RFR (S): JDK-8028710: G1 does not retire allocation buffers after reference processing work

[Thomas Schatzl]

Hi all,

  can I have some reviews for the following bugfix?

When doing parallel reference processing, its allocation buffers are not
retired. This causes problems with the not-retired buffer being wrongly
formatted (however I could not trigger a bug because of that since that
area does not contain live objects, and G1 always uses one of the
liveness bitmaps for walking the heap).

Further, and typically more importantly, it inflates the PLAB sizes as
the space wasted by these buffers (typically a lot compared to the
actually number of copied bytes) is not accounted for when PLAB resizing
occurs.

You may notice that this CR adds the check that the PLABs must be
retired (in G1ParGCAllocBuffer::~G1ParGCAllocBuffer): this is because
when the problem had been identified, there has been one, which was part
of JDK-6976350. That change has been reverted since though.

The change further fixes a few areas which lead to that issue: first
G1ParGCAllocBuffer hides but not overrides the set_buf() and retire()
methods, which, when the latter is called via
ParGCAllocBuffer::flush_stats_and_retire() leads to a call of the wrong
retire() method. So the _retired member of G1ParGCAllocBuffer is never
set to true.
So these two methods were made virtual in ParGCAllocBuffer.

Another issue fixed is that the initial value of _retired should be
true, as buffers are assigned lazily.

Finally, to avoid missing calls to
ParGCAllocBuffer::retire_alloc_buffers(), I put the call to it into the
destructor of G1ParScanThreadState so that it does not need to be called
manually any more (which means that its visibility changed to private).

Webrev:
http://cr.openjdk.java.net/~tschatzl/8028710/webrev/

CR:
https://bugs.openjdk.java.net/browse/JDK-8028710

Testing:
no perf regressions on any standard benchmark without parallel reference
processing with g1 and pargc on a few platforms, jprt

Thanks,
Thomas