Pools renamed to FixedCapacityPools. Other fixed capacity collections added. Queues, concurrent queues and standard stacks replaced with fixed capacity stacks. MethodImplOptions.AggressiveInlining and .AggressiveOptimization added to simple methods and getters / setters. Variables optimized in few places.

PerfectNumbers.Core/ArrayPools.cs

@@ -1,104 +0,0 @@
-using System.Runtime.CompilerServices;
-
-namespace PerfectNumbers.Core
-{
-	public static class Pools
-	{
-		private const int PoolCapacity = 10240; //PerfectNumberConstants.DefaultSmallPrimeFactorSlotCount << 8;
-
-		[ThreadStatic]
-		private static IntArrayPool? _intArrayPool;
-		internal static IntArrayPool ExclusiveIntArray
-		{
-			[MethodImpl(MethodImplOptions.AggressiveInlining)]
-			get
-			{
-				return _intArrayPool ??= new(PoolCapacity);
-			}
-		}
-
-
-		[ThreadStatic]
-		private static UlongArrayPool? _ulongArrayPool;
-		internal static UlongArrayPool ExclusiveUlongArray
-		{
-			[MethodImpl(MethodImplOptions.AggressiveInlining)]
-			get
-			{
-				return _ulongArrayPool ??= new(PoolCapacity);
-			}
-		}
-	}
-
-    public struct IntArrayPool(int poolCapacity)
-	{
-        private readonly int[][] _arrays = new int[poolCapacity][];
-        private int _count = 0;
-
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public int[] Rent(int length)
-        {
-            int available = _count;
-            if (available == 0)
-            {
-                return new int[length];
-            }
-
-            int[][] arrays = _arrays;
-            available--;
-            _count = available;
-
-            int[] candidate = arrays[available];
-			return candidate.Length >= length ? candidate : new int[length];
-		}
-
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Return(int[] array)
-        {
-            int[][] arrays = _arrays;
-            int available = _count;
-
-            if (available < arrays.Length)
-            {
-				arrays[available] = array;
-				_count = available + 1;
-            }
-        }
-    }
-
-    public struct UlongArrayPool(int poolCapacity)
-	{
-        private readonly ulong[][] _arrays = new ulong[poolCapacity][];
-        private int _count = 0;
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ulong[] Rent(int length)
-        {
-            int available = _count;
-            if (available == 0)
-            {
-                return new ulong[length];
-            }
-
-            ulong[][] arrays = _arrays;
-            available--;
-            _count = available;
-
-            ulong[] candidate = arrays[available];
-			return candidate.Length >= length ? candidate : new ulong[length];
-		}
-
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Return(ulong[] array)
-        {
-            ulong[][] arrays = _arrays;
-            int available = _count;
-
-            if (available < arrays.Length)
-            {
-				arrays[available] = array;
-				_count = available + 1;
-            }
-        }
-    }
-}

PerfectNumbers.Core/ConcurrentFixedCapacityStack.cs

@@ -0,0 +1,77 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+public sealed class ConcurrentFixedCapacityStack<T>(int capacity)
+{
+	private static readonly FixedCapacityArrayPool<T> _arrayPool = FixedCapacityPools<T>.ExclusiveArray;
+
+	private readonly T[] _items = _arrayPool.Rent(capacity);
+	private readonly int _capacity = capacity;
+
+	private int _count = 0;
+	public int Count
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => Volatile.Read(ref _count);
+	}
+
+	public int Capacity
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _capacity;
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Clear() => Interlocked.Exchange(ref _count, 0);
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public T? Pop() => TryPop(out T? item) ? item : default;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public bool TryPop(out T? item)
+	{
+		while (true)
+		{
+			int observed = Volatile.Read(ref _count);
+			if (observed == 0)
+			{
+				item = default;
+				return false;
+			}
+
+			int next = observed - 1;
+			if (Interlocked.CompareExchange(ref _count, next, observed) == observed)
+			{
+				item = _items[next];
+				return true;
+			}
+		}
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Push(T item)
+	{
+		int next = Interlocked.Increment(ref _count);
+		if (next <= _capacity)
+		{
+			_items[next - 1] = item;
+			return;
+		}
+
+		Interlocked.Decrement(ref _count);
+		throw new InvalidOperationException("Stack is full.");
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public static void Return(in T[] items) => _arrayPool.Return(items);
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public T[] ToArray()
+	{
+		int count = Volatile.Read(ref _count);
+		T[] result = _arrayPool.Rent(count);
+		Array.Copy(_items, result, count);
+		return result;
+	}
+}

PerfectNumbers.Core/Cpu/MersenneNumberDivisorByDivisorCpuTester.cs

@@ -157,18 +157,8 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
         }
     }
 
-	public IMersenneNumberDivisorByDivisorTester.IDivisorScanSession CreateDivisorSession(PrimeOrderCalculatorAccelerator gpu)
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-	{
+	public IMersenneNumberDivisorByDivisorTester.IDivisorScanSession CreateDivisorSession(PrimeOrderCalculatorAccelerator gpu) => ThreadStaticPools.RentMersenneCpuDivisorSession(gpu, _orderDevice);
-		MersenneCpuDivisorScanSession? session = ThreadStaticPools.RentMersenneCpuDivisorSession();
-		
-		if (session is not null)
-		{
-			session.Configure(gpu, _orderDevice);
-			return session;
-		}
-
-		return new MersenneCpuDivisorScanSession(gpu, _orderDevice);
-	}
 
 	private bool CheckDivisors(
         PrimeOrderCalculatorAccelerator gpu,
@@ -564,7 +554,7 @@ private bool CheckDivisors64(
 		out ulong foundDivisor)
 	{
 		bool canAdvance = step <= limit;
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		if (!canAdvance)
 		{
 			if (divisor <= limit && residueStepper.IsAdmissible())

PerfectNumbers.Core/DivisorCycleCache.cs

@@ -55,6 +55,7 @@ public sealed class DivisorCycleCache
 	{
 		if (!skipPrimeOrderHeuristic)
 		{
+			// TODO: Implement array-free version of GetCycleLengthsCpu so that we don't pay the array costs for single checks.
 			Span<ulong> result = stackalloc ulong[1];
 			ReadOnlySpan<ulong> singleDivisor = stackalloc ulong[1] { divisor };
 			GetCycleLengthsCpu(singleDivisor, result);
@@ -82,7 +83,7 @@ public sealed class DivisorCycleCache
 			return cycleLength;
 		}
 
