dotnet · timcassell · Apr 15, 2024
diff --git a/src/BenchmarkDotNet/Engines/Engine.cs b/src/BenchmarkDotNet/Engines/Engine.cs
@@ -3,6 +3,7 @@
 using System.Globalization;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Threading;
 using BenchmarkDotNet.Characteristics;
 using BenchmarkDotNet.Jobs;
 using BenchmarkDotNet.Portability;
@@ -214,31 +215,56 @@ private ClockSpan Measure(Action<long> action, long invokeCount)
 
         private (GcStats, ThreadingStats, double) GetExtraStats(IterationData data)
         {
-            // we enable monitoring after main target run, for this single iteration which is executed at the end
-            // so even if we enable AppDomain monitoring in separate process
-            // it does not matter, because we have already obtained the results!
-            EnableMonitoring();
+            // Warm up the measurement functions before starting the actual measurement.
+            DeadCodeEliminationHelper.KeepAliveWithoutBoxing(GcStats.ReadInitial());
+            DeadCodeEliminationHelper.KeepAliveWithoutBoxing(GcStats.ReadFinal());
 
             IterationSetupAction(); // we run iteration setup first, so even if it allocates, it is not included in the results
 
             var initialThreadingStats = ThreadingStats.ReadInitial(); // this method might allocate
             var exceptionsStats = new ExceptionsStats(); // allocates
             exceptionsStats.StartListening(); // this method might allocate
-            var initialGcStats = GcStats.ReadInitial();
 
-            WorkloadAction(data.InvokeCount / data.UnrollFactor);
+#if !NET7_0_OR_GREATER
+            if (RuntimeInformation.IsNetCore && Environment.Version.Major is >= 3 and <= 6 && RuntimeInformation.IsTieredJitEnabled)
+            {
+                // #1542
+                // We put the current thread to sleep so tiered jit can kick in, compile its stuff,
+                // and NOT allocate anything on the background thread when we are measuring allocations.
+                // This is only an issue on netcoreapp3.0 to net6.0. Tiered jit allocations were "fixed" in net7.0
+                // (maybe not completely eliminated forever, but at least reduced to a point where measurements are much more stable),
+                // and netcoreapp2.X uses only GetAllocatedBytesForCurrentThread which doesn't capture the tiered jit allocations.
+                Thread.Sleep(TimeSpan.FromMilliseconds(500));
+            }
+#endif
 
-            exceptionsStats.Stop();
-            var finalGcStats = GcStats.ReadFinal();
+            // GC collect before measuring allocations.
+            ForceGcCollect();
+            GcStats gcStats;
+            using (FinalizerBlocker.MaybeStart())
+            {
+                gcStats = MeasureWithGc(data.InvokeCount / data.UnrollFactor);
+            }
+
+            exceptionsStats.Stop(); // this method might (de)allocate
             var finalThreadingStats = ThreadingStats.ReadFinal();
 
             IterationCleanupAction(); // we run iteration cleanup after collecting GC stats
 
             var totalOperationsCount = data.InvokeCount * OperationsPerInvoke;
-            GcStats gcStats = (finalGcStats - initialGcStats).WithTotalOperations(totalOperationsCount);
-            ThreadingStats threadingStats = (finalThreadingStats - initialThreadingStats).WithTotalOperations(data.InvokeCount * OperationsPerInvoke);
+            return (gcStats.WithTotalOperations(totalOperationsCount),
+                (finalThreadingStats - initialThreadingStats).WithTotalOperations(totalOperationsCount),
+                exceptionsStats.ExceptionsCount / (double)totalOperationsCount);
+        }
 
-            return (gcStats, threadingStats, exceptionsStats.ExceptionsCount / (double)totalOperationsCount);
+        // Isolate the allocation measurement and skip tier0 jit to make sure we don't get any unexpected allocations.
+        [MethodImpl(MethodImplOptions.NoInlining | CodeGenHelper.AggressiveOptimizationOption)]
+        private GcStats MeasureWithGc(long invokeCount)
+        {
+            var initialGcStats = GcStats.ReadInitial();
+            WorkloadAction(invokeCount);
+            var finalGcStats = GcStats.ReadFinal();
+            return finalGcStats - initialGcStats;
         }
 
         private void RandomizeManagedHeapMemory()
@@ -267,7 +293,7 @@ private void GcCollect()
             ForceGcCollect();
         }
 
