Binary search for large bucket count histograms

src/OpenTelemetry/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+* Use binary search for histograms with 50 or more supplied boundaries.
+  ([#3252](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3252))
+
 * `TracerProviderSDK` modified for spans with remote parent. For such spans
   activity will be created irrespective of SamplingResult, to maintain context
   propagation.
@@ -52,7 +55,7 @@ Released 2022-May-16
 
 * Exposed public setters for `LogRecord.State`, `LogRecord.StateValues`,
   and `LogRecord.FormattedMessage`.
- ([#3217](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3217))
+  ([#3217](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3217))
 
 ## 1.3.0-beta.1
 

src/OpenTelemetry/Metrics/HistogramBuckets.cs

@@ -15,6 +15,8 @@
 // </copyright>
 
 using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
 
 namespace OpenTelemetry.Metrics
 {
@@ -24,6 +26,8 @@ namespace OpenTelemetry.Metrics
     // Note: Does not implement IEnumerable<> to prevent accidental boxing.
     public class HistogramBuckets
     {
+        internal const int DefaultBoundaryCountForBinarySearch = 50;
+
         internal readonly double[] ExplicitBounds;
 
         internal readonly long[] RunningBucketCounts;
@@ -36,9 +40,38 @@ public class HistogramBuckets
 
         internal int IsCriticalSectionOccupied = 0;
 
+        private readonly BucketLookupNode bucketLookupTreeRoot;
+
+        private readonly Func<double, int> findHistogramBucketIndex;
+
         internal HistogramBuckets(double[] explicitBounds)
         {
             this.ExplicitBounds = explicitBounds;
+            this.findHistogramBucketIndex = this.FindBucketIndexLinear;
+            if (explicitBounds != null && explicitBounds.Length >= DefaultBoundaryCountForBinarySearch)
+            {
+                this.bucketLookupTreeRoot = ConstructBalancedBST(explicitBounds, 0, explicitBounds.Length);
+                this.findHistogramBucketIndex = this.FindBucketIndexBinary;
+
+                static BucketLookupNode ConstructBalancedBST(double[] values, int min, int max)
+                {
+                    if (min == max)
+                    {
+                        return null;
+                    }
+
+                    int median = min + ((max - min) / 2);
+                    return new BucketLookupNode
+                    {
+                        Index = median,
+                        UpperBoundInclusive = values[median],
+                        LowerBoundExclusive = median > 0 ? values[median - 1] : double.NegativeInfinity,
+                        Left = ConstructBalancedBST(values, min, median),
+                        Right = ConstructBalancedBST(values, median + 1, max),
+                    };
+                }
+            }
+
             this.RunningBucketCounts = explicitBounds != null ? new long[explicitBounds.Length + 1] : null;
             this.SnapshotBucketCounts = explicitBounds != null ? new long[explicitBounds.Length + 1] : new long[0];
         }
@@ -57,6 +90,55 @@ internal HistogramBuckets Copy()
             return copy;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal int FindBucketIndex(double value)
+        {
+            return this.findHistogramBucketIndex(value);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal int FindBucketIndexBinary(double value)
+        {
+            BucketLookupNode current = this.bucketLookupTreeRoot;
+
+            Debug.Assert(current != null, "Bucket root was null.");
+
+            do
+            {
+                if (value <= current.LowerBoundExclusive)
+                {
+                    current = current.Left;
+                }
+                else if (value > current.UpperBoundInclusive)
+                {
+                    current = current.Right;
+                }
+                else
+                {
+                    return current.Index;
+                }
+            }
+            while (current != null);
+
+            return this.ExplicitBounds.Length;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal int FindBucketIndexLinear(double value)
+        {
+            int i;
+            for (i = 0; i < this.ExplicitBounds.Length; i++)
+            {
+                // Upper bound is inclusive
+                if (value <= this.ExplicitBounds[i])
+                {
+                    break;
+                }
+            }
+
+            return i;
+        }
+
         /// <summary>
         /// Enumerates the elements of a <see cref="HistogramBuckets"/>.
         /// </summary>
@@ -104,5 +186,18 @@ public bool MoveNext()
                 return false;
             }
         }
+
+        private sealed class BucketLookupNode
+        {
+            public double UpperBoundInclusive { get; set; }
+
+            public double LowerBoundExclusive { get; set; }
+
+            public int Index { get; set; }
+
+            public BucketLookupNode Left { get; set; }
+
+            public BucketLookupNode Right { get; set; }
+        }
     }
 }

src/OpenTelemetry/Metrics/MetricPoint.cs

@@ -324,15 +324,7 @@ internal void Update(double number)
 
                 case AggregationType.Histogram:
                     {
-                        int i;
-                        for (i = 0; i < this.histogramBuckets.ExplicitBounds.Length; i++)
-                        {
-                            // Upper bound is inclusive
-                            if (number <= this.histogramBuckets.ExplicitBounds[i])
-                            {
-                                break;
-                            }
-                        }
+                        int i = this.histogramBuckets.FindBucketIndex(number);
 
