RFR: 8204686: Dynamic parallel reference processing support for Parallel GC

[Albert Mingkun Yang]

On Fri, 28 May 2021 15:26:51 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

> we can then no longer read the flag ParallelRefProcEnabled to see if we are going to do parallel ref processing

Given the semantics of `ParallelRefProcEnabled` is "Enable parallel reference processing whenever possible", I think it's fine to forgo that guarantee, `ParallelRefProcEnabled == true` *iff* parallel ref processing.

That being said, I think a possibly better direction is to remove the ambiguity in the semantics, "Perform parallel reference processing", and keep the check for `ParallelGCThreads > 1` before setting `ParallelRefProcEnabled` to `true`.

PS: the current patch looks fine to me as well.

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PR: https://git.openjdk.java.net/jdk/pull/4253