Add concurrency to exponential histogram

- Rescaling is not frequent behaviour (for cumulative scale should normalize after initial few recording, for delta it could increase/decrease but would be mostly stable). Instead of opting for a full synchronization on writes (which would have less performance), the choice of ReadWriteLock is chosen.

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/internal/Base2ExponentialHistogram.java

@@ -22,6 +22,7 @@
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.LongAdder;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.stream.Collectors;
 
 import io.micrometer.common.lang.Nullable;
@@ -49,6 +50,21 @@
  */
 public abstract class Base2ExponentialHistogram implements Histogram {
 
+    /*
+     * This is an alternative to full-blown synchronization on the Histogram which would
+     * severely affect the performance under high concurrent cases.
+     *
+     * Read lock guards adding data into histogram i,e multiple values can be recorded at
+     * the same time for a given scale. If scale needs to be changed, write-lock need to
+     * be acquired and all the operations are blocked until new scale is determined and
+     * updated with.
+     */
+    private final ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
+
+    private final ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
+
+    private final ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();
+
     private final int maxScale;
 
     private final int maxBucketsCount;
@@ -131,13 +147,33 @@ public HistogramSnapshot takeSnapshot(final long count, final double total, fina
     }
 
     /**
-     * Returns the snapshot of current recorded values.
+     * Returns the snapshot of current recorded values. This method is thread-safe and
+     * block concurrent recordings to the underlying histogram.
      */
-    ExponentialHistogramSnapShot getCurrentValuesSnapshot() {
-        return (circularCountHolder.isEmpty() && zeroCount.longValue() == 0)
-                ? DefaultExponentialHistogramSnapShot.getEmptySnapshotForScale(scale)
-                : new DefaultExponentialHistogramSnapShot(scale, zeroCount.longValue(), zeroThreshold,
-                        new ExponentialBuckets(getOffset(), getBucketCounts()), EMPTY_EXPONENTIAL_BUCKET);
+    ExponentialHistogramSnapShot getCurrentValuesSnapshot(final boolean shouldResetHistogram) {
+        writeLock.lock();
+        try {
+            ExponentialHistogramSnapShot exponentialHistogramSnapShot = getExponentialHistogramSnapShot();
+            if (shouldResetHistogram)
+                reset();
+            return exponentialHistogramSnapShot;
+        }
+        finally {
+            writeLock.unlock();
+        }
+    }
+
+    private ExponentialHistogramSnapShot getExponentialHistogramSnapShot() {
+        ExponentialHistogramSnapShot exponentialHistogramSnapShot;
+        if (circularCountHolder.isEmpty() && zeroCount.longValue() == 0) {
+            // Will only be possible for a delta variant
+            exponentialHistogramSnapShot = DefaultExponentialHistogramSnapShot.getEmptySnapshotForScale(scale);
+        }
+        else {
+            exponentialHistogramSnapShot = new DefaultExponentialHistogramSnapShot(scale, zeroCount.longValue(),
+                    zeroThreshold, new ExponentialBuckets(getOffset(), getBucketCounts()), EMPTY_EXPONENTIAL_BUCKET);
+        }
+        return exponentialHistogramSnapShot;
     }
 
     /**
@@ -168,24 +204,46 @@ public void recordDouble(double value) {
             return;
         }
 
-        int index = base2IndexProvider.getIndexForValue(value);
-        if (!circularCountHolder.increment(index, 1)) {
-            synchronized (this) {
-                int downScaleFactor = getDownScaleFactor(index);
+        recordInternal(value);
+    }
+
+    private void recordInternal(double value) {
+        boolean hasRecorded;
+        readLock.lock();
+        try {
+            hasRecorded = incrementValue(value);
+        }
+        finally {
+            readLock.unlock();
+        }
+
+        if (!hasRecorded) {
+            // If recording fails, then we MIGHT have to re-scale. Try re-scaling.
+            writeLock.lock();
+            try {
+                final int downScaleFactor = getDownScaleFactor(base2IndexProvider.getIndexForValue(value));
                 downScale(downScaleFactor);
-                index = base2IndexProvider.getIndexForValue(value);
-                circularCountHolder.increment(index, 1);
+                // Record the value within the write lock to guarantee write.
+                incrementValue(value);
+            }
+            finally {
+                writeLock.unlock();
             }
         }
     }
 
+    private boolean incrementValue(final double value) {
+        return circularCountHolder.increment(base2IndexProvider.getIndexForValue(value), 1);
+    }
+
     /**
      * Reduces the scale of the histogram by downScaleFactor. The buckets are merged to
      * align with the exponential scale.
      * @param downScaleFactor - the factor to downscale this histogram.
      */
     private void downScale(int downScaleFactor) {
         if (downScaleFactor == 0) {
+            // Should never happen.
             return;
         }
 