                         var sw = default(SpinWait);
                         while (true)

test/Benchmarks/Metrics/HistogramBenchmarks.cs

@@ -25,49 +25,50 @@
 
 /*
 BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
-Intel Core i7-9700 CPU 3.00GHz, 1 CPU, 8 logical and 8 physical cores
-.NET SDK=6.0.200
-  [Host]     : .NET 6.0.2 (6.0.222.6406), X64 RyuJIT
-  DefaultJob : .NET 6.0.2 (6.0.222.6406), X64 RyuJIT
-
-
-|                      Method | BoundCount |      Mean |    Error |   StdDev | Allocated |
-|---------------------------- |----------- |----------:|---------:|---------:|----------:|
-|            HistogramHotPath |         10 |  45.27 ns | 0.384 ns | 0.359 ns |         - |
-|  HistogramWith1LabelHotPath |         10 |  89.99 ns | 0.373 ns | 0.312 ns |         - |
-| HistogramWith3LabelsHotPath |         10 | 185.34 ns | 3.184 ns | 3.667 ns |         - |
-| HistogramWith5LabelsHotPath |         10 | 266.69 ns | 1.391 ns | 1.301 ns |         - |
-| HistogramWith7LabelsHotPath |         10 | 323.20 ns | 1.834 ns | 1.531 ns |         - |
-|            HistogramHotPath |         20 |  48.69 ns | 0.347 ns | 0.307 ns |         - |
-|  HistogramWith1LabelHotPath |         20 |  93.84 ns | 0.696 ns | 0.651 ns |         - |
-| HistogramWith3LabelsHotPath |         20 | 189.82 ns | 1.208 ns | 1.071 ns |         - |
-| HistogramWith5LabelsHotPath |         20 | 269.23 ns | 2.027 ns | 1.693 ns |         - |
-| HistogramWith7LabelsHotPath |         20 | 329.92 ns | 1.272 ns | 1.128 ns |         - |
-|            HistogramHotPath |         50 |  55.73 ns | 0.339 ns | 0.317 ns |         - |
-|  HistogramWith1LabelHotPath |         50 | 100.38 ns | 0.455 ns | 0.425 ns |         - |
-| HistogramWith3LabelsHotPath |         50 | 200.02 ns | 1.011 ns | 0.844 ns |         - |
-| HistogramWith5LabelsHotPath |         50 | 279.94 ns | 1.595 ns | 1.492 ns |         - |
-| HistogramWith7LabelsHotPath |         50 | 346.88 ns | 1.064 ns | 0.943 ns |         - |
-|            HistogramHotPath |        100 |  66.39 ns | 0.167 ns | 0.148 ns |         - |
-|  HistogramWith1LabelHotPath |        100 | 114.98 ns | 1.340 ns | 1.253 ns |         - |
-| HistogramWith3LabelsHotPath |        100 | 220.52 ns | 1.723 ns | 1.528 ns |         - |
-| HistogramWith5LabelsHotPath |        100 | 299.10 ns | 1.950 ns | 1.629 ns |         - |
-| HistogramWith7LabelsHotPath |        100 | 356.25 ns | 2.153 ns | 1.798 ns |         - |
+Intel Core i7-1065G7 CPU 1.30GHz, 1 CPU, 8 logical and 4 physical cores
+.NET SDK=6.0.203
+  [Host]     : .NET 6.0.5 (6.0.522.21309), X64 RyuJIT
+  DefaultJob : .NET 6.0.5 (6.0.522.21309), X64 RyuJIT
+
+
+|                      Method | BoundCount |      Mean |     Error |    StdDev | Allocated |
+|---------------------------- |----------- |----------:|----------:|----------:|----------:|
+|            HistogramHotPath |         10 |  55.07 ns |  0.664 ns |  1.091 ns |         - |
+|  HistogramWith1LabelHotPath |         10 | 108.66 ns |  1.324 ns |  1.174 ns |         - |
+| HistogramWith3LabelsHotPath |         10 | 193.79 ns |  3.261 ns |  3.349 ns |         - |
+| HistogramWith5LabelsHotPath |         10 | 279.44 ns |  4.608 ns |  3.848 ns |         - |
+| HistogramWith7LabelsHotPath |         10 | 334.28 ns |  6.650 ns |  5.895 ns |         - |
+|            HistogramHotPath |         49 |  68.27 ns |  0.744 ns |  0.581 ns |         - |
+|  HistogramWith1LabelHotPath |         49 | 125.55 ns |  2.265 ns |  2.518 ns |         - |
+| HistogramWith3LabelsHotPath |         49 | 207.95 ns |  4.023 ns |  3.951 ns |         - |
+| HistogramWith5LabelsHotPath |         49 | 293.45 ns |  5.689 ns |  5.842 ns |         - |
+| HistogramWith7LabelsHotPath |         49 | 362.19 ns |  5.610 ns |  6.003 ns |         - |
+|            HistogramHotPath |         50 |  69.64 ns |  1.422 ns |  1.330 ns |         - |
+|  HistogramWith1LabelHotPath |         50 | 118.15 ns |  2.040 ns |  1.908 ns |         - |
+| HistogramWith3LabelsHotPath |         50 | 250.31 ns |  4.617 ns |  9.326 ns |         - |
+| HistogramWith5LabelsHotPath |         50 | 335.31 ns |  3.904 ns |  3.461 ns |         - |
+| HistogramWith7LabelsHotPath |         50 | 398.02 ns |  6.815 ns |  6.374 ns |         - |
+|            HistogramHotPath |       1000 |  94.05 ns |  1.890 ns |  2.100 ns |         - |
+|  HistogramWith1LabelHotPath |       1000 | 148.57 ns |  2.055 ns |  1.822 ns |         - |
+| HistogramWith3LabelsHotPath |       1000 | 661.78 ns | 11.599 ns | 20.314 ns |         - |
+| HistogramWith5LabelsHotPath |       1000 | 761.54 ns | 15.049 ns | 16.727 ns |         - |
+| HistogramWith7LabelsHotPath |       1000 | 830.14 ns | 16.063 ns | 17.853 ns |         - |
 */
 