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor);
 		cycleLength = MersenneDivisorCycles.CalculateCycleLengthCpu(divisor, divisorData, skipPrimeOrderHeuristic: true);
 		divisorPool.Return(divisorData);
@@ -93,6 +94,7 @@ public sealed class DivisorCycleCache
 	{
 		if (!skipPrimeOrderHeuristic)
 		{
+			// TODO: Implement array-free version of GetCycleLengthsHybrid so that we don't pay the array costs for single checks.
 			Span<ulong> result = stackalloc ulong[1];
 			ReadOnlySpan<ulong> singleDivisor = stackalloc ulong[1] { divisor };
 			GetCycleLengthsHybrid(gpu, singleDivisor, result);
@@ -120,7 +122,7 @@ public sealed class DivisorCycleCache
 			return cycleLength;
 		}
 
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor);
 		cycleLength = MersenneDivisorCycles.CalculateCycleLengthHybrid(gpu, divisor, divisorData, skipPrimeOrderHeuristic: true);
 		divisorPool.Return(divisorData);
@@ -158,7 +160,7 @@ public sealed class DivisorCycleCache
 			return cycleLength;
 		}
 
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor);
 		cycleLength = MersenneDivisorCycles.CalculateCycleLengthGpu(gpu, divisor, divisorData, skipPrimeOrderHeuristic: true);
 		divisorPool.Return(divisorData);
@@ -346,7 +348,7 @@ public sealed class DivisorCycleCache
 	[MethodImpl(MethodImplOptions.AggressiveInlining)]
 	private static void ComputeCyclesCpu(ReadOnlySpan<ulong> divisors, Span<ulong> cycles, ReadOnlySpan<int> indices)
 	{
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		for (int i = 0; i < indices.Length; i++)
 		{
 			int targetIndex = indices[i];
@@ -360,7 +362,7 @@ public sealed class DivisorCycleCache
 	[MethodImpl(MethodImplOptions.AggressiveInlining)]
 	private static void ComputeCyclesHybrid(PrimeOrderCalculatorAccelerator gpu, ReadOnlySpan<ulong> divisors, Span<ulong> cycles, ReadOnlySpan<int> indices)
 	{
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		for (int i = 0; i < indices.Length; i++)
 		{
 			int targetIndex = indices[i];

PerfectNumbers.Core/FixedCapacityArrayPool.cs

@@ -0,0 +1,40 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core
+{
+	public struct FixedCapacityArrayPool<T>(int poolCapacity)
+	{
+        private readonly T[][] _arrays = new T[poolCapacity][];
+        private int _count = 0;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public T[] Rent(int length)
+        {
+            int available = _count;
+            if (available == 0)
+            {
+                return new T[length];
+            }
+
+            T[][] arrays = _arrays;
+            available--;
+            _count = available;
+
+            T[] candidate = arrays[available];
+			return candidate.Length >= length ? candidate : new T[length];
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public void Return(T[] array)
+        {
+            T[][] arrays = _arrays;
+            int available = _count;
+
+            if (available < arrays.Length)
+            {
+				arrays[available] = array;
+				_count = available + 1;
+            }
+        }
+    }
+}

PerfectNumbers.Core/FixedCapacityPools.cs

@@ -0,0 +1,37 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core
+{
+	public static class FixedCapacityPools<T>
+	{
+		[ThreadStatic]
+		private static FixedCapacityArrayPool<T>? _arrayPool;
+		internal static FixedCapacityArrayPool<T> ExclusiveArray
+		{
+
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+			get => _arrayPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
+		}
+	}
+
+	public static class FixedCapacityPools
+	{
+		[ThreadStatic]
+		private static IntArrayPool? _intArrayPool;
+		internal static IntArrayPool ExclusiveIntArray
+		{
+
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+			get => _intArrayPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
+		}
+
+		[ThreadStatic]
+		private static UlongArrayPool? _ulongArrayPool;
+		internal static UlongArrayPool ExclusiveUlongArray
+		{
+
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+			get => _ulongArrayPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
+		}
+	}
+}

PerfectNumbers.Core/FixedCapacityStack.cs

@@ -0,0 +1,46 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+public sealed class FixedCapacityStack<T>(int capacity)
+{
+	private static readonly FixedCapacityArrayPool<T> _arrayPool = FixedCapacityPools<T>.ExclusiveArray;
+
+	public int Count = 0;
+	private T[] _items = _arrayPool.Rent(capacity);
+
+	private int _capacity = capacity;
+	public int Capacity
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _capacity;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		set
+		{
+			if (value < _capacity)
+			{
+				_arrayPool.Return(_items);
+				_items = _arrayPool.Rent(value);
+				_capacity = value;
+			}
+		}
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Clear() => Count = 0;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public T? Pop() => Count > 0 ? _items[Count--] : default;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	public void Push(T item)
+	{
+		int newCount = Count + 1;
+		if (newCount < _items.Length)
+		{
+			_items[newCount] = item;
+			Count = newCount;
+		}
+	}
+}

PerfectNumbers.Core/Gpu/Accelerators/AcceleratorStreamPool.cs

@@ -1,4 +1,4 @@
-using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
 using ILGPU.Runtime;
 
 namespace PerfectNumbers.Core.Gpu.Accelerators;
@@ -7,13 +7,14 @@ public static class AcceleratorStreamPool
 {
 	private static readonly Accelerator[] _accelerators = AcceleratorPool.Shared.Accelerators;
 	private static readonly SemaphoreSlim[] _locks = new SemaphoreSlim[PerfectNumberConstants.RollingAccelerators];
-	private static readonly ConcurrentQueue<AcceleratorStream>[] _streams = new ConcurrentQueue<AcceleratorStream> [PerfectNumberConstants.RollingAccelerators];
+	private static readonly ConcurrentFixedCapacityStack<AcceleratorStream>[] _streams = new ConcurrentFixedCapacityStack<AcceleratorStream>[PerfectNumberConstants.RollingAccelerators];
 
 	private static SemaphoreSlim CreateLock() => new(PerfectNumberConstants.ThreadsByAccelerator);
 
