Reduce memory footprint of StepBucketHistogram

micrometer-core/src/main/java/io/micrometer/core/instrument/distribution/FixedBoundaryHistogram.java

@@ -27,14 +27,32 @@ class FixedBoundaryHistogram {
 
     private final boolean isCumulativeBucketCounts;
 
+    /**
+     * Creates a FixedBoundaryHistogram which tracks the count of values for each bucket
+     * bound).
+     * @param buckets - sorted bucket boundaries
+     * @param isCumulativeBucketCounts - whether the count values should be cumulative
+     * count of lower buckets and current bucket.
+     */
     FixedBoundaryHistogram(double[] buckets, boolean isCumulativeBucketCounts) {
         this.buckets = buckets;
         this.values = new AtomicLongArray(buckets.length);
         this.isCumulativeBucketCounts = isCumulativeBucketCounts;
     }
 
-    long countAtValue(double value) {
-        int index = Arrays.binarySearch(buckets, value);
+    double[] getBuckets() {
+        return this.buckets;
+    }
+
+    /**
+     * Returns the number of values that was recorded between previous bucket and the
+     * queried bucket (upper bound inclusive)
+     * @param bucket - the bucket to find values for
+     * @return 0 if bucket is not a valid bucket otherwise number of values recorded
+     * between (index(bucket) - 1, bucket]
+     */
+    private long countAtBucket(double bucket) {
+        int index = Arrays.binarySearch(buckets, bucket);
         if (index < 0)
             return 0;
         return values.get(index);
@@ -53,18 +71,20 @@ void record(long value) {
     }
 
     /**
-     * The least bucket that is less than or equal to a sample.
+     * The least bucket that is less than or equal to a valueToRecord. Returns -1, if the
+     * valueToRecord is greater than the highest bucket.
      */
-    int leastLessThanOrEqualTo(double key) {
+    // VisibleForTesting
+    int leastLessThanOrEqualTo(long valueToRecord) {
         int low = 0;
         int high = buckets.length - 1;
 
         while (low <= high) {
             int mid = (low + high) >>> 1;
-            double value = buckets[mid];
-            if (value < key)
+            double bucket = buckets[mid];
+            if (bucket < valueToRecord)
                 low = mid + 1;
-            else if (value > key)
+            else if (bucket > valueToRecord)
                 high = mid - 1;
             else
                 return mid; // exact match
@@ -73,28 +93,49 @@ else if (value > key)
         return low < buckets.length ? low : -1;
     }
 
-    Iterator<CountAtBucket> countsAtValues(Iterator<Double> values) {
+    Iterator<CountAtBucket> countsAtValues(Iterator<Double> buckets) {
         return new Iterator<CountAtBucket>() {
             private double cumulativeCount = 0.0;
 
             @Override
             public boolean hasNext() {
-                return values.hasNext();
+                return buckets.hasNext();
             }
 
             @Override
             public CountAtBucket next() {
-                double value = values.next();
-                double count = countAtValue(value);
+                double bucket = buckets.next();
+                double count = countAtBucket(bucket);
                 if (isCumulativeBucketCounts) {
                     cumulativeCount += count;
-                    return new CountAtBucket(value, cumulativeCount);
+                    return new CountAtBucket(bucket, cumulativeCount);
                 }
                 else {
-                    return new CountAtBucket(value, count);
+                    return new CountAtBucket(bucket, count);
                 }
             }
         };
     }
 
+    /**
+     * Returns the list of {@link CountAtBucket} for each of the buckets tracked by this
+     * histogram.
+     */
+    CountAtBucket[] getCountsAtBucket() {
+        CountAtBucket[] countAtBuckets = new CountAtBucket[this.buckets.length];
+        long cumulativeCount = 0;
+
+        for (int i = 0; i < this.buckets.length; i++) {
+            final long valueAtCurrentBucket = values.get(i);
+            if (isCumulativeBucketCounts) {
+                cumulativeCount += valueAtCurrentBucket;
+                countAtBuckets[i] = new CountAtBucket(buckets[i], cumulativeCount);
+            }
+            else {
+                countAtBuckets[i] = new CountAtBucket(buckets[i], valueAtCurrentBucket);
+            }
+        }
+        return countAtBuckets;
+    }
+
 }

micrometer-core/src/main/java/io/micrometer/core/instrument/distribution/StepBucketHistogram.java

@@ -19,8 +19,6 @@
 import io.micrometer.core.instrument.config.InvalidConfigurationException;
 import io.micrometer.core.instrument.step.StepValue;
 
-import java.util.Arrays;
-import java.util.Iterator;
 import java.util.NavigableSet;
 import java.util.Objects;
 import java.util.function.Supplier;
@@ -36,16 +34,14 @@ public class StepBucketHistogram extends StepValue<CountAtBucket[]> implements H
 
     private final FixedBoundaryHistogram fixedBoundaryHistogram;
 
-    private final double[] buckets;
-
     public StepBucketHistogram(Clock clock, long stepMillis, DistributionStatisticConfig distributionStatisticConfig,
             boolean supportsAggregablePercentiles, boolean isCumulativeBucketCounts) {
         super(clock, stepMillis, getEmptyCounts(
                 getBucketsFromDistributionStatisticConfig(distributionStatisticConfig, supportsAggregablePercentiles)));
 
