RFR: 8254027: gc/g1/TestHumongousConcurrentStartUndo.java failed with "'Concurrent Mark Cycle' missing from stdout/stderr"

[Thomas Schatzl]

Hi all,

  can I get reviews for this change to the test to verify concurrent mark/undo cycles to make it more stable?

In the failing scenario, the second concurrent cycle that would typically result in a complete concurrent mark would
start with too few strongly reachable humongous objects, so instead g1 performs an Undo cycle, resulting in that
message.

The changes make sure that if the second concurrent start pause starts there are always enough strongly reachable
humongous objects there to disallow an undo cycle (because the reason we started the cycle has been allocation of that
amount of humongous objects that are all guaranteed to be kept reachable).

Testing: could not reproduce the failure, but 2k runs on linux/aarch64 did not show an issue.

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/523/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=523&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254027
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/523.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/523/head:pull/523

PR: https://git.openjdk.java.net/jdk/pull/523