 
 	// private static int _rented;
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public static AcceleratorStream Rent(int acceleratorIndex)
 	{
 		var streamLock = _locks[acceleratorIndex];
@@ -21,22 +22,23 @@ public static class AcceleratorStreamPool
 
 		streamLock.Wait();
 
-		return queue.TryDequeue(out var stream)
+		return queue.Pop() is { } stream
 			? stream
 			: _accelerators[acceleratorIndex].CreateStream();
 	}
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public static void Return(int acceleratorIndex, AcceleratorStream stream)
 	{
 		var streamLock = _locks[acceleratorIndex];
 		var queue = _streams[acceleratorIndex];
-		queue.Enqueue(stream);
+		queue.Push(stream);
 		streamLock.Release();
 	}
 
 	public static void WarmUp(int acceleratorIndex)
 	{
-		_streams[acceleratorIndex] = new();
+		_streams[acceleratorIndex] = new(PerfectNumberConstants.DefaultPoolCapacity);
 		_locks[acceleratorIndex] = CreateLock();
 	}
 }

PerfectNumbers.Core/Gpu/Accelerators/PrimeOrderCalculatorAccelerator.cs

@@ -1,4 +1,3 @@
-using System.Collections.Concurrent;
 using System.Runtime.CompilerServices;
 using ILGPU;
 using ILGPU.Runtime;
@@ -11,34 +10,31 @@ public sealed class PrimeOrderCalculatorAccelerator
 	#region Pool
 
 	// [ThreadStatic]
-	private static ConcurrentQueue<PrimeOrderCalculatorAccelerator>? _pool;
+	private static ConcurrentFixedCapacityStack<PrimeOrderCalculatorAccelerator>? _pool;
 
 	[MethodImpl(MethodImplOptions.AggressiveInlining)]
 	public static PrimeOrderCalculatorAccelerator Rent(int primeTesterCapacity)
 	{
-		var pool = _pool ??= [];
+		var pool = _pool ??= new(PerfectNumberConstants.DefaultThreadPoolCapacity);
-		if (!pool.TryDequeue(out var gpu))
+		if (pool.Pop() is { } gpu)
-		{
-			int acceleratorIndex = AcceleratorPool.Shared.Rent();
-			gpu = new(acceleratorIndex, PerfectNumberConstants.DefaultFactorsBuffer, primeTesterCapacity, PerfectNumberConstants.DefaultSmallPrimeFactorSlotCount, PerfectNumberConstants.DefaultSpecialMaxFactorCapacity);
-		}
-		else
 		{
 			gpu.EnsureCapacity(PerfectNumberConstants.DefaultFactorsBuffer, primeTesterCapacity);
-		}
-
 			return gpu;
 		}
 
+		int acceleratorIndex = AcceleratorPool.Shared.Rent();
+		return new(acceleratorIndex, PerfectNumberConstants.DefaultFactorsBuffer, primeTesterCapacity, PerfectNumberConstants.DefaultSmallPrimeFactorSlotCount, PerfectNumberConstants.DefaultSpecialMaxFactorCapacity);
+	}
+
 	[MethodImpl(MethodImplOptions.AggressiveInlining)]
-	public static void Return(PrimeOrderCalculatorAccelerator gpu) => _pool!.Enqueue(gpu);
+	public static void Return(PrimeOrderCalculatorAccelerator gpu) => _pool!.Push(gpu);
 
 	internal static void Clear()
 	{
 		var pool = _pool;
 		if (pool != null)
 		{
-			while (pool.TryDequeue(out var gpu))
+			while (pool.Pop() is { } gpu)
 			{
 				gpu.Dispose();
 			}
@@ -50,7 +46,7 @@ public sealed class PrimeOrderCalculatorAccelerator
 		var pool = _pool;
 		if (pool != null)
 		{
-			while (pool.TryDequeue(out var gpu))
+			while (pool.Pop() is { } gpu)
 			{
 				gpu.Dispose();
 			}

PerfectNumbers.Core/Gpu/MersenneNumberDivisorByDivisorGpuTester.cs

@@ -22,9 +22,9 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 	private BigInteger _minK = BigInteger.One;
 	// private bool _isConfigured;
 
-	private readonly ConcurrentBag<MersenneNumberDivisorByDivisorAccelerator>[] _resourcePool = [.. AcceleratorPool.Shared.Accelerators.Select(x => new ConcurrentBag<MersenneNumberDivisorByDivisorAccelerator>())];
+	private readonly ConcurrentFixedCapacityStack<MersenneNumberDivisorByDivisorAccelerator>[] _resourcePool = [.. AcceleratorPool.Shared.Accelerators.Select(x => new ConcurrentFixedCapacityStack<MersenneNumberDivisorByDivisorAccelerator>(PerfectNumberConstants.DefaultPoolCapacity))];
 
-	private readonly ConcurrentBag<DivisorScanSession> _sessionPool = [];
+	private readonly ConcurrentFixedCapacityStack<DivisorScanSession> _sessionPool = new(PerfectNumberConstants.DefaultPoolCapacity);
 
 	public int GpuBatchSize
 	{
@@ -272,7 +272,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 		ArrayView1D<ulong, Stride1D.Dense> exponentViewDevice = exponentBuffer.View;
 		ArrayView1D<byte, Stride1D.Dense> hitsView = hitsBuffer.View;
 		ArrayView1D<int, Stride1D.Dense> hitIndexView = hitIndexBuffer.View;
-		Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+		FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 		static bool ProcessRange(
 			PrimeOrderCalculatorAccelerator gpu,
 			ulong prime,
@@ -291,7 +291,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 			Span<int> offsetSpan,
 			Span<int> countSpan,
 			Span<ulong> cycleSpan,
-			Queue<MontgomeryDivisorData> divisorPool,
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool,
 			int chunkCountBaseline,
 			int acceleratorIndex,
 			ref bool composite,
@@ -598,6 +598,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 			_kernel = KernelUtil.GetKernel(accelerator.LoadAutoGroupedStreamKernel<Index1D, ArrayView<GpuDivisorPartialData>, ArrayView<int>, ArrayView<int>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>, ArrayView<int>>(DivisorByDivisorKernels.CheckKernel)).CreateLauncherDelegate<Action<AcceleratorStream, Index1D, ArrayView<GpuDivisorPartialData>, ArrayView<int>, ArrayView<int>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>, ArrayView<int>>>();
 		}
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		internal void Reset()
 		{
 		}
@@ -706,15 +707,13 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 			return sentinel == 0;
 		}
 
