zentol commented on a change in pull request #8887: [FLINK-12982][metrics] 
improve DescriptiveStatisticsHistogramStatistics performance
URL: https://github.com/apache/flink/pull/8887#discussion_r314324918
 
 

 ##########
 File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/metrics/DescriptiveStatisticsHistogramStatistics.java
 ##########
 @@ -19,53 +19,174 @@
 
 import org.apache.flink.metrics.HistogramStatistics;
 
+import org.apache.commons.math3.exception.MathIllegalArgumentException;
 import org.apache.commons.math3.stat.descriptive.DescriptiveStatistics;
+import org.apache.commons.math3.stat.descriptive.UnivariateStatistic;
+import org.apache.commons.math3.stat.descriptive.moment.SecondMoment;
+import org.apache.commons.math3.stat.descriptive.moment.StandardDeviation;
+import org.apache.commons.math3.stat.descriptive.rank.Percentile;
+import org.apache.commons.math3.stat.ranking.NaNStrategy;
 
 import java.util.Arrays;
 
 /**
  * DescriptiveStatistics histogram statistics implementation returned by 
{@link DescriptiveStatisticsHistogram}.
- * The statistics class wraps a {@link DescriptiveStatistics} instance and 
forwards the method calls accordingly.
+ *
+ * <p>The statistics takes a point-in-time snapshot of a {@link 
DescriptiveStatistics} instance and
+ * allows optimised metrics retrieval from this.
  */
 public class DescriptiveStatisticsHistogramStatistics extends 
HistogramStatistics {
-       private final DescriptiveStatistics descriptiveStatistics;
+       private final CommonMetricsSnapshot statisticsSummary = new 
CommonMetricsSnapshot();
 
        public DescriptiveStatisticsHistogramStatistics(DescriptiveStatistics 
latencyHistogram) {
-               this.descriptiveStatistics = latencyHistogram;
+               latencyHistogram.apply(statisticsSummary);
        }
 
        @Override
        public double getQuantile(double quantile) {
-               return descriptiveStatistics.getPercentile(quantile * 100);
+               return statisticsSummary.getPercentile(quantile * 100);
        }
 
        @Override
        public long[] getValues() {
-               return 
Arrays.stream(descriptiveStatistics.getValues()).mapToLong(i -> (long) 
i).toArray();
+               return Arrays.stream(statisticsSummary.getValues()).mapToLong(i 
-> (long) i).toArray();
        }
 
        @Override
        public int size() {
-               return (int) descriptiveStatistics.getN();
+               return (int) statisticsSummary.getCount();
        }
 
        @Override
        public double getMean() {
-               return descriptiveStatistics.getMean();
+               return statisticsSummary.getMean();
        }
 
        @Override
        public double getStdDev() {
-               return descriptiveStatistics.getStandardDeviation();
+               return statisticsSummary.getStandardDeviation();
        }
 
        @Override
        public long getMax() {
-               return (long) descriptiveStatistics.getMax();
+               return (long) statisticsSummary.getMax();
        }
 
        @Override
        public long getMin() {
-               return (long) descriptiveStatistics.getMin();
+               return (long) statisticsSummary.getMin();
+       }
+
+       /**
+        * Function to extract several commonly used metrics in an optimised 
way, i.e. with as few runs
+        * over the data / calculations as possible.
+        *
+        * <p>Note that calls to {@link #evaluate(double[])} or {@link 
#evaluate(double[], int, int)}
+        * will not return a value but instead populate this class so that 
further values can be
+        * retrieved from it.
+        */
+       private static class CommonMetricsSnapshot implements 
UnivariateStatistic {
+               long count = 0;
 
 Review comment:
   these should all be private since we access them through a getter anyway

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