@@ -286,12 +344,12 @@ private List<Long> getBucketCounts() {
      * Reset the current values and possibly increase the scale based on current recorded
      * values;
      */
-    synchronized void reset() {
+    // VisibleForTesting
+    void reset() {
         int upscaleFactor = getUpscaleFactor();
         if (upscaleFactor > 0) {
             this.updateScale(this.scale + upscaleFactor);
         }
-
         this.circularCountHolder.reset();
         this.zeroCount.reset();
     }

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/internal/CircularCountHolder.java

@@ -81,8 +81,19 @@ else if (index < startIndex) {
             startIndex = index;
         }
 
-        counts.addAndGet(getRelativeIndex(index), incrementBy);
-        return true;
+        final int relativeIndex = getRelativeIndex(index);
+        if (relativeIndex >= 0 && relativeIndex < length) {
+            counts.addAndGet(relativeIndex, incrementBy);
+            return true;
+        }
+        /*
+         * This should not happen if the writes are fully exclusive but that would mean a
+         * poor performance. This case only possible (rarely) during initial writes on th
+         * histogram where there might be concurrent threads modifying the index and if it
+         * not fully synchronized externally. In this case, we would notify the caller
+         * that write has failed and let the caller take a call on what to do.
+         */
+        return false;
     }
 
     private int getRelativeIndex(int index) {

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/internal/CumulativeBase2ExponentialHistogram.java

@@ -59,8 +59,8 @@ public ExponentialHistogramSnapShot getLatestExponentialHistogramSnapshot() {
     }
 
     @Override
-    synchronized void takeExponentialHistogramSnapShot() {
-        this.exponentialHistogramSnapShot = getCurrentValuesSnapshot();
+    void takeExponentialHistogramSnapShot() {
+        this.exponentialHistogramSnapShot = getCurrentValuesSnapshot(false);
     }
 
 }

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/internal/DeltaBase2ExponentialHistogram.java

@@ -66,7 +66,7 @@ public ExponentialHistogramSnapShot getLatestExponentialHistogramSnapshot() {
     }
 
     @Override
-    synchronized void takeExponentialHistogramSnapShot() {
+    void takeExponentialHistogramSnapShot() {
         stepExponentialHistogramSnapShot.poll();
     }
 
@@ -82,12 +82,8 @@ public StepExponentialHistogramSnapShot(final Clock clock, final long stepMillis
         }
 
         @Override
-        protected synchronized Supplier<ExponentialHistogramSnapShot> valueSupplier() {
-            return () -> {
-                ExponentialHistogramSnapShot latestSnapShot = getCurrentValuesSnapshot();
-                reset();
-                return latestSnapShot;
-            };
+        protected Supplier<ExponentialHistogramSnapShot> valueSupplier() {
+            return () -> getCurrentValuesSnapshot(true);
         }
 
         @Override

implementations/micrometer-registry-otlp/src/test/java/io/micrometer/registry/otlp/internal/Base2ExponentialHistogramTest.java

@@ -51,8 +51,8 @@ void testRecordDouble() {
         // 1 Always belongs to index 0.
         base2ExponentialHistogram.recordDouble(1.000000000001);
         assertThat(base2ExponentialHistogram.getScale()).isEqualTo(MAX_SCALE);
-        assertThat(base2ExponentialHistogram.getCurrentValuesSnapshot().zeroCount()).isZero();
-        assertThat(getAllBucketsCountSum(base2ExponentialHistogram.getCurrentValuesSnapshot())).isEqualTo(1);
+        assertThat(base2ExponentialHistogram.getCurrentValuesSnapshot(false).zeroCount()).isZero();
+        assertThat(getAllBucketsCountSum(base2ExponentialHistogram.getCurrentValuesSnapshot(false))).isEqualTo(1);
     }
 
     @Test
@@ -66,7 +66,7 @@ void testRecordTimeBased() {
                                                                               // calling
                                                                               // recordDouble(2).
 
