Code changes

src/OpenTelemetry/Metrics/AggregatorStore.cs

@@ -109,22 +109,25 @@ internal AggregatorStore(
 
         this.emitOverflowAttribute = emitOverflowAttribute;
 
+        var reservedMetricPointsCount = 1;
+
         if (emitOverflowAttribute)
         {
             // Setting metricPointIndex to 1 as we would reserve the metricPoints[1] for overflow attribute.
             // Newer attributes should be added starting at the index: 2
             this.metricPointIndex = 1;
+            reservedMetricPointsCount++;
         }
 
         if (this.OutputDelta)
         {
-            this.availableMetricPoints = new Queue<int>(maxMetricPoints - 1);
+            this.availableMetricPoints = new Queue<int>(maxMetricPoints - reservedMetricPointsCount);
 
             // There is no overload which only takes capacity as the parameter
             // Using the DefaultConcurrencyLevel defined in the ConcurrentDictionary class: https://github.com/dotnet/runtime/blob/v7.0.5/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs#L2020
-            // We expect at the most (maxMetricPoints - 1) * 2 entries- one for sorted and one for unsorted input
+            // We expect at the most (maxMetricPoints - reservedMetricPointsCount) * 2 entries- one for sorted and one for unsorted input
             this.tagsToMetricPointIndexDictionaryDelta =
-                new ConcurrentDictionary<Tags, LookupData>(concurrencyLevel: Environment.ProcessorCount, capacity: (maxMetricPoints - 1) * 2);
+                new ConcurrentDictionary<Tags, LookupData>(concurrencyLevel: Environment.ProcessorCount, capacity: (maxMetricPoints - reservedMetricPointsCount) * 2);
 
             this.metricPointReclamationThreshold = maxMetricPoints * 3 / 4;
 
@@ -792,7 +795,7 @@ private int RemoveStaleEntriesAndGetAvailableMetricPointRare(LookupData lookupDa
         // Try to remove stale entries from dictionary
         // Get the index for a new MetricPoint (it could be self-claimed or from another thread that added a fresh entry)
         // If self-claimed, then add a fresh entry to the dictionary
-        // If an available MetricPoint is found, then increment the ReferenceCount
+        // If an available MetricPoint is found, then only increment the ReferenceCount
 
         // Delete the entry for these Tags and get another MetricPoint.
         lock (this.tagsToMetricPointIndexDictionaryDelta)

src/OpenTelemetry/Metrics/LookupData.cs

@@ -22,7 +22,7 @@ internal sealed class LookupData
     public Tags SortedTags;
     public Tags GivenTags;
 
-    public LookupData(int index, Tags sortedTags, Tags givenTags)
+    public LookupData(int index, in Tags sortedTags, in Tags givenTags)
     {
         this.Index = index;
         this.SortedTags = sortedTags;

src/OpenTelemetry/Metrics/MetricPoint.cs

@@ -28,8 +28,9 @@ public struct MetricPoint
 {
     // Represents the number of update threads using this MetricPoint at any given point of time.
     // If the value is equal to int.MinValue which is -2147483648, it means that this MetricPoint is available for reuse.
-    // We never increment the ReferenceCount for MetricPoint with no tags (index == 0) but we always decrement it (in the Update methods).
-    // This should be fine. ReferenceCount doesn't matter for MetricPoint with no tags as it is never reclaimed.
+    // We never increment the ReferenceCount for MetricPoint with no tags (index == 0) and the MetricPoint for overflow attribute,
+    // but we always decrement it (in the Update methods). This should be fine.
+    // ReferenceCount doesn't matter for MetricPoint with no tags and overflow attribute as they are never reclaimed.
     internal int ReferenceCount;
 
     // When the AggregatorStore is reclaiming MetricPoints, this serves the purpose of validating the a given thread is using the right

src/OpenTelemetry/Metrics/Tags.cs

@@ -18,7 +18,7 @@ namespace OpenTelemetry.Metrics;
 
 internal readonly struct Tags : IEquatable<Tags>
 {
-    public static Tags EmptyTags = new Tags(Array.Empty<KeyValuePair<string, object>>());
+    public static readonly Tags EmptyTags = new Tags(Array.Empty<KeyValuePair<string, object>>());
 
     private readonly int hashCode;
 

test/Benchmarks/Metrics/MetricsBenchmarks.cs

@@ -22,27 +22,27 @@
 using OpenTelemetry.Tests;
 
