RFR: 8254623: gc/g1/TestHumongousConcurrentStartUndo.java still fails sometimes

[Thomas Schatzl]

Hi all,

  can I have reviews for this change of the gc/g1/TestHumongousConcurrentStartUndo.java test to make it even more
  resilient to timing?

Previously the test tried to create a stable initial condition by performing a full gc before that test step, but the
problem is about when the first concurrent mark happens while building up the large set of humongous object kept alive:
if it happens too early, and the concurrent undo operation takes too long, the next concurrent start will just be
another undo because the large set of humongous objects will already be unreferenced and may as well cause a concurrent
undo and not a concurrent mark.

The solution I came up with is that *after* creating all the humongous objects of the large set trigger an extra
concurrent mark (and wait for its completion). That one (or some before) must have been a concurrent mark because at
least at that point the heap must have been occupied by more than the IHOP value.

Testing: 2k iterations on failing platform (always aarch64-linux) without issues.

Thanks,
  Thomas

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/632/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=632&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254623
  Stats: 10 lines in 1 file changed: 6 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/632.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/632/head:pull/632

PR: https://git.openjdk.java.net/jdk/pull/632