-        private static void ForceGcCollect()
+        internal static void ForceGcCollect()
         {
             GC.Collect();
             GC.WaitForPendingFinalizers();
@@ -278,15 +304,6 @@ private static void ForceGcCollect()
 
         public void WriteLine() => Host.WriteLine();
 
-        private static void EnableMonitoring()
-        {
-            if (RuntimeInformation.IsOldMono) // Monitoring is not available in Mono, see http://stackoverflow.com/questions/40234948/how-to-get-the-number-of-allocated-bytes-in-mono
-                return;
-
-            if (RuntimeInformation.IsFullFramework)
-                AppDomain.MonitoringIsEnabled = true;
-        }
-
         [UsedImplicitly]
         public static class Signals
         {
@@ -309,5 +326,56 @@ private static readonly Dictionary<string, HostSignal> MessagesToSignals
             public static bool TryGetSignal(string message, out HostSignal signal)
                 => MessagesToSignals.TryGetValue(message, out signal);
         }
+
+        // Very long key and value so this shouldn't be used outside of unit tests.
+        internal const string UnitTestBlockFinalizerEnvKey = "BENCHMARKDOTNET_UNITTEST_BLOCK_FINALIZER_FOR_MEMORYDIAGNOSER";
+        internal const string UnitTestBlockFinalizerEnvValue = UnitTestBlockFinalizerEnvKey + "_ACTIVE";
+
+        // To prevent finalizers interfering with allocation measurements for unit tests,
+        // we block the finalizer thread until we've completed the measurement.
+        // https://github.com/dotnet/runtime/issues/101536#issuecomment-2077647417
+        private readonly struct FinalizerBlocker : IDisposable
+        {
+            private readonly ManualResetEventSlim hangEvent;
+
+            private FinalizerBlocker(ManualResetEventSlim hangEvent) => this.hangEvent = hangEvent;
+
+            private sealed class Impl
+            {
+                private readonly ManualResetEventSlim hangEvent = new (false);
+                private readonly ManualResetEventSlim enteredFinalizerEvent = new (false);
+
+                ~Impl()
+                {
+                    enteredFinalizerEvent.Set();
+                    hangEvent.Wait();
+                }
+
+                [MethodImpl(MethodImplOptions.NoInlining)]
+                internal static (ManualResetEventSlim hangEvent, ManualResetEventSlim enteredFinalizerEvent) CreateWeakly()
+                {
+                    var impl = new Impl();
+                    return (impl.hangEvent, impl.enteredFinalizerEvent);
+                }
+            }
+
+            internal static FinalizerBlocker MaybeStart()
+            {
+                if (Environment.GetEnvironmentVariable(UnitTestBlockFinalizerEnvKey) != UnitTestBlockFinalizerEnvValue)
+                {
+                    return default;
+                }
+                var (hangEvent, enteredFinalizerEvent) = Impl.CreateWeakly();
+                do
+                {
+                    GC.Collect();
+                    // Do NOT call GC.WaitForPendingFinalizers.
+                }
+                while (!enteredFinalizerEvent.IsSet);
+                return new FinalizerBlocker(hangEvent);
+            }
+
+            public void Dispose() => hangEvent?.Set();
+        }
     }
 }
diff --git a/src/BenchmarkDotNet/Engines/GcStats.cs b/src/BenchmarkDotNet/Engines/GcStats.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Reflection;
+using System.Runtime.CompilerServices;
 using BenchmarkDotNet.Columns;
 using BenchmarkDotNet.Jobs;
 using BenchmarkDotNet.Portability;
@@ -106,9 +107,10 @@ public int GetCollectionsCount(int generation)
             return AllocatedBytes <= AllocationQuantum ? 0L : AllocatedBytes;
         }
 
+        // Skip tier0 jit to make sure we don't get any unexpected allocations in this method.
+        [MethodImpl(CodeGenHelper.AggressiveOptimizationOption)]
         public static GcStats ReadInitial()
         {
-            // this will force GC.Collect, so we want to do this before collecting collections counts
             long? allocatedBytes = GetAllocatedBytes();
 
             return new GcStats(
@@ -119,15 +121,14 @@ public static GcStats ReadInitial()
                 0);
         }
 
