code: final int idlePerActive() {
          // Approximate at powers of two for small values, saturate past 4
          int p = parallelism;
          int a = p + (int)(ctl >> AC_SHIFT);
          return (a > (p >>>= 1) ? 0 :
                  a > (p >>>= 1) ? 1 :
                  a > (p >>>= 1) ? 2 :
                  a > (p >>>= 1) ? 4 :
                  8);
      }
docs: Returns the approximate (non-atomic) number of idle threads per
  active thread to offset steal queue size for method
  ForkJoinTask.getSurplusQueuedTaskCount().