RFR: 8271722: [TESTBUG] gc/g1/TestMixedGCLiveThreshold.java can fail if G1 Full GC uses >1 workers

[Richard Reingruber]

I tried to make this pr is dependent on #4968. Hope this works...

The change fixes a test issue in gc/g1/TestMixedGCLiveThreshold.java by adding
the option `-XX:ParallelGCThreads=1`. This prevents the full gc from running
with more than one worker because if it would do that, live data could be
distributed into several regions and one of them could be selected for
collection which is unexpected with `-XX:G1MixedGCLiveThresholdPercent=25`.

Please refer to the JBS-Item for more details.

Thanks, Richard.

-------------

Depends on: https://git.openjdk.java.net/jdk/pull/4968

Commit messages:
 - 8271722: [TESTBUG] gc/g1/TestMixedGCLiveThreshold.java can fail if G1 Full GC uses >1 workers

Changes: https://git.openjdk.java.net/jdk/pull/4971/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4971&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271722
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4971.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4971/head:pull/4971

PR: https://git.openjdk.java.net/jdk/pull/4971