-		public void Return()
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		{
+		public void Return() => _owner._sessionPool.Push(this);
-			_owner._sessionPool.Add(this);
 	}
-	}
-
 
 	public ulong DivisorLimit
 	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		get
 		{
 			// The EvenPerfectBitScanner driver configures the GPU tester before exposing it to callers, so the previous synchronization guard remains commented out.
@@ -732,22 +731,22 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 		}
 	}
 
-
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-
 	private MersenneNumberDivisorByDivisorAccelerator RentBatchResources(int acceleratorIndex, int capacity)
 	{
 		var queue = _resourcePool[acceleratorIndex];
 		var accelerator = AcceleratorPool.Shared.Accelerators[acceleratorIndex];
-		if (!queue.TryTake(out MersenneNumberDivisorByDivisorAccelerator? resources))
+		if (queue.Pop() is { } resources)
 		{
-			return new MersenneNumberDivisorByDivisorAccelerator(accelerator, capacity);
-		}
-
 			resources.EnsureCapacity(accelerator, capacity);
 			return resources;
 		}
 
-	private void ReturnBatchResources(int acceleratorIndex, MersenneNumberDivisorByDivisorAccelerator resources) => _resourcePool[acceleratorIndex].Add(resources);
+		return new MersenneNumberDivisorByDivisorAccelerator(accelerator, capacity);
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	private void ReturnBatchResources(int acceleratorIndex, MersenneNumberDivisorByDivisorAccelerator resources) => _resourcePool[acceleratorIndex].Push(resources);
 
 	private sealed class AcceleratorReferenceComparer : IEqualityComparer<Accelerator>
 	{
@@ -758,6 +757,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 		public int GetHashCode(Accelerator obj) => RuntimeHelpers.GetHashCode(obj);
 	}
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public ulong GetAllowedMaxDivisor(ulong prime)
 	{
 		// The configuration is immutable after setup, so the cached limit can be read without locking.
@@ -774,6 +774,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 		return ComputeAllowedMaxDivisorGpu(prime, _divisorLimit);
 	}
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public IMersenneNumberDivisorByDivisorTester.IDivisorScanSession CreateDivisorSession(PrimeOrderCalculatorAccelerator gpu)
 	{
 		// The tester is configured once at startup and the session pool relies on thread-safe collections, so the previous synchronization guard stays commented out.
@@ -793,7 +794,7 @@ public sealed partial class MersenneNumberDivisorByDivisorGpuTester : IMersenneN
 		//     return new DivisorScanSession(this);
 		// }
 
-		if (_sessionPool.TryTake(out DivisorScanSession? session))
+		if (_sessionPool.Pop() is { } session)
 		{
 			session.Reset();
 			return session;

PerfectNumbers.Core/IntArrayPool.cs

@@ -0,0 +1,40 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core
+{
+	public struct IntArrayPool(int poolCapacity)
+	{
+        private readonly int[][] _arrays = new int[poolCapacity][];
+        private int _count = 0;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public int[] Rent(int length)
+        {
+            int available = _count;
+            if (available == 0)
+            {
+                return new int[length];
+            }
+
+            int[][] arrays = _arrays;
+            available--;
+            _count = available;
+
+            int[] candidate = arrays[available];
+			return candidate.Length >= length ? candidate : new int[length];
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public void Return(int[] array)
+        {
+            int[][] arrays = _arrays;
+            int available = _count;
+
+            if (available < arrays.Length)
+            {
+				arrays[available] = array;
+				_count = available + 1;
+            }
+        }
+    }
+}

PerfectNumbers.Core/MersenneDivisorCycles.cs

@@ -394,7 +394,7 @@ public class MersenneDivisorCycles
 		if (divisor <= ulong.MaxValue)
 		{
 			ulong divisor64 = (ulong)divisor;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor64);
 			ulong cycle = CalculateCycleLengthCpu(divisor64, divisorData);
 			divisorPool.Return(divisorData);
@@ -441,7 +441,7 @@ public class MersenneDivisorCycles
 		if (divisor <= ulong.MaxValue)
 		{
 			ulong divisor64 = (ulong)divisor;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor64);
 			ulong cycle = CalculateCycleLengthHybrid(gpu, divisor64, divisorData);
 			divisorPool.Return(divisorData);
@@ -489,7 +489,7 @@ public class MersenneDivisorCycles
 		if (divisor <= ulong.MaxValue)
 		{
 			ulong divisor64 = (ulong)divisor;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			MontgomeryDivisorData divisorData = divisorPool.FromModulus(divisor64);
 			ulong cycle = CalculateCycleLengthGpu(gpu, divisor64, divisorData);
 			divisorPool.Return(divisorData);
@@ -1161,7 +1161,6 @@ public class MersenneDivisorCycles
 
 			ulong localStart = threadStart;
 			ulong localEnd = threadEnd;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			tasks[taskIndex] = Task.Run(() =>
 			{
 				ulong rangeLength = localEnd - localStart + 1UL;
@@ -1169,6 +1168,7 @@ public class MersenneDivisorCycles
 				// (ulong divisor, ulong cycle)[] localCycles = ArrayPool<(ulong, ulong)>.Shared.Rent(checked((int)rangeLength));
 				int localCycleIndex = 0;
 
+				FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 				var gpu = PrimeOrderCalculatorAccelerator.Rent(1);
 				for (ulong divisor = localStart; divisor <= localEnd; divisor++)
 				{

PerfectNumbers.Core/MersennePrimeFactorTester.cs

@@ -1,4 +1,3 @@
-using System.Collections.Concurrent;
 using System.Runtime.CompilerServices;
 using System.Numerics;
 using UInt128 = System.UInt128;
@@ -185,36 +184,40 @@ public static class MersennePrimeFactorTester
 		return result;
 	}
 
