使用IOStatistics API进行统计收集

if x.samples = 0:
    y
else if y.samples = 0 :
    x
else:
    samples' = x.samples + y.samples
    mean' = (x.mean * x.samples) + (y.mean * y.samples) / samples'
    (samples', mean')

public final class MeanStatistic implements Serializable, Cloneable {
  /**
   * Arithmetic mean.
   */
  private double mean;

  /**
   * Number of samples used to calculate
   * the mean.
   */
  private long samples;

  /**
   * Get the mean value.
   * @return the mean
   */
  public double getMean() {
    return mean;
  }

  /**
   * Get the sample count.
   * @return the sample count; 0 means empty
   */
  public long getSamples() {
    return samples;
  }

  /**
   * Is a statistic empty?
   * @return true if the sample count is 0
   */
  public boolean isEmpty() {
    return samples == 0;
  }
   /**
   * Add another mean statistic to create a new statistic.
   * When adding two statistics, if either is empty then
   * a copy of the non-empty statistic is returned.
   * If both are empty then a new empty statistic is returned.
   *
   * @param other other value
   * @return the aggregate mean
   */
  public MeanStatistic add(final MeanStatistic other) {
    /* Implementation elided. */
  }
  @Override
  public int hashCode() {
    return Objects.hash(mean, samples);
  }

  @Override
  public boolean equals(final Object o) {
    if (this == o) { return true; }
    if (o == null || getClass() != o.getClass()) { return false; }
    MeanStatistic that = (MeanStatistic) o;
    if (this.isEmpty()) {
      return that.isEmpty();
    }
    return Double.compare(that.mean, mean) == 0 &&
        samples == that.samples;
  }

  @Override
  public MeanStatistic clone() {
    return new MeanStatistic(this);
  }

  public MeanStatistic copy() {
    return new MeanStatistic(this);
  }

}

  public static <X extends IOStatistics & Serializable> X
      snapshotIOStatistics(IOStatistics statistics)