 namespace Benchmarks.Metrics
 {
     public class HistogramBenchmarks
     {
-        private const int MaxValue = 1000;
+        private const int MaxValue = 10000;
         private readonly Random random = new();
         private readonly string[] dimensionValues = new string[] { "DimVal1", "DimVal2", "DimVal3", "DimVal4", "DimVal5", "DimVal6", "DimVal7", "DimVal8", "DimVal9", "DimVal10" };
         private Histogram<long> histogram;
         private MeterProvider provider;
         private Meter meter;
         private double[] bounds;
 
-        [Params(10, 20, 50, 100)]
+        // Note: Values related to `HistogramBuckets.DefaultHistogramCountForBinarySearch`
+        [Params(10, 49, 50, 1000)]
         public int BoundCount { get; set; }
 
         [GlobalSetup]

test/OpenTelemetry.Tests/Metrics/AggregatorTest.cs

@@ -104,6 +104,46 @@ public void HistogramDistributeToAllBucketsCustom()
             Assert.Equal(boundaries.Length + 1, actualCount);
         }
 
+        [Fact]
+        public void HistogramBinaryBucketTest()
+        {
+            // Arrange
+            // Bounds = (-Inf, 0] (0, 1], ... (49, +Inf)
+            var boundaries = new double[HistogramBuckets.DefaultBoundaryCountForBinarySearch];
+            for (var i = 0; i < boundaries.Length; i++)
+            {
+                boundaries[i] = i;
+            }
+
+            var histogramPoint = new MetricPoint(this.aggregatorStore, AggregationType.Histogram, null, null, boundaries);
+
+            // Act
+            histogramPoint.Update(-1);
+            histogramPoint.Update(boundaries[0]);
+            histogramPoint.Update(boundaries[boundaries.Length - 1]);
+            for (var i = 0.5; i < boundaries.Length; i++)
+            {
+                histogramPoint.Update(i);
+            }
+
+            histogramPoint.TakeSnapshot(true);
+
+            // Assert
+            var index = 0;
+            foreach (var histogramMeasurement in histogramPoint.GetHistogramBuckets())
+            {
+                var expectedCount = 1;
+
+                if (index == 0 || index == boundaries.Length - 1)
+                {
+                    expectedCount = 2;
+                }
+
+                Assert.Equal(expectedCount, histogramMeasurement.BucketCount);
+                index++;
+            }
+        }
+
         [Fact]
         public void HistogramWithOnlySumCount()
         {