-    private static readonly ConcurrentBag<List<ulong>> ListPool = new();
+	private static readonly ConcurrentFixedCapacityStack<List<ulong>> ListPool = new(PerfectNumberConstants.DefaultPoolCapacity);
-    private static readonly ConcurrentBag<Dictionary<ulong, byte>> DictPool = new();
+	private static readonly ConcurrentFixedCapacityStack<Dictionary<ulong, byte>> DictPool = new(PerfectNumberConstants.DefaultPoolCapacity);
-    private static readonly Dictionary<ulong, ulong[]> FactorCache = new();
-    private static readonly object FactorCacheLock = new();
 
-    private static List<ulong> RentList() => ListPool.TryTake(out var list) ? list : new List<ulong>(4);
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	private static List<ulong> RentList() => ListPool.Pop() is { } list ? list : new List<ulong>(4);
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	private static void ReturnList(List<ulong> list)
 	{
 		list.Clear();
-        ListPool.Add(list);
+		ListPool.Push(list);
 	}
 
-    private static Dictionary<ulong, byte> RentDictionary() => DictPool.TryTake(out var dict) ? dict : new Dictionary<ulong, byte>();
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	private static Dictionary<ulong, byte> RentDictionary() => DictPool.Pop() is { } dict ? dict : [];
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	private static void ReturnDictionary(Dictionary<ulong, byte> dict)
 	{
 		dict.Clear();
-        DictPool.Add(dict);
+		DictPool.Push(dict);
 	}
 
-    private static readonly ConcurrentBag<Dictionary<UInt128, byte>> DictPool128 = new();
+	private static readonly ConcurrentFixedCapacityStack<Dictionary<UInt128, byte>> DictPool128 = new(PerfectNumberConstants.DefaultPoolCapacity);
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	private static Dictionary<UInt128, byte> RentDictionary128() =>
-        DictPool128.TryTake(out var dict) ? dict : new Dictionary<UInt128, byte>();
+	DictPool128.Pop() is { } dict ? dict : [];
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	private static void ReturnDictionary128(Dictionary<UInt128, byte> dict)
 	{
 		dict.Clear();
-        DictPool128.Add(dict);
+		DictPool128.Push(dict);
 	}
 
 	private static ulong[] Factor64(ulong n, CancellationToken ct)
@@ -233,10 +236,10 @@ public static class MersennePrimeFactorTester
 		}
 
 		var dict = RentDictionary();
-        var queue = new Queue<ulong>(); // TODO: Replace this Queue with the pooled stack from the
+		var queue = ThreadStaticPools.RentUlongStack(PerfectNumberConstants.DefaultPoolCapacity); // TODO: Replace this Queue with the pooled stack from the
 																								  // FactorizationBenchmarks fast path so factoring large
 																								  // composites avoids per-node allocations.
-        queue.Enqueue(n);
+		queue.Push(n);
 
 		while (queue.Count != 0)
 		{
@@ -245,7 +248,7 @@ public static class MersennePrimeFactorTester
 				break;
 			}
 
-            ulong m = queue.Dequeue();
+			ulong m = queue.Pop();
 			if (m == 1UL)
 			{
 				continue;
@@ -273,10 +276,11 @@ public static class MersennePrimeFactorTester
 				break;
 			}
 
-            queue.Enqueue(d);
+			queue.Push(d);
-            queue.Enqueue(m / d);
+			queue.Push(m / d);
 		}
 
+		ThreadStaticPools.ReturnUlongStack(queue);
 		ulong[] result = [.. dict.Keys];
 		// TODO: Rent the result array from ArrayPool once the factoring pipeline consumes spans so we
 		// can recycle buffers across large factorizations.
@@ -375,24 +379,9 @@ public static class MersennePrimeFactorTester
 				}
 
 				x = x.MulMod64(x, n).AddMod64(c, n);
-                if (ct.IsCancellationRequested)
-                {
-                    return 0UL;
-                }
-
 				y = y.MulMod64(y, n).AddMod64(c, n);
-                if (ct.IsCancellationRequested)
-                {
-                    return 0UL;
-                }
-
 				y = y.MulMod64(y, n).AddMod64(c, n);
 				diff = x > y ? x - y : y - x;
-                if (ct.IsCancellationRequested)
-                {
-                    return 0UL;
-                }
-
 				d = Gcd(diff, n, ct);
 			}
 

PerfectNumbers.Core/MontgomeryDivisorDataPool.cs

@@ -5,10 +5,14 @@ namespace PerfectNumbers.Core;
 public static class MontgomeryDivisorDataPool
 {
 	[ThreadStatic]
-	private static Queue<MontgomeryDivisorData>? _pool;
+	private static FixedCapacityStack<MontgomeryDivisorData>? _pool;
-	public static Queue<MontgomeryDivisorData> Shared => _pool ??= new();
+	public static FixedCapacityStack<MontgomeryDivisorData> Shared
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _pool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
+	}
 
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	private static ulong ComputeMontgomeryResidue(UInt128 value, ulong modulus) => (ulong)(value % modulus);
 
 	private static ulong ComputeMontgomeryNPrime(ulong modulus)
@@ -24,10 +28,11 @@ public static class MontgomeryDivisorDataPool
 		return unchecked(0UL - inv);
 	}
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public static MontgomeryDivisorData Rent(ulong modulus, ulong nPrime, ulong montgomeryOne, ulong montgomeryTwo, ulong montgomeryTwoSquared)
 	{
-		var pool = _pool ??= new();
+		var pool = _pool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
-		if (pool.TryDequeue(out var pooled))
+		if (pool.Pop() is { } pooled)
 		{
 			pooled.Modulus = modulus;
 			pooled.NPrime = nPrime;
@@ -40,12 +45,12 @@ public static class MontgomeryDivisorDataPool
 		return new(modulus, nPrime, montgomeryOne, montgomeryTwo, montgomeryTwoSquared);
 	}
 
-	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-	public static void Return(MontgomeryDivisorData divisorData) => _pool!.Enqueue(divisorData);
+	public static void Return(MontgomeryDivisorData divisorData) => _pool!.Push(divisorData);
 
 
-	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    public static MontgomeryDivisorData FromModulus(this Queue<MontgomeryDivisorData> queue, ulong modulus)
+	public static MontgomeryDivisorData FromModulus(this FixedCapacityStack<MontgomeryDivisorData> queue, ulong modulus)
 	{
 		ulong nPrime = ComputeMontgomeryNPrime(modulus);
 		ulong montgomeryOne = ComputeMontgomeryResidue(UInt128Numbers.OneShiftedLeft64, modulus);
@@ -60,10 +65,10 @@ public static class MontgomeryDivisorDataPool
 			montgomeryTwoSquared);
 	}
 
