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

[Bengt Rutisson]

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
>
> On 2/13/2015 9:42 AM, Karen Kinnear wrote:
>> 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.
>>
>> thanks,
>> Karen
>>
>> 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
>>>>
>