RFR: 8266073: Regression ~2% in Derby after 8261804 [v3]

[Leo Korinth]

On Wed, 12 May 2021 17:11:15 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

>> My change in 8261804 made processing_is_mt() to always be false when _active_mt_degree is one. This is better because we then need not to execute on another thread. However, at a few places we do not change the _active_mt_degree if processing_is_mt() is false. We could then be stuck at 1 thread. I now change the code to always change the mt degree. 
>> 
>> Derby does no longer regress for g1, and tier 1 passes. I am running tier 1-3 at the moment.
>
> Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix indentation

My fix is using `ParallelRefProcEnabled` instead of `rp->processing_is_mt()` as `rp->processing_is_mt()` can return false even if `ParallelRefProcEnabled` is true if only one thread is set at the moment. I also set the thread-count directly after calculating it and then use `rp->processing_is_mt()` to decide if we should use a work gang (we should not use a work gang if only one thread is used even if `ParallelRefProcEnabled` is true.

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PR: https://git.openjdk.java.net/jdk/pull/3972