-	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-	public static MontgomeryDivisorData Rent(this Queue<MontgomeryDivisorData> queue, ulong modulus, ulong nPrime, ulong montgomeryOne, ulong montgomeryTwo, ulong montgomeryTwoSquared)
+	public static MontgomeryDivisorData Rent(this FixedCapacityStack<MontgomeryDivisorData> queue, ulong modulus, ulong nPrime, ulong montgomeryOne, ulong montgomeryTwo, ulong montgomeryTwoSquared)
 	{
-		if (queue.TryDequeue(out var pooled))
+		if (queue.Pop() is { } pooled)
 		{
 			pooled.Modulus = modulus;
 			pooled.NPrime = nPrime;
@@ -76,6 +81,6 @@ public static class MontgomeryDivisorDataPool
 		return new(modulus, nPrime, montgomeryOne, montgomeryTwo, montgomeryTwoSquared);
 	}
 
-	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-	public static void Return(this Queue<MontgomeryDivisorData> queue, MontgomeryDivisorData divisorData) => queue.Enqueue(divisorData);
+	public static void Return(this FixedCapacityStack<MontgomeryDivisorData> queue, MontgomeryDivisorData divisorData) => queue.Push(divisorData);
 }

PerfectNumbers.Core/PartialFactorResult.cs

@@ -1,3 +1,5 @@
+using System.Numerics;
+
 namespace PerfectNumbers.Core
 {
 	public sealed class PartialFactorResult
@@ -77,7 +79,7 @@ namespace PerfectNumbers.Core
 				return;
 			}
 
-			int newSize = sourceCount + 1;
+			int newSize = (int)BitOperations.RoundUpToPowerOf2((uint)sourceCount + 1U);
 			ulong[] extendedFactors = sourceFactors.Length >= newSize
 				? sourceFactors
 				: ResizeFactors(sourceFactors, sourceCount, newSize);
@@ -87,6 +89,7 @@ namespace PerfectNumbers.Core
 				? sourceExponents
 				: ResizeExponents(sourceCount, newSize, sourceExponents);
 
+			newSize = sourceCount + 1;
 			extendedFactors[sourceCount] = prime;
 			extendedExponents[sourceCount] = 1;
 			Array.Sort(extendedFactors, extendedExponents, 0, newSize);
@@ -101,24 +104,24 @@ namespace PerfectNumbers.Core
 
 		private static int[] ResizeExponents(int sourceCount, int newSize, in int[] sourceExponents)
 		{
-			int[] extendedExponents = Pools.ExclusiveIntArray.Rent(newSize);
+			int[] extendedExponents = FixedCapacityPools.ExclusiveIntArray.Rent(newSize);
 			Array.Copy(sourceExponents, 0, extendedExponents, 0, sourceCount);
-			Pools.ExclusiveIntArray.Return(sourceExponents);
+			FixedCapacityPools.ExclusiveIntArray.Return(sourceExponents);
 			return extendedExponents;
 		}
 
 		private static ulong[] ResizeFactors(in ulong[] sourceFactors, int sourceCount, int newSize)
 		{
-			ulong[] extendedFactors = Pools.ExclusiveUlongArray.Rent(newSize);
+			ulong[] extendedFactors = FixedCapacityPools.ExclusiveUlongArray.Rent(newSize);
 			Array.Copy(sourceFactors, 0, extendedFactors, 0, sourceCount);
-			Pools.ExclusiveUlongArray.Return(sourceFactors);
+			FixedCapacityPools.ExclusiveUlongArray.Return(sourceFactors);
 			return extendedFactors;
 		}
 
 		private void InitializeWithPrime(ulong prime)
 		{
-			ulong[] factors = Pools.ExclusiveUlongArray.Rent(1);
+			ulong[] factors = FixedCapacityPools.ExclusiveUlongArray.Rent(1);
-			int[] exponents = Pools.ExclusiveIntArray.Rent(1);
+			int[] exponents = FixedCapacityPools.ExclusiveIntArray.Rent(1);
 			factors[0] = prime;
 			exponents[0] = 1;
 			Factors = factors;
@@ -142,8 +145,8 @@ namespace PerfectNumbers.Core
 					Factors = null;
 					Exponents = null;
 
-					Pools.ExclusiveUlongArray.Return(factors);
+					FixedCapacityPools.ExclusiveUlongArray.Return(factors);
-					Pools.ExclusiveIntArray.Return(exponents);
+					FixedCapacityPools.ExclusiveIntArray.Return(exponents);
 				}
 
 				_next = s_poolHead;

PerfectNumbers.Core/PerfectNumberConstants.cs

@@ -7,12 +7,15 @@ public static class PerfectNumberConstants
     public const ulong BiggestKnownEvenPerfectP = 136279841UL;
 	public const int ConsoleInterval = 100_000;
 	public const int DefaultFactorsBuffer = 32;
+	public const int DefaultPoolCapacity = 10_240;
 	public const int DefaultSmallPrimeFactorSlotCount = 64;
+	public const int DefaultThreadPoolCapacity = 131_072;
 	public const int DefaultSpecialMaxFactorCapacity = 1024;
     public const ulong ExtraDivisorCycleSearchLimit = 64UL;
 	public static int GpuRatio = 16384 - 1;
 	public const int MaxQForDivisorCycles = 4_000_000;
     public static readonly uint PrimesLimit = 1_000_000; //(ulong)Array.MaxLength;// 1_000_000;
+	public const int DefaultStringBuilderCapacity = 10_240;
 	public const int MaxOddPowersCount = 128;
 	public const int PooledArrayThreshold = 64;
 	public const int Pow2WindowSize = 8;

PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs

@@ -108,7 +108,7 @@ internal static partial class PrimeOrderCalculator
 			}
 
 			ulong prime64 = (ulong)prime;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			divisorData = divisorPool.FromModulus(prime64);
 			ulong order64 = CalculateCpu(prime64, previous, divisorData, config);
 			divisorPool.Return(divisorData);
@@ -226,7 +226,7 @@ internal static partial class PrimeOrderCalculator
 	private static UInt128 FinishStrictlyWideCpu(in UInt128 prime, in MontgomeryDivisorData? divisorData)
 	{
 		UInt128 phi = prime - UInt128.One;
-		Queue<Dictionary<UInt128, int>> uInt128IntDictionaryPool = ThreadStaticPools.UInt128IntDictionaryPool;
+		FixedCapacityStack<Dictionary<UInt128, int>> uInt128IntDictionaryPool = ThreadStaticPools.UInt128IntDictionaryPool;
 		Dictionary<UInt128, int> counts = uInt128IntDictionaryPool.Rent(capacity: 8);
 		FactorCompletelyWide(phi, counts);
 		int entryCount = counts.Count;
@@ -236,7 +236,7 @@ internal static partial class PrimeOrderCalculator
 			return phi;
 		}
 
-		Queue<List<KeyValuePair<UInt128, int>>> keyValuePairUInt128IntegerPool = ThreadStaticPools.KeyValuePairUInt128IntegerPool;
+		FixedCapacityStack<List<KeyValuePair<UInt128, int>>> keyValuePairUInt128IntegerPool = ThreadStaticPools.KeyValuePairUInt128IntegerPool;
 		var entries = keyValuePairUInt128IntegerPool.Rent(entryCount);
 		entries.AddRange(counts);
 		uInt128IntDictionaryPool.Return(counts);
@@ -446,7 +446,7 @@ internal static partial class PrimeOrderCalculator
 		}
 
 		int capacity = config.MaxPowChecks <= 0 ? 64 : config.MaxPowChecksCapacity;
-		Queue<List<UInt128>> uInt128ListPool = ThreadStaticPools.UInt128ListPool;
+		FixedCapacityStack<List<UInt128>> uInt128ListPool = ThreadStaticPools.UInt128ListPool;
 		List<UInt128> candidates = uInt128ListPool.Rent(capacity);
 		FactorEntry128[] factorArray = orderFactors.Factors!;
 
@@ -580,8 +580,8 @@ internal static partial class PrimeOrderCalculator
 
 		int length = factors.Count;
 		int newLength = length + 1;
-		ulong[] tempFactorsArray = Pools.ExclusiveUlongArray.Rent(newLength);
+		ulong[] tempFactorsArray = FixedCapacityPools.ExclusiveUlongArray.Rent(newLength);
-		int[] tempExponentsArray = Pools.ExclusiveIntArray.Rent(newLength);
+		int[] tempExponentsArray = FixedCapacityPools.ExclusiveIntArray.Rent(newLength);
 
 		Span<ulong> ulongBuffer = tempFactorsArray;
 		factorSpan.CopyTo(ulongBuffer);
@@ -629,7 +629,7 @@ internal static partial class PrimeOrderCalculator
 				continue;
 			}
 
-			heapCandidateArray ??= Pools.ExclusiveUlongArray.Rent(ExponentHardLimit);
+			heapCandidateArray ??= FixedCapacityPools.ExclusiveUlongArray.Rent(ExponentHardLimit);
 			heapEvaluationArray ??= ThreadStaticPools.BoolPool.Rent(ExponentHardLimit);
 			ProcessExponentLoweringPrime(heapCandidateArray.AsSpan(0, exponent), heapEvaluationArray.AsSpan(0, exponent), ref order, primeFactor, exponent, ref stepper);
 		}
@@ -638,11 +638,11 @@ internal static partial class PrimeOrderCalculator
 
 		if (heapCandidateArray is not null)
 		{
-			Pools.ExclusiveUlongArray.Return(heapCandidateArray);
+			FixedCapacityPools.ExclusiveUlongArray.Return(heapCandidateArray);
 			ThreadStaticPools.BoolPool.Return(heapEvaluationArray!);
 		}
 
-		Pools.ExclusiveUlongArray.Return(tempFactorsArray);
+		FixedCapacityPools.ExclusiveUlongArray.Return(tempFactorsArray);
 		return order;
 	}
 
@@ -789,7 +789,7 @@ internal static partial class PrimeOrderCalculator
 				continue;
 			}
 
-			heapCandidateArray ??= Pools.ExclusiveUlongArray.Rent(ExponentHardLimit);
+			heapCandidateArray ??= FixedCapacityPools.ExclusiveUlongArray.Rent(ExponentHardLimit);
 			if (ValidateOrderForFactor(heapCandidateArray.AsSpan(0, exponent), primeFactor, exponent, order, ref stepper))
 			{
 				violates = true;
@@ -801,7 +801,7 @@ internal static partial class PrimeOrderCalculator
 
 		if (heapCandidateArray is not null)
 		{
-			Pools.ExclusiveUlongArray.Return(heapCandidateArray);
+			FixedCapacityPools.ExclusiveUlongArray.Return(heapCandidateArray);
 		}
 
 		return !violates;
@@ -1229,7 +1229,7 @@ internal static partial class PrimeOrderCalculator
 				continue;
 			}
 
-			heapCandidateArray ??= Pools.ExclusiveUlongArray.Rent(ExponentHardLimit);
+			heapCandidateArray ??= FixedCapacityPools.ExclusiveUlongArray.Rent(ExponentHardLimit);
 			if (CheckCandidateViolation(heapCandidateArray.AsSpan(0, exponent), primeFactor, exponent, candidate, prime, ref powUsed, powBudget, ref stepper))
 			{
 				violates = true;
@@ -1241,7 +1241,7 @@ internal static partial class PrimeOrderCalculator
 
 		if (heapCandidateArray is not null)
 		{
-			Pools.ExclusiveUlongArray.Return(heapCandidateArray);
+			FixedCapacityPools.ExclusiveUlongArray.Return(heapCandidateArray);
 		}
 
 		factorization.Dispose();
@@ -1334,9 +1334,6 @@ internal static partial class PrimeOrderCalculator
 	private static bool Pow2EqualsOneCpu(ulong exponent, in MontgomeryDivisorData divisorData)
 		=> exponent.Pow2MontgomeryModWindowedConvertToStandardCpu(divisorData) == 1UL;
 
