RFR: 8224660: Parallel GC: Use WorkGang (2: MarksFromRootsTask)

[Kim Barrett]

Leo and I discussed my issues with the task claiming in the first
version. We agreed to use SequentialSubTasksDone in this WorkGang
conversion project, rather than gating that project on fixing various
issues we have with the existing subtask claiming mechanisms. We'll
just squint and ignore those issues for this project, and file some
RFEs.

http://cr.openjdk.java.net/~lkorinth/workgang/1/_8224660-Parallel-GC-Use-WorkGang-2-MarksFromRootsTask/
http://cr.openjdk.java.net/~lkorinth/workgang/1/_8224660-Parallel-GC-Use-WorkGang-2-MarksFromRootsTask-fixup-1/

These look good.

Just one minor naming change requested:

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src/hotspot/share/gc/parallel/psParallelCompact.cpp
2114 static void mark_from_roots_task(ParallelRootType::Value root_type, uint which) {

Instead of "which", please stay with "worker_id" or "worker_i", as
used in the caller and more or less everywhere else.

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