double phi() {
      final DescriptiveStatistics ds = new DescriptiveStatistics(heartbeatIntervals);
      double mean = ds.getMean();
      double std = ds.getStandardDeviation();

      /* ensure the std is valid */
      std = Math.max(std, minHeartbeatStd);

      /* add tolerance specified by acceptableHeartbeatPause */
      mean += acceptableHeartbeatPause;

      return p(timeElapsedSinceLastHeartbeat, mean, std);
    }
@return phi value given the heartbeat interval history
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/manager/load/cache/detector/PhiAccrualDetector.java