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

Tom Benson tom.benson at oracle.com
Fri Feb 13 17:16:15 UTC 2015

Note that not taking this bit for the age field would open the door to 
reducing the alignment of the JavaThread*.  It's the fact that there was 
already an unclaimed bit there (in the 64-bit version) that made the age 
size increase seem more reasonable.

However, I'd propose not changing that, either, at least for the 64-bit 
version, so that when someone finally claims "the bit" it doesn't need 
to be undone.  For the 32-bit version, it's less clear cut, but I'd 
still lean toward leaving it as is.

On 2/13/2015 11:37 AM, Bengt Rutisson wrote:
> Hi,
> On 2015-02-13 16:37, Tom Benson wrote:
>> Hi,
>> Based on comments here and elsewhere on possible future uses for this 
>> unused bit (in the 64-bit version), I'm more inclined to close both 
>> 6764713 and 6719225 with no change.  With a comment along the lines 
>> of "evolution of the JVM since the time the age field was reduced has 
>> revealed potentially more valuable uses of the bit."
> This sounds like a good approach in my view. I think we can leave the 
> age at 4 bits. In my view the main issue with the aging is that our 
> heuristics for adjusting the tenuring threshold are not always 
> reliable. Sometimes the threshold gets stuck at the max value etc. I 
> prefer to close these bug reports as suggested above and if we want to 
> improve the tenuring we should work on the heuristics instead.
> Thanks for digging these bug reports up, Tom! We should probably have 
> brought them up for discussion and closing them a long time ago.
> Thanks,
> Bengt
>> However, if there are supporters of a larger MaxTenuringThreshold 
>> lurking, I'd like to hear their point of view as well.
>> Thanks,
>> Tom

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