-        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.zeroCount()).isZero();
         assertThat(currentSnapshot.scale()).isLessThan(MAX_SCALE);
         assertThat(getAllBucketsCountSum(currentSnapshot)).isEqualTo(2);
@@ -84,13 +84,13 @@ void testRecordTimeBasedInSeconds() {
         // This should be same as calling recordDouble(0.05).
         base2ExponentialHistogram.recordLong(Duration.ofMillis(50).toNanos());
 
-        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.zeroCount()).isZero();
         assertThat(currentSnapshot.scale()).isLessThan(MAX_SCALE);
         assertThat(getAllBucketsCountSum(currentSnapshot)).isEqualTo(2);
 
         base2ExponentialHistogram.recordLong(Duration.ofMillis(90).toNanos());
-        currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.scale()).isEqualTo(1);
         assertThat(getAllBucketsCountSum(currentSnapshot)).isEqualTo(3);
     }
@@ -101,7 +101,7 @@ void testZeroThreshHold() {
         base2ExponentialHistogram.recordDouble(0.0);
         base2ExponentialHistogram.recordDouble(0.5);
 
-        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.zeroThreshold()).isLessThan(1).isGreaterThan(0);
         assertThat(currentSnapshot.zeroCount()).isEqualTo(2);
         assertThat(currentSnapshot.scale()).isEqualTo(MAX_SCALE);
@@ -113,7 +113,7 @@ void testZeroThreshHold() {
         base2ExponentialHistogramWithZeroAsMin.recordDouble(Math.nextUp(0.0));
 
         ExponentialHistogramSnapShot snapshotWithZeroAsMin = base2ExponentialHistogramWithZeroAsMin
-            .getCurrentValuesSnapshot();
+            .getCurrentValuesSnapshot(false);
         assertThat(snapshotWithZeroAsMin.zeroThreshold()).isEqualTo(0.0);
         assertThat(snapshotWithZeroAsMin.zeroCount()).isEqualTo(1);
         assertThat(snapshotWithZeroAsMin.scale()).isEqualTo(MAX_SCALE);
@@ -124,7 +124,7 @@ void testZeroThreshHold() {
     void testDownScale() {
         base2ExponentialHistogram.recordDouble(1.0001);
 
-        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.zeroCount()).isZero();
         assertThat(currentSnapshot.scale()).isEqualTo(MAX_SCALE);
         assertThat(getAllBucketsCountSum(currentSnapshot)).isEqualTo(1);
@@ -182,11 +182,11 @@ void testUpscale() {
 
     @Test
     void testValuesAtIndices() {
-        ExponentialHistogramSnapShot currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentValueSnapshot.positive().bucketCounts()).isEmpty();
 
         base2ExponentialHistogram.recordDouble(1.0001);
-        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentValueSnapshot.positive().offset()).isZero();
         assertThat(currentValueSnapshot.positive().bucketCounts().get(0)).isEqualTo(1);
         assertThat(currentValueSnapshot.positive().bucketCounts()).filteredOn(value -> value == 0).isEmpty();
@@ -195,7 +195,7 @@ void testValuesAtIndices() {
 
         base2ExponentialHistogram.recordDouble(1.0076);
         base2ExponentialHistogram.recordDouble(1.008);
-        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentValueSnapshot.positive().offset()).isZero();
         assertThat(base2ExponentialHistogram.getScale()).isEqualTo(MAX_SCALE);
         assertThat(currentValueSnapshot.positive().bucketCounts().get(0)).isEqualTo(1);
@@ -206,7 +206,7 @@ void testValuesAtIndices() {
         // We will record a value that will downscale by 1 and this should merge 2
         // consecutive buckets into one.
         base2ExponentialHistogram.recordDouble(1.012);
-        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentValueSnapshot.positive().offset()).isZero();
         assertThat(base2ExponentialHistogram.getScale()).isEqualTo(MAX_SCALE - 1);
         assertThat(currentValueSnapshot.positive().bucketCounts().get(0)).isEqualTo(2);
@@ -216,7 +216,7 @@ void testValuesAtIndices() {
 
         // The base will reduced by a factor of more than one,
         base2ExponentialHistogram.recordDouble(4);
-        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentValueSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentValueSnapshot.positive().offset()).isZero();
         assertThat(base2ExponentialHistogram.getScale()).isEqualTo(3);
         assertThat(currentValueSnapshot.positive().bucketCounts().get(0)).isEqualTo(5);
@@ -242,15 +242,15 @@ void reset() {
         base2ExponentialHistogram.recordDouble(1);
         base2ExponentialHistogram.recordDouble(2);
 
-        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        ExponentialHistogramSnapShot currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         final int intialScale = currentSnapshot.scale();
         assertThat(currentSnapshot.zeroCount()).isEqualTo(1);
         assertThat(currentSnapshot.scale()).isLessThan(MAX_SCALE);
         assertThat(currentSnapshot.positive().offset()).isNotZero();
         assertThat(getAllBucketsCountSum(currentSnapshot)).isEqualTo(2);
 
         base2ExponentialHistogram.reset();
-        currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot();
+        currentSnapshot = base2ExponentialHistogram.getCurrentValuesSnapshot(false);
         assertThat(currentSnapshot.zeroCount()).isZero();
         assertThat(currentSnapshot.scale()).isEqualTo(intialScale);
         assertThat(currentSnapshot.positive().offset()).isZero();