24 bits.  This is something I'd especially
>> like feedback on.  Running reference server performance tests, I saw no
>> impact from this change.  We *could* make this change 64-bit-only, and
>> leave the age field at 4 bits for the 32-bit version.  If we did so, we
>> could also decrease the alignment required for the JavaThread* to 512
>> from the current 1024.
>> The comment changes imply that the bits available for the JavaThread*
>> have been reduced by 1, and that the alignment is now stronger, but
>> neither is true.  The comments have been corrected to match the
>> alignment that was already enforced.
>> Three tests needed to be corrected to match the new limits.  These check
>> the maximum valid values, what value represents NeverTenure, and so on.
>> Thanks,
>> Tom

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