 /*
-BenchmarkDotNet=v0.13.5, OS=Windows 11 (10.0.23424.1000)
+BenchmarkDotNet v0.13.6, Windows 11 (10.0.23424.1000)
 Intel Core i7-9700 CPU 3.00GHz, 1 CPU, 8 logical and 8 physical cores
-.NET SDK=7.0.203
-  [Host]     : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
-  DefaultJob : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
+.NET SDK 7.0.400
+  [Host]     : .NET 7.0.10 (7.0.1023.36312), X64 RyuJIT AVX2
+  DefaultJob : .NET 7.0.10 (7.0.1023.36312), X64 RyuJIT AVX2
 
 
 |                    Method | AggregationTemporality |      Mean |    Error |   StdDev | Allocated |
 |-------------------------- |----------------------- |----------:|---------:|---------:|----------:|
-|            CounterHotPath |             Cumulative |  21.62 ns | 0.201 ns | 0.188 ns |         - |
-| CounterWith1LabelsHotPath |             Cumulative |  71.12 ns | 0.509 ns | 0.476 ns |         - |
-| CounterWith3LabelsHotPath |             Cumulative | 156.46 ns | 1.512 ns | 1.340 ns |         - |
-| CounterWith5LabelsHotPath |             Cumulative | 235.59 ns | 1.273 ns | 1.190 ns |         - |
-| CounterWith6LabelsHotPath |             Cumulative | 264.38 ns | 2.671 ns | 2.368 ns |         - |
-| CounterWith7LabelsHotPath |             Cumulative | 302.73 ns | 1.558 ns | 1.457 ns |         - |
-|            CounterHotPath |                  Delta |  27.11 ns | 0.194 ns | 0.172 ns |         - |
-| CounterWith1LabelsHotPath |                  Delta |  90.23 ns | 0.270 ns | 0.225 ns |         - |
-| CounterWith3LabelsHotPath |                  Delta | 165.63 ns | 0.671 ns | 0.524 ns |         - |
-| CounterWith5LabelsHotPath |                  Delta | 254.29 ns | 0.897 ns | 0.795 ns |         - |
-| CounterWith6LabelsHotPath |                  Delta | 281.36 ns | 1.066 ns | 0.945 ns |         - |
-| CounterWith7LabelsHotPath |                  Delta | 316.55 ns | 3.161 ns | 2.957 ns |         - |
+|            CounterHotPath |             Cumulative |  21.61 ns | 0.084 ns | 0.078 ns |         - |
+| CounterWith1LabelsHotPath |             Cumulative |  69.08 ns | 0.261 ns | 0.244 ns |         - |
+| CounterWith3LabelsHotPath |             Cumulative | 149.77 ns | 0.549 ns | 0.486 ns |         - |
+| CounterWith5LabelsHotPath |             Cumulative | 236.47 ns | 1.684 ns | 1.493 ns |         - |
+| CounterWith6LabelsHotPath |             Cumulative | 276.48 ns | 1.442 ns | 1.349 ns |         - |
+| CounterWith7LabelsHotPath |             Cumulative | 294.09 ns | 2.354 ns | 2.202 ns |         - |
+|            CounterHotPath |                  Delta |  27.32 ns | 0.380 ns | 0.355 ns |         - |
+| CounterWith1LabelsHotPath |                  Delta |  80.83 ns | 0.219 ns | 0.183 ns |         - |
+| CounterWith3LabelsHotPath |                  Delta | 162.48 ns | 1.053 ns | 0.985 ns |         - |
+| CounterWith5LabelsHotPath |                  Delta | 255.48 ns | 1.807 ns | 1.602 ns |         - |
+| CounterWith6LabelsHotPath |                  Delta | 281.75 ns | 2.761 ns | 2.583 ns |         - |
+| CounterWith7LabelsHotPath |                  Delta | 310.29 ns | 1.817 ns | 1.611 ns |         - |
 */
 
 namespace Benchmarks.Metrics;