Parallel GC now ergonomically determines the optimal number of threads to use for processing `java.lang.ref.Reference` instances during garbage collection. The option `-XX:ParallelRefProcEnabled` is now `true` (enabled) by default.

The change improves this phase of the garbage collection pause significantly on machines with more than one thread available for garbage collection.

If you experience increased garbage collection pauses, you can revert to the original behavior by specifying `-XX:-ParallelRefProcEnabled` on the command line.

The ergonomics of `java.lang.ref.Reference` processing can be tuned by using the experimental option `-XX:ReferencesPerThread` (default value: 1000).