RFR: 8248314: Parallel: Parallelize parallel full gc Adjust Roots phase

[Albert Mingkun Yang]

Testing using specjvm2008: the worse cases goes from ~19ms to ~7ms.

before
[0.482s][info][gc,phases] GC(1) Adjust Roots 1.304ms
[0.489s][info][gc,phases] GC(3) Adjust Roots 0.474ms
[182.277s][info][gc,phases] GC(160) Adjust Roots 2.740ms
[214.946s][info][gc,phases] GC(222) Adjust Roots 1.709ms
[300.323s][info][gc,phases] GC(1035) Adjust Roots 2.141ms
[349.820s][info][gc,phases] GC(1408) Adjust Roots 2.197ms
[412.236s][info][gc,phases] GC(1715) Adjust Roots 5.117ms
[422.092s][info][gc,phases] GC(1721) Adjust Roots 5.606ms
[424.787s][info][gc,phases] GC(1725) Adjust Roots 12.448ms
[660.648s][info][gc,phases] GC(1870) Adjust Roots 19.632ms
[778.819s][info][gc,phases] GC(1872) Adjust Roots 8.234ms
[1667.836s][info][gc,phases] GC(1971) Adjust Roots 5.042ms

after
[0.520s][info][gc,phases] GC(1) Adjust Roots 3.996ms
[0.525s][info][gc,phases] GC(3) Adjust Roots 0.437ms
[211.693s][info][gc,phases] GC(178) Adjust Roots 2.682ms
[274.137s][info][gc,phases] GC(823) Adjust Roots 1.751ms
[315.286s][info][gc,phases] GC(1435) Adjust Roots 2.213ms
[409.044s][info][gc,phases] GC(1986) Adjust Roots 3.261ms
[419.851s][info][gc,phases] GC(1992) Adjust Roots 4.337ms
[423.395s][info][gc,phases] GC(1996) Adjust Roots 4.967ms
[637.759s][info][gc,phases] GC(2151) Adjust Roots 7.613ms
[734.576s][info][gc,phases] GC(2153) Adjust Roots 5.029ms
[929.933s][info][gc,phases] GC(2155) Adjust Roots 4.284ms
[1666.213s][info][gc,phases] GC(2312) Adjust Roots 4.272ms

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

Commit messages:
 - adjust

Changes: https://git.openjdk.java.net/jdk/pull/2794/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2794&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248314
  Stats: 84 lines in 3 files changed: 46 ins; 4 del; 34 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2794.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2794/head:pull/2794

PR: https://git.openjdk.java.net/jdk/pull/2794