code: private void cleanup() {
          if ((++counter & 0xFF) != 0) { // (++counter % 256) != 0
              return;
          }
  
          final long currentTimeNanos = System.nanoTime();
          if (currentTimeNanos - lastCleanupTimeNanos < CLEANUP_INTERVAL_NANOS) {
              return;
          }
  
          for (final Iterator<State> i = map.values().iterator(); i.hasNext();) {
              final State state = i.next();
              final boolean remove;
  
              synchronized (state) {
                  remove = state.allActiveRequests == 0 &&
                           currentTimeNanos - state.lastActivityTimeNanos >= CLEANUP_INTERVAL_NANOS;
              }
  
              if (remove) {
                  i.remove();
              }
          }
  
          lastCleanupTimeNanos = System.nanoTime();
      }
docs: Cleans up empty entries with no activity for more than 1 minute. For reduced overhead, we perform this
  only when 1) the last clean-up was more than 1 minute ago and 2) the number of acquisitions % 256 is 0.