RFR(s): 6764713: Enlarge the age field in object headers to allow a higher MaxTenuringThreshold
tom.benson at oracle.com
Thu Feb 12 20:14:10 UTC 2015
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.
JBS bug: https://bugs.openjdk.java.net/browse/JDK-6764713
Testing: JPRT and reference server performance tests
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.
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