RFR (XXS): 8048085: Aborting marking just before remark results in useless additional clearing of the next mark bitmap

[Thomas Schatzl]

Hi all,

  can I have reviews for the following minor performance fix noticed
during work on JDK-8038423 (G1: Decommit memory within the heap)?

The situation is that when a full GC aborts concurrent marking, the
concurrent mark thread needlessly clears the next mark bitmap again
(concurrently this time) in ConcurrentMark::abort().

The fix is to skip this phase in this case.

An alternative, always keeping this phase but not doing this bitmap
clear at Full GC abort (JDK-7098512) would be possible. However doing
that would make JDK-8048084 (sending out reviews soon) much harder as we
would need to keep track of regions that have become unavailable.

This change also does not change anything about duration of Full GC. So
I would prefer to postpone JDK-7098512.

Webrev:
http://cr.openjdk.java.net/~tschatzl/8048085/webrev/

CR:
https://bugs.openjdk.java.net/browse/JDK-8048085

Testing:
JPRT, running a lot of adhoc-test runs as part of the 8038423 changes.

Thanks,
  Thomas