Release Note: Shenandoah reduces the default number of GC threads

In previous releases, Shenandoah GC defaulted to the large number of concurrent and parallel GC threads to do the work. In many cases, this lead to taking a significant CPU toll when GC was running. In worst case, the system can be saturated with the GC work alone.

In this release, the default number of concurrent and parallel Shenandoah GC threads is trimmed down to 1/4 and 1/2 of CPU threads, respectively. Users are advised to check if their installations benefit from these new defaults, or adjust `-XX:ConcGCThreads=#` or `-XX:ParallelGCThreads=#` if needed.