+        // Skip tier0 jit to make sure we don't get any unexpected allocations in this method.
+        [MethodImpl(CodeGenHelper.AggressiveOptimizationOption)]
         public static GcStats ReadFinal()
         {
             return new GcStats(
                 GC.CollectionCount(0),
                 GC.CollectionCount(1),
                 GC.CollectionCount(2),
-
-                // this will force GC.Collect, so we want to do this after collecting collections counts
-                // to exclude this single full forced collection from results
                 GetAllocatedBytes(),
                 0);
         }
@@ -136,17 +137,16 @@ public static GcStats ReadFinal()
         public static GcStats FromForced(int forcedFullGarbageCollections)
             => new GcStats(forcedFullGarbageCollections, forcedFullGarbageCollections, forcedFullGarbageCollections, 0, 0);
 
+        // Skip tier0 jit to make sure we don't get any unexpected allocations in this method.
+        [MethodImpl(CodeGenHelper.AggressiveOptimizationOption)]
         private static long? GetAllocatedBytes()
         {
             // we have no tests for WASM and don't want to risk introducing a new bug (https://github.com/dotnet/BenchmarkDotNet/issues/2226)
             if (RuntimeInformation.IsWasm)
                 return null;
 
-            // "This instance Int64 property returns the number of bytes that have been allocated by a specific
-            // AppDomain. The number is accurate as of the last garbage collection." - CLR via C#
-            // so we enforce GC.Collect here just to make sure we get accurate results
-            GC.Collect();
-
+            // Do NOT call GC.Collect() here, as it causes finalizers to run and possibly allocate. https://github.com/dotnet/runtime/issues/101536#issuecomment-2077533242
+            // Instead, we call it before we start the measurement in the Engine.
 #if NET6_0_OR_GREATER
             return GC.GetTotalAllocatedBytes(precise: true);
 #else
@@ -218,9 +218,7 @@ private static long CalculateAllocationQuantumSize()
                     break;
                 }
 
-                GC.Collect();
-                GC.WaitForPendingFinalizers();
-                GC.Collect();
+                Engine.ForceGcCollect();
 
                 result = GC.GetTotalMemory(false);
                 var tmp = new object();

diff --git a/src/BenchmarkDotNet/Portability/RuntimeInformation.cs b/src/BenchmarkDotNet/Portability/RuntimeInformation.cs
@@ -29,47 +29,47 @@ internal static class RuntimeInformation
         internal const string ReleaseConfigurationName = "RELEASE";
         internal const string Unknown = "?";
 
+        // Many of these checks allocate and/or are expensive to compute. We store the results in static readonly fields to keep Engine non-allocating.
+        // Static readonly fields are used instead of properties to avoid an extra getter method call that might not be tier1 jitted.
+        // This class is internal, so we don't need to expose these as properties.
+
         /// <summary>
         /// returns true for both the old (implementation of .NET Framework) and new Mono (.NET 6+ flavour)
         /// </summary>
-        public static bool IsMono { get; } =
-            Type.GetType("Mono.RuntimeStructs") != null; // it allocates a lot of memory, we need to check it once in order to keep Engine non-allocating!
+        public static readonly bool IsMono = Type.GetType("Mono.RuntimeStructs") != null;
 
-        public static bool IsOldMono { get; } = Type.GetType("Mono.Runtime") != null;
+        public static readonly bool IsOldMono = Type.GetType("Mono.Runtime") != null;
 
-        public static bool IsNewMono { get; } = IsMono && !IsOldMono;
+        public static readonly bool IsNewMono = IsMono && !IsOldMono;
 
-        public static bool IsFullFramework =>
+        public static readonly bool IsFullFramework =
 #if NET6_0_OR_GREATER
+            // This could be const, but we want to avoid unreachable code warnings.
             false;
 #else
             FrameworkDescription.StartsWith(".NET Framework", StringComparison.OrdinalIgnoreCase);
 #endif
 
-        [PublicAPI]
-        public static bool IsNetNative => FrameworkDescription.StartsWith(".NET Native", StringComparison.OrdinalIgnoreCase);
+        public static readonly bool IsNetNative = FrameworkDescription.StartsWith(".NET Native", StringComparison.OrdinalIgnoreCase);
 
