Feature/2005 large composite disposable perf (#2092)

Author: idg10

Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/GroupByCompletion.cs

@@ -0,0 +1,71 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT License.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Reactive.Linq;
+
+using BenchmarkDotNet.Attributes;
+
+namespace Benchmarks.System.Reactive
+{
+    /// <summary>
+    /// Completion of a wide fan-out/in scenario.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This was added to address https://github.com/dotnet/reactive/issues/2005 in which completion
+    /// takes longer and longer to handle as the number of groups increases.
+    /// </para>
+    /// <para>
+    /// The queries in this benchmark represent the common 'fan out/in' pattern in Rx. It is often
+    /// useful to split a stream into groups to enable per-group processing, and then to recombine
+    /// the data back into a single stream. These benchmarks don't do any per-group processing, so
+    /// they might look pointless, but we're trying to measure the minimum unavoidable overhead
+    /// that any code using this technique will encounter.
+    /// </para>
+    /// </remarks>
+    [MemoryDiagnoser]
+    public class GroupByCompletion
+    {
+        private IObservable<int> observable;
+
+        [Params(200_000, 1_000_000)]
+        public int NumberOfSamples { get; set; }
+
+        [Params(10, 100, 1_000, 10_000, 100_000, 150_000, 200_000)]
+        public int NumberOfGroups { get; set; }
+
+        [GlobalSetup]
+        public void GlobalSetup()
+        {
+            var data = new int[NumberOfSamples];
+            for (var i = 0; i < data.Length; ++i)
+            {
+                data[i] = i;
+            }
+
+            observable = data.ToObservable();
+        }
+
+        [Benchmark]
+        public void GroupBySelectMany()
+        {
+            var numberOfGroups = NumberOfGroups;
+
+            observable!.GroupBy(value => value % numberOfGroups)
+                .SelectMany(groupOfInts => groupOfInts)
+                .Subscribe(intValue => { });
+        }
+
+        [Benchmark]
+        public void GroupByMerge()
+        {
+            var numberOfGroups = NumberOfGroups;
+
+            observable!.GroupBy(value => value % numberOfGroups)
+                .Merge()
+                .Subscribe(intValue => { });
+        }
+    }
+}

Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/Program.cs

@@ -27,7 +27,8 @@ private static void Main()
                 typeof(ScalarScheduleBenchmark),
                 typeof(StableCompositeDisposableBenchmark),
                 typeof(SubjectBenchmark),
-                typeof(ComparisonAsyncBenchmark)
+                typeof(ComparisonAsyncBenchmark),
+                typeof(GroupByCompletion)
 #if (CURRENT)
                 ,typeof(AppendPrependBenchmark)
                 ,typeof(PrependVsStartWtihBenchmark)