RFR: 8266073: Regression ~2% in Derby after 8261804 [v2]

[Leo Korinth]

> 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:

  Fixup after comment by Stefan

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3972/files
  - new: https://git.openjdk.java.net/jdk/pull/3972/files/c8bf51f9..35b602d9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3972&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3972&range=00-01

  Stats: 12 lines in 1 file changed: 5 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3972.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3972/head:pull/3972

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