RFR: 8280396: G1: Full gc mark stack draining should prefer to make work available to other threads

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes a significant performance problem with g1 full gc marking when the overflow queue is in use? Basically this case causes the threads never share their work from the overflow queue, resulting in very bad parallelization in some cases.

This is a problem that has been fixed already in [JDK-8152438](https://bugs.openjdk.java.net/browse/JDK-8152438), but when looking at this code on g1 and parallel full gc, they both showed that issue.

This is the change for g1.

The CR shows some numbers how bad this can be.

Thanks,
  Thomas

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/7175/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7175&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280396
  Stats: 19 lines in 1 file changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7175.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7175/head:pull/7175

PR: https://git.openjdk.java.net/jdk/pull/7175