From eda3c2128af972bf7fe74c020cd4e836fdf0ab8b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Lema=C5=84czyk?= <marek@lemanczyk.eu>
Date: Sun, 19 Oct 2025 21:51:22 +0200
Subject: [PATCH] Align task scheduler defaults with guidance

---
 PerfectNumbers.Core/TaskThreadPool.cs         | 116 ++++++++++++++++++
 PerfectNumbers.Core/UnboundedTaskScheduler.cs |  46 ++++---
 2 files changed, 145 insertions(+), 17 deletions(-)
 create mode 100644 PerfectNumbers.Core/TaskThreadPool.cs

diff --git a/PerfectNumbers.Core/TaskThreadPool.cs b/PerfectNumbers.Core/TaskThreadPool.cs
new file mode 100644
index 0000000..80a106a
--- /dev/null
+++ b/PerfectNumbers.Core/TaskThreadPool.cs
@@ -0,0 +1,116 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace PerfectNumbers.Core;
+
+internal sealed class TaskThreadPool : IDisposable
+{
+    private readonly struct WorkItem
+    {
+        public WorkItem(Task task)
+        {
+            Task = task;
+        }
+
+        public Task Task { get; }
+    }
+
+    private readonly ConcurrentQueue<WorkItem> _pendingTasks = new();
+    private readonly SemaphoreSlim _availableTasks = new(0);
+    private readonly Thread[] _threads;
+    private readonly Action<Task> _taskExecutor;
+    private int _disposed;
+
+    public TaskThreadPool(int minimumThreads, Action<Task> taskExecutor)
+    {
+        _taskExecutor = taskExecutor ?? throw new ArgumentNullException(nameof(taskExecutor));
+        _threads = new Thread[minimumThreads];
+
+        for (int i = 0; i < minimumThreads; i++)
+        {
+            Thread thread = new(WorkerLoop)
+            {
+                IsBackground = true,
+                Name = $"{nameof(TaskThreadPool)}-{i + 1}"
+            };
+
+            _threads[i] = thread;
+            thread.Start();
+        }
+    }
+
+    public int ThreadCount => _threads.Length;
+
+    public void Queue(Task task)
+    {
+        if (Volatile.Read(ref _disposed) != 0)
+        {
+            throw new ObjectDisposedException(nameof(TaskThreadPool));
+        }
+
+        _pendingTasks.Enqueue(new WorkItem(task));
+        _availableTasks.Release();
+    }
+
+    public IEnumerable<Task> GetScheduledTasks()
+    {
+        WorkItem[] snapshot = _pendingTasks.ToArray();
+
+        if (snapshot.Length == 0)
+        {
+            return Array.Empty<Task>();
+        }
+
+        Task[] tasks = new Task[snapshot.Length];
+
+        for (int i = 0; i < snapshot.Length; i++)
+        {
+            tasks[i] = snapshot[i].Task;
+        }
+
+        return tasks;
+    }
+
+    public void Dispose()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+        {
+            return;
+        }
+
+        for (int i = 0; i < _threads.Length; i++)
+        {
+            _availableTasks.Release();
+        }
+
+        foreach (Thread thread in _threads)
+        {
+            thread.Join();
+        }
+
+        _availableTasks.Dispose();
+    }
+
+    private void WorkerLoop()
+    {
+        while (true)
+        {
+            _availableTasks.Wait();
+
+            if (Volatile.Read(ref _disposed) != 0)
+            {
+                return;
+            }
+
+            if (!_pendingTasks.TryDequeue(out WorkItem workItem))
+            {
+                continue;
+            }
+
+            _taskExecutor(workItem.Task);
+        }
+    }
+}
diff --git a/PerfectNumbers.Core/UnboundedTaskScheduler.cs b/PerfectNumbers.Core/UnboundedTaskScheduler.cs
index 56e6f49..ec9209f 100644
--- a/PerfectNumbers.Core/UnboundedTaskScheduler.cs
+++ b/PerfectNumbers.Core/UnboundedTaskScheduler.cs
@@ -1,27 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
 namespace PerfectNumbers.Core;
 
 public sealed class UnboundedTaskScheduler : TaskScheduler
 {
-	public static UnboundedTaskScheduler Instance { get; } = new();
+    public static UnboundedTaskScheduler Instance { get; } = new();
 
-	private UnboundedTaskScheduler()
-	{
-		ThreadPool.SetMaxThreads(100_000, 100_000);
-		ThreadPool.SetMinThreads(10_340, 100);
-	}
+    private readonly TaskThreadPool _threadPool;
 
-	public override int MaximumConcurrencyLevel => int.MaxValue;
+    private UnboundedTaskScheduler()
+    {
+        _threadPool = new TaskThreadPool(10_340, ExecuteTask);
+    }
 
-	protected override void QueueTask(Task task)
-	{
-		ThreadPool.UnsafeQueueUserWorkItem( (state) =>
-		{
-			var (scheduler, task) = ((UnboundedTaskScheduler, Task))state!;
-			scheduler.TryExecuteTask(task);
-		}, (this, task));
-	}
+    public override int MaximumConcurrencyLevel => int.MaxValue;
 
-	protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => TryExecuteTask(task);
+    protected override void QueueTask(Task task)
+    {
+        _threadPool.Queue(task);
+    }
 
-	protected override IEnumerable<Task>? GetScheduledTasks() => null;
+    private void ExecuteTask(Task task)
+    {
+        TryExecuteTask(task);
+    }
+
+    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
+    {
+        return TryExecuteTask(task);
+    }
+
+    protected override IEnumerable<Task>? GetScheduledTasks()
+    {
+        return _threadPool.GetScheduledTasks();
+    }
 }