RFR(S): 8048179: Early reclaim of large objects that are referenced by a few objects

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that improves eager reclaim
efficiency at no (measureable) cost?

Until that change we only ever considered humongous objects with
absolutely no references for eager reclaim. This change widens that
restriction by allowing humongous objects with a few references as
candidates.

This works simply by pushing remembered set cards of such humongous
objects into the DCQS at the beginning of evacuation. The update rs
phase will automatically (and in parallel!) determine whether there is
still a reference to an object or not.

Humongous objects that do not contain any remembered set entries after
gc (and the other conditions that prevent reclamation do not hold like
reference from young gen) that object is reclaimed.

I arbitrarily defined "having a few references" as a region having only
sparse remembered set entries.

There is a new global G1ReclaimDeadHumongousObjectsWithRefsAtYoungGC to
turn off this particular feature of early reclamation in case it takes
too long. The default value is true, i.e. it is turned on by default. 

All benchmarks we have looked at showed that there is no particular
pause time impact by this change if it has been ineffective, however
there is a new phase "Humongous Register" in the log that can be used to
determine the impact.

Some benchmarks show some improvement in overall performance due to the
feature reclaiming temporary objects much faster.

There is also some cleanup in the debug messages enabled by
G1TraceReclaimDeadHumongousObjectsAtYoungGC.

CR:
https://bugs.openjdk.java.net/browse/JDK-8048179
Webrev:
http://cr.openjdk.java.net/~tschatzl/8048179/webrev.0/ 
Testing:
jprt, a few benchmarks

Thanks,
  Thomas