-        this.buckets = getBucketsFromDistributionStatisticConfig(distributionStatisticConfig,
-                supportsAggregablePercentiles);
-        this.fixedBoundaryHistogram = new FixedBoundaryHistogram(buckets, isCumulativeBucketCounts);
+        this.fixedBoundaryHistogram = new FixedBoundaryHistogram(
+                getBucketsFromDistributionStatisticConfig(distributionStatisticConfig, supportsAggregablePercentiles),
+                isCumulativeBucketCounts);
     }
 
     @Override
@@ -66,13 +62,9 @@ public HistogramSnapshot takeSnapshot(long count, double total, double max) {
     @Override
     protected Supplier<CountAtBucket[]> valueSupplier() {
         return () -> {
-            CountAtBucket[] countAtBuckets = new CountAtBucket[buckets.length];
+            CountAtBucket[] countAtBuckets;
             synchronized (fixedBoundaryHistogram) {
-                final Iterator<CountAtBucket> iterator = fixedBoundaryHistogram
-                    .countsAtValues(Arrays.stream(buckets).iterator());
-                for (int i = 0; i < countAtBuckets.length; i++) {
-                    countAtBuckets[i] = iterator.next();
-                }
+                countAtBuckets = fixedBoundaryHistogram.getCountsAtBucket();
                 fixedBoundaryHistogram.reset();
             }
             return countAtBuckets;
@@ -81,7 +73,7 @@ protected Supplier<CountAtBucket[]> valueSupplier() {
 
     @Override
     protected CountAtBucket[] noValue() {
-        return getEmptyCounts(buckets);
+        return getEmptyCounts(this.fixedBoundaryHistogram.getBuckets());
     }
 
     private static CountAtBucket[] getEmptyCounts(double[] buckets) {

micrometer-core/src/test/java/io/micrometer/core/instrument/distribution/FixedBoundaryHistogramTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2024 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.core.instrument.distribution;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class FixedBoundaryHistogramTest {
+
+    private static final double[] BUCKET_BOUNDS = new double[] { 1, 10, 100 };
+
+    private FixedBoundaryHistogram fixedBoundaryHistogram;
+
+    @BeforeEach
+    void setup() {
+        fixedBoundaryHistogram = new FixedBoundaryHistogram(BUCKET_BOUNDS, false);
+    }
+
+    @Test
+    void testGetBuckets() {
+        assertThat(fixedBoundaryHistogram.getBuckets()).containsExactly(BUCKET_BOUNDS);
+    }
+
+    @ParameterizedTest
+    @MethodSource("valuedIndexProvider")
+    void testLeastLessThanOrEqualTo(long value, int expectedIndex) {
+        assertThat(fixedBoundaryHistogram.leastLessThanOrEqualTo(value)).isEqualTo(expectedIndex);
+    }
+
+    private static Stream<Arguments> valuedIndexProvider() {
+        return Stream.of(Arguments.of(0, 0), Arguments.of(1, 0), Arguments.of(2, 1), Arguments.of(5, 1),
+                Arguments.of(10, 1), Arguments.of(11, 2), Arguments.of(90, 2), Arguments.of(100, 2),
+                Arguments.of(101, -1), Arguments.of(Long.MAX_VALUE, -1));
+    }
+
+    @Test
+    void testReset() {
+        fixedBoundaryHistogram.record(1);
+        fixedBoundaryHistogram.record(10);
+        fixedBoundaryHistogram.record(100);
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).allMatch(countAtBucket -> countAtBucket.count() == 1);
+        fixedBoundaryHistogram.reset();
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).allMatch(countAtBucket -> countAtBucket.count() == 0);
+    }
+
+    @Test
+    void testCountsAtBucket() {
+        fixedBoundaryHistogram.record(1);
+        fixedBoundaryHistogram.record(10);
+        fixedBoundaryHistogram.record(100);
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).allMatch(countAtBucket -> countAtBucket.count() == 1);
+        fixedBoundaryHistogram.reset();
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).allMatch(countAtBucket -> countAtBucket.count() == 0);
+        fixedBoundaryHistogram.record(0);
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).containsExactly(new CountAtBucket(1.0, 1),
+                new CountAtBucket(10.0, 0), new CountAtBucket(100.0, 0));
+    }
+
+    @Test
+    void testCumulativeCounts() {
+        fixedBoundaryHistogram = new FixedBoundaryHistogram(BUCKET_BOUNDS, true);
+        fixedBoundaryHistogram.record(1);
+        fixedBoundaryHistogram.record(10);
+        fixedBoundaryHistogram.record(100);
+        assertThat(fixedBoundaryHistogram.getCountsAtBucket()).containsExactly(new CountAtBucket(1.0, 1),
+                new CountAtBucket(10.0, 2), new CountAtBucket(100.0, 3));
+    }
+
+}

micrometer-core/src/test/java/io/micrometer/core/instrument/distribution/StepHistogramTest.java → micrometer-core/src/test/java/io/micrometer/core/instrument/distribution/StepBucketHistogramTest.java

@@ -23,7 +23,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-class StepHistogramTest {
+class StepBucketHistogramTest {
 
     MockClock clock = new MockClock();