Add heuristic to trigger GC to finalize dead threads and clean up han… (#10413)

Add heuristic to trigger GC to finalize dead threads and clean up handles and memory

A thread that is started allocates some handles in Thread::AllocHandles(). After it terminates, the managed thread object needs to be collected by a GC for the handles to be released. Some applications that are mostly idle but have a timer ticking periodically will cause a thread pool thread to be created on each tick, since the thread pool preserves threads for a maximum of 20 seconds currently. Over time the number of handles accumulates to a high value. Thread creation adds some memory pressure, but that does not force a GC until a sufficiently large handle count, and for some mostly idle apps, that can be very long. The same issue arises with directly starting threads as well.

Fixes #6602:
- Track a dead thread count separately from the current dead thread count. This is the count that will contribute to triggering a GC, once it reaches a threshold. The count is tracked separately so that it may be reset to zero when a max-generation GC occurs, preventing dead threads that survive a GC due to references from contributing to triggering a GC again in this fashion.
- If the count exceeds a threshold, enumerate dead threads to see which GC generation the corresponding managed thread objects are in. If the duration of time since the last GC of the desired generation also exceeds a threshold, trigger a preferably non-blocking GC on the finalizer thread.
- Removed a couple of handles and some code paths specific to user-requested thread suspension, which is not supported on CoreCLR

2 files changed, 75 insertions, 0 deletions

diff --git a/tests/src/baseservices/threading/DeadThreads/DeadThreads.cs b/tests/src/baseservices/threading/DeadThreads/DeadThreads.cs
new file mode 100644
index 0000000000..cc9f054dea
--- /dev/null
+++ b/tests/src/baseservices/threading/DeadThreads/DeadThreads.cs
@@ -0,0 +1,43 @@
+// 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.Runtime.CompilerServices;
+using System.Threading;
+
+public class DeadThreads
+{
+    /// <summary>
+    /// A sanity test that exercises code paths relevant to the heuristic that triggers GCs based on dead thread count and time
+    /// elapsed since a previous GC. See https://github.com/dotnet/coreclr/pull/10413.
+    /// 
+    /// This test suite runs with the following environment variables relevant to this test (see .csproj):
+    ///     set COMPlus_Thread_DeadThreadCountThresholdForGCTrigger=8
+    ///     set COMPlus_Thread_DeadThreadGCTriggerPeriodMilliseconds=3e8 // 1000
+    /// </summary>
+    private static void GCTriggerSanityTest()
+    {
+        var testDuration = TimeSpan.FromSeconds(8);
+        var startTime = DateTime.UtcNow;
+        do
+        {
+            StartNoOpThread();
+            Thread.Sleep(1);
+        } while (DateTime.UtcNow - startTime < testDuration);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void StartNoOpThread()
+    {
+        var t = new Thread(() => { });
+        t.IsBackground = true;
+        t.Start();
+    }
+
+    public static int Main()
+    {
+        GCTriggerSanityTest();
+        return 100;
+    }
+}
diff --git a/tests/src/baseservices/threading/DeadThreads/DeadThreads.csproj b/tests/src/baseservices/threading/DeadThreads/DeadThreads.csproj
new file mode 100644
index 0000000000..b9c27581a9
--- /dev/null
+++ b/tests/src/baseservices/threading/DeadThreads/DeadThreads.csproj
@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <AssemblyName>DeadThreads</AssemblyName>
+    <ProjectGuid>{649A239B-AB4B-4E20-BDCA-3BA736E8B790}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <!-- Default configurations to help VS understand the configurations -->
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|x64'">
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|x64'">
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="DeadThreads.cs" />
+  </ItemGroup>
+  <PropertyGroup>
+    <CLRTestBatchPreCommands><![CDATA[
+$(CLRTestBatchPreCommands)
+set COMPlus_Thread_DeadThreadCountThresholdForGCTrigger=8
+set COMPlus_Thread_DeadThreadGCTriggerPeriodMilliseconds=3e8
+]]></CLRTestBatchPreCommands>
+    <BashCLRTestPreCommands><![CDATA[
+$(BashCLRTestPreCommands)
+export COMPlus_Thread_DeadThreadCountThresholdForGCTrigger=8
+export COMPlus_Thread_DeadThreadGCTriggerPeriodMilliseconds=3e8
+]]></BashCLRTestPreCommands>
+  </PropertyGroup>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
+</Project>