Lock-free updates for Histogram (#2951)

Author: utpilla

src/OpenTelemetry/CHANGELOG.md

@@ -2,9 +2,13 @@
 
 ## Unreleased
 
-* Make `IResourceDetector` public to allow custom implementations of resource detectors.
+* Make `IResourceDetector` public to allow custom implementations of resource
+  detectors.
   ([2897](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2897))
 
+* Perf improvement for Histogram, by implementing lock-free updates.
+  ([2951](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2951))
+
 ## 1.2.0-rc2
 
 Released 2022-Feb-02

src/OpenTelemetry/Metrics/HistogramBuckets.cs

@@ -32,6 +32,8 @@ public class HistogramBuckets
 
         internal double SnapshotSum;
 
+        internal int IsCriticalSectionOccupied = 0;
+
         internal HistogramBuckets(double[] explicitBounds)
         {
             this.ExplicitBounds = explicitBounds;

src/OpenTelemetry/Metrics/MetricPoint.cs

@@ -323,11 +323,23 @@ internal void Update(double number)
                             }
                         }
 
-                        lock (this.histogramBuckets.LockObject)
+                        var sw = default(SpinWait);
+                        while (true)
                         {
-                            this.runningValue.AsLong++;
-                            this.histogramBuckets.RunningSum += number;
-                            this.histogramBuckets.RunningBucketCounts[i]++;
+                            if (Interlocked.Exchange(ref this.histogramBuckets.IsCriticalSectionOccupied, 1) == 0)
+                            {
+                                unchecked
+                                {
+                                    this.runningValue.AsLong++;
+                                    this.histogramBuckets.RunningSum += number;
+                                    this.histogramBuckets.RunningBucketCounts[i]++;
+                                }
+
+                                this.histogramBuckets.IsCriticalSectionOccupied = 0;
+                                break;
+                            }
+
+                            sw.SpinOnce();
                         }
 
                         break;