RFR [L][4/7]: 8180415: Rebuild remembered sets during the concurrent cycle

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that implements the bulk of the
"rebuild remembered sets concurrently" mechanism?

This change modifies the management of remembered sets so that those
are only maintained when they are useful. This roughly means:
- always maintain remembered sets for young gen regions
- (almost) always maintain remembered sets for humongous regions (for
eager reclaim) as long as they meet the eager reclaim criteria.
- never create remembered sets for archive regions
- do not create remembered sets for any regions during full gc
- do not create remembered sets for regions with evacuation failure

(The latter two decisions may not be optimal, but the idea was to
strictly only maintain remembered sets for regions we are going to try
to evacuate soon)

- create remembered sets for old gen regions after finishing marking
(starting at the Remark pause), iff

  - the liveness of these regions indicate so (liveness <=
G1MixedGCLiveThresholdPercent)
  - any (non-objArray) humongous region that does not have a remembered
set yet (eg. because of full gc)

(Please see the new G1RemSetTrackingPolicy implementation for exact
details)

During the following "Rebuild Remembered Set" phase, that replaces the
"Create Live Data" phase, all non-young regions that may contain
outgoing pointers (i.e. excluding closed archive regions), G1 scans the
live objects in the regions between bottom and TARS
("top_at_rebuild_start" - yes, that's new :) for inter-region
references and add them to the appropriate remembered set.

The TARS, as the name indicates, is the top-value at the start of
remark, and the regions contain live data between this value and bottom
that needs to be scanned during this rebuild phase.

The reason for a new indicator is that nTAMS is *not* sufficient:
during marking there might have been more allocations in the old gen
that also need to be scanned for new references.

All references above TARS (and below the region's top) will be handled
by the existing barrier mechanism.

After rebuild, during the Cleanup pause, we create the new (candidate)
collection set, eventually filtering out more regions, like humongous
ones that exceed the eager reclaim criteria.

The current state of the remembered set for a given region is tracked
within the remembered set for that region (HeapRegionRememberedSet
class): a remembered set can be either Untracked, Updating or Complete.
Untracked means that we do not manage a remembered set for this region,
Complete means just that, and Updating means that we are currently in
the process of updating the remembered set. This distinction is
relevant for evacuation, e.g. we obviously must not evacuate not-
Complete remembered sets.

Note that this change contains one single temporary ugly hack that
allows G1 to work until the next change :) - so in this change G1
determines whether there will be a mixed gc phase during the Cleanup
pause. It needs to determine this information at that time, instead of
previously during concurrent cleanup, to, if there is no mixed gc
coming, drop all remembered sets to not maintain them further.

For this reason, and to avoid duplicate recalculation that may yield
different results after dropping remembered sets, there is a new flag
mixed_gc_pending in CollectorState, making it even more complicated.
That one is going away in the next change, promised. :)

Some imho useful messages about remembered set tracking are printed
using "gc, remset, tracking" log tags, both in debug and trace level.

CR:
https://bugs.openjdk.java.net/browse/JDK-8180415
Webrev:
http://cr.openjdk.java.net/~tschatzl/8180415/webrev/index.html
Testing:
hs-tier 1-5, etc. etc.

Thanks,
  Thomas