-        public static bool IsNetCore
-            => ((Environment.Version.Major >= 5) || FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase))
-               && !string.IsNullOrEmpty(typeof(object).Assembly.Location);
+        public static readonly bool IsNetCore =
+            ((Environment.Version.Major >= 5) || FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase))
+                && !string.IsNullOrEmpty(typeof(object).Assembly.Location);
 
-        public static bool IsNativeAOT
-            => Environment.Version.Major >= 5
-               && string.IsNullOrEmpty(typeof(object).Assembly.Location) // it's merged to a single .exe and .Location returns null
-               && !IsWasm; // Wasm also returns "" for assembly locations
+        public static readonly bool IsNativeAOT =
+            Environment.Version.Major >= 5
+                && string.IsNullOrEmpty(typeof(object).Assembly.Location) // it's merged to a single .exe and .Location returns null
+                && !IsWasm; // Wasm also returns "" for assembly locations
 
 #if NET6_0_OR_GREATER
         [System.Runtime.Versioning.SupportedOSPlatformGuard("browser")]
-#endif
-        public static bool IsWasm =>
-#if NET6_0_OR_GREATER
-            OperatingSystem.IsBrowser();
+        public static readonly bool IsWasm = OperatingSystem.IsBrowser();
 #else
-            IsOSPlatform(OSPlatform.Create("BROWSER"));
+        public static readonly bool IsWasm = IsOSPlatform(OSPlatform.Create("BROWSER"));
 #endif
 
 #if NETSTANDARD2_0
-        public static bool IsAot { get; } = IsAotMethod(); // This allocates, so we only want to call it once statically.
+        public static readonly bool IsAot = IsAotMethod();
 
         private static bool IsAotMethod()
         {
@@ -87,11 +87,22 @@ private static bool IsAotMethod()
             return false;
         }
 #else
-        public static bool IsAot => !System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeCompiled;
+        public static readonly bool IsAot = !System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeCompiled;
 #endif
 
-        public static bool IsRunningInContainer => string.Equals(Environment.GetEnvironmentVariable("DOTNET_RUNNING_IN_CONTAINER"), "true");
-
+        public static readonly bool IsTieredJitEnabled =
+            IsNetCore
+            && (Environment.Version.Major < 3
+                // Disabled by default in netcoreapp2.X, check if it's enabled.
+                ? Environment.GetEnvironmentVariable("COMPlus_TieredCompilation") == "1"
+                || Environment.GetEnvironmentVariable("DOTNET_TieredCompilation") == "1"
+                || (AppContext.TryGetSwitch("System.Runtime.TieredCompilation", out bool isEnabled) && isEnabled)
+                // Enabled by default in netcoreapp3.0+, check if it's disabled.
+                : Environment.GetEnvironmentVariable("COMPlus_TieredCompilation") != "0"
+                && Environment.GetEnvironmentVariable("DOTNET_TieredCompilation") != "0"
+                && (!AppContext.TryGetSwitch("System.Runtime.TieredCompilation", out isEnabled) || isEnabled));
+
+        public static readonly bool IsRunningInContainer = string.Equals(Environment.GetEnvironmentVariable("DOTNET_RUNNING_IN_CONTAINER"), "true");
 
         internal static string GetArchitecture() => GetCurrentPlatform().ToString();
 

diff --git a/tests/BenchmarkDotNet.IntegrationTests/BenchmarkDotNet.IntegrationTests.csproj b/tests/BenchmarkDotNet.IntegrationTests/BenchmarkDotNet.IntegrationTests.csproj
@@ -18,6 +18,8 @@
     <Content Include="xunit.runner.json">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </Content>
+    <!-- Disable EventSource to stabilize MemoryDiagnoserTests. https://github.com/dotnet/BenchmarkDotNet/pull/2562#issuecomment-2081317379 -->
+    <RuntimeHostConfigurationOption Include="System.Diagnostics.Tracing.EventSource.IsSupported" Value="false" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\BenchmarkDotNet.IntegrationTests.ConfigPerAssembly\BenchmarkDotNet.IntegrationTests.ConfigPerAssembly.csproj" />