Update AggregatorStore to reclaim unused MetricPoints for Delta aggregation temporality

src/OpenTelemetry/Metrics/LookupData.cs

@@ -0,0 +1,32 @@
+// <copyright file="LookupData.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// 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
+//
+//     http://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.
+// </copyright>
+
+namespace OpenTelemetry.Metrics
+{
+    internal class LookupData
+    {
+        public int Index;
+        public Tags SortedTags;
+        public Tags GivenTags;
+
+        public LookupData(int index, Tags sortedTags, Tags givenTags)
+        {
+            this.Index = index;
+            this.SortedTags = sortedTags;
+            this.GivenTags = givenTags;
+        }
+    }
+}

src/OpenTelemetry/Metrics/MetricPoint.cs

@@ -26,6 +26,17 @@ namespace OpenTelemetry.Metrics
     /// </summary>
     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.
+        internal int ReferenceCount;
+
+        // When the AggregatorStore is reclaiming MetricPoints, this serves the purpose of validating the a given thread is using the right
+        // MetricPoint for update by checking it against what as added in the Dictionary. Also, when a thread finds out that the MetricPoint
+        // that its using is already reclaimed, this helps avoid sorting of the tags for adding a new Dictionary entry.
+        internal LookupData? LookupData;
+
         // TODO: Ask spec to define a default value for this.
         private const int DefaultSimpleReservoirPoolSize = 10;
 
@@ -49,17 +60,25 @@ internal MetricPoint(
             KeyValuePair<string, object>[] tagKeysAndValues,
             double[] histogramExplicitBounds,
             int exponentialHistogramMaxSize,
-            int exponentialHistogramMaxScale)
+            int exponentialHistogramMaxScale,
+            LookupData? lookupData = null)
         {
             Debug.Assert(aggregatorStore != null, "AggregatorStore was null.");
             Debug.Assert(histogramExplicitBounds != null, "Histogram explicit Bounds was null.");
 
+            if (aggregatorStore!.OutputDelta)
+            {
+                Debug.Assert(lookupData != null, "LookupData was null.");
+            }
+
             this.aggType = aggType;
             this.Tags = new ReadOnlyTagCollection(tagKeysAndValues);
             this.runningValue = default;
             this.snapshotValue = default;
             this.deltaLastValue = default;
             this.MetricPointStatus = MetricPointStatus.NoCollectPending;
+            this.ReferenceCount = 0;
+            this.LookupData = lookupData;
 
             ExemplarReservoir? reservoir = null;
             if (this.aggType == AggregationType.HistogramWithBuckets ||
@@ -420,6 +439,11 @@ internal void Update(long number)
             // TODO: For Delta, this can be mitigated
             // by ignoring Zero points
             this.MetricPointStatus = MetricPointStatus.CollectPending;
+
+            if (this.aggregatorStore.OutputDelta)
+            {
+                Interlocked.Decrement(ref this.ReferenceCount);
+            }
         }
 
         internal void UpdateWithExemplar(long number, ReadOnlySpan<KeyValuePair<string, object>> tags, bool isSampled)
@@ -548,6 +572,11 @@ internal void UpdateWithExemplar(long number, ReadOnlySpan<KeyValuePair<string,
             // TODO: For Delta, this can be mitigated
             // by ignoring Zero points
             this.MetricPointStatus = MetricPointStatus.CollectPending;
+
+            if (this.aggregatorStore.OutputDelta)
+            {
+                Interlocked.Decrement(ref this.ReferenceCount);
+            }
         }
 
         internal void Update(double number)
@@ -639,6 +668,11 @@ internal void Update(double number)
             // TODO: For Delta, this can be mitigated
             // by ignoring Zero points
             this.MetricPointStatus = MetricPointStatus.CollectPending;
+
+            if (this.aggregatorStore.OutputDelta)
+            {
+                Interlocked.Decrement(ref this.ReferenceCount);
+            }
         }
 
         internal void UpdateWithExemplar(double number, ReadOnlySpan<KeyValuePair<string, object>> tags, bool isSampled)
@@ -775,6 +809,11 @@ internal void UpdateWithExemplar(double number, ReadOnlySpan<KeyValuePair<string
             // TODO: For Delta, this can be mitigated
             // by ignoring Zero points
             this.MetricPointStatus = MetricPointStatus.CollectPending;
+
+            if (this.aggregatorStore.OutputDelta)
+            {
+                Interlocked.Decrement(ref this.ReferenceCount);
+            }
         }
 
         internal void TakeSnapshot(bool outputDelta)

src/OpenTelemetry/Metrics/Tags.cs

@@ -18,6 +18,8 @@ namespace OpenTelemetry.Metrics
 {
     internal readonly struct Tags : IEquatable<Tags>
     {
+        public static Tags EmptyTags = new Tags(Array.Empty<KeyValuePair<string, object>>());
+
         private readonly int hashCode;
 
         public Tags(KeyValuePair<string, object>[] keyValuePairs)

test/Benchmarks/Metrics/MetricsBenchmarks.cs

@@ -31,18 +31,18 @@
 
 |                    Method | AggregationTemporality |      Mean |    Error |   StdDev | Allocated |
 |-------------------------- |----------------------- |----------:|---------:|---------:|----------:|
-|            CounterHotPath |             Cumulative |  17.06 ns | 0.113 ns | 0.094 ns |         - |
-| CounterWith1LabelsHotPath |             Cumulative |  71.47 ns | 1.464 ns | 2.100 ns |         - |
-| CounterWith3LabelsHotPath |             Cumulative | 162.04 ns | 2.469 ns | 2.188 ns |         - |
-| CounterWith5LabelsHotPath |             Cumulative | 237.30 ns | 2.884 ns | 2.698 ns |         - |
-| CounterWith6LabelsHotPath |             Cumulative | 269.41 ns | 4.087 ns | 3.623 ns |         - |
-| CounterWith7LabelsHotPath |             Cumulative | 303.01 ns | 5.313 ns | 4.970 ns |         - |
-|            CounterHotPath |                  Delta |  17.30 ns | 0.350 ns | 0.310 ns |         - |
-| CounterWith1LabelsHotPath |                  Delta |  70.96 ns | 0.608 ns | 0.539 ns |         - |
-| CounterWith3LabelsHotPath |                  Delta | 156.55 ns | 3.139 ns | 3.358 ns |         - |
-| CounterWith5LabelsHotPath |                  Delta | 247.14 ns | 4.703 ns | 5.598 ns |         - |
-| CounterWith6LabelsHotPath |                  Delta | 271.30 ns | 5.310 ns | 5.215 ns |         - |
-| CounterWith7LabelsHotPath |                  Delta | 309.02 ns | 5.934 ns | 5.828 ns |         - |
+|            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 |         - |
 */
 
 namespace Benchmarks.Metrics