-	private static readonly ulong[] _emptyUlongArray = [];
-	private static readonly int[] _emptyIntArray = [];
-
 	[MethodImpl(MethodImplOptions.AggressiveOptimization)]
 	private static PartialFactorResult PartialFactorCpu(ulong value, in PrimeOrderCalculatorConfig config)
 	{
@@ -1369,7 +1366,7 @@ internal static partial class PrimeOrderCalculator
 			if (config.PollardRhoMilliseconds > 0)
 			{
 				long deadlineTimestamp = CreateDeadlineTimestamp(config.PollardRhoMilliseconds);
-				Stack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(PrimeOrderConstants.GpuSmallPrimeFactorSlots);
+				FixedCapacityStack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(PrimeOrderConstants.GpuSmallPrimeFactorSlots);
 				compositeStack.Push(remaining);
 
 				CollectFactorsCpu(primeSlots, exponentSlots, ref factorCount, pending, compositeStack, deadlineTimestamp, out limitReached);
@@ -1395,6 +1392,11 @@ internal static partial class PrimeOrderCalculator
 		bool cofactorContainsComposite = false;
 		int pendingCount = pending.Count;
 		int index = 0;
+		if (pending.Capacity > PrimeOrderConstants.GpuSmallPrimeFactorSlots)
+		{
+			Console.WriteLine($"Initial capacity is too small. Required capacity: {pending.Capacity}");
+		}
+
 		for (; index < pendingCount; index++)
 		{
 			PendingEntry entry = pending[index];
@@ -1470,8 +1472,8 @@ internal static partial class PrimeOrderCalculator
 		// 	goto ReturnResult;
 		// }
 
-		ulong[] factorsArray = Pools.ExclusiveUlongArray.Rent(factorCount);
+		ulong[] factorsArray = FixedCapacityPools.ExclusiveUlongArray.Rent(factorCount);
-		int[] exponentsArray = Pools.ExclusiveIntArray.Rent(factorCount);
+		int[] exponentsArray = FixedCapacityPools.ExclusiveIntArray.Rent(factorCount);
 
 		for (int i = 0; i < factorCount; i++)
 		{
@@ -1500,7 +1502,7 @@ internal static partial class PrimeOrderCalculator
 	}
 
 	[MethodImpl(MethodImplOptions.AggressiveOptimization)]
-	private static void CollectFactorsCpu(Span<ulong> primeSlots, Span<int> exponentSlots, ref int factorCount, List<PendingEntry> pending, Stack<ulong> compositeStack, long deadlineTimestamp, out bool pollardRhoDeadlineReached)
+	private static void CollectFactorsCpu(Span<ulong> primeSlots, Span<int> exponentSlots, ref int factorCount, List<PendingEntry> pending, FixedCapacityStack<ulong> compositeStack, long deadlineTimestamp, out bool pollardRhoDeadlineReached)
 	{
 		while (compositeStack.Count > 0)
 		{

PerfectNumbers.Core/PrimeOrderCalculator.Gpu.cs

@@ -117,7 +117,7 @@ internal static partial class PrimeOrderCalculator
 			}
 
 			ulong prime64 = (ulong)prime;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			divisorData = divisorPool.FromModulus(prime64);
 			ulong order64 = CalculateGpu(gpu, prime64, previous, divisorData, config);
 			divisorPool.Return(divisorData);
@@ -377,7 +377,7 @@ internal static partial class PrimeOrderCalculator
 			if (config.PollardRhoMilliseconds > 0)
 			{
 				long deadlineTimestamp = CreateDeadlineTimestamp(config.PollardRhoMilliseconds);
-				Stack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(4);
+				FixedCapacityStack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(4);
 				compositeStack.Push(remaining);
 
 				CollectFactorsCpu(primeSlots, exponentSlots, ref factorCount, pending, compositeStack, deadlineTimestamp, out limitReached);

PerfectNumbers.Core/PrimeOrderCalculator.Hybrid.cs

@@ -67,7 +67,7 @@ internal static partial class PrimeOrderCalculator
 			}
 
 			ulong prime64 = (ulong)prime;
-			Queue<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
+			FixedCapacityStack<MontgomeryDivisorData> divisorPool = MontgomeryDivisorDataPool.Shared;
 			divisorData = divisorPool.FromModulus(prime64);
 			ulong order64 = CalculateHybrid(gpu, prime64, previous, divisorData, config);
 			divisorPool.Return(divisorData);
@@ -83,7 +83,7 @@ internal static partial class PrimeOrderCalculator
 	}
 
 	[MethodImpl(MethodImplOptions.AggressiveOptimization)]
-	private static void CollectFactorsHybrid(PrimeOrderCalculatorAccelerator gpu, Span<ulong> primeSlots, Span<int> exponentSlots, ref ulong[] primeSlotArray, ref int[] exponentSlotArray, ref int factorCount, List<PendingEntry> pending, Stack<ulong> compositeStack, long deadlineTimestamp, out bool pollardRhoDeadlineReached)
+	private static void CollectFactorsHybrid(PrimeOrderCalculatorAccelerator gpu, Span<ulong> primeSlots, Span<int> exponentSlots, ref int factorCount, List<PendingEntry> pending, FixedCapacityStack<ulong> compositeStack, long deadlineTimestamp, out bool pollardRhoDeadlineReached)
 	{
 		while (compositeStack.Count > 0)
 		{
@@ -169,10 +169,10 @@ internal static partial class PrimeOrderCalculator
 			if (config.PollardRhoMilliseconds > 0)
 			{
 				long deadlineTimestamp = CreateDeadlineTimestamp(config.PollardRhoMilliseconds);
-				Stack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(4);
+				FixedCapacityStack<ulong> compositeStack = ThreadStaticPools.RentUlongStack(4);
 				compositeStack.Push(remaining);
 
-				CollectFactorsHybrid(gpu, primeSlots, exponentSlots, primeSlotArray: ref primeSlotArray, ref exponentSlotArray, ref factorCount, pending, compositeStack, deadlineTimestamp, out limitReached);
+				CollectFactorsHybrid(gpu, primeSlots, exponentSlots, ref factorCount, pending, compositeStack, deadlineTimestamp, out limitReached);
 
 				if (limitReached)
 				{

PerfectNumbers.Core/StringBuilderPool.cs

@@ -1,23 +1,21 @@
-using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
 using System.Text;
 
 namespace PerfectNumbers.Core;
 
 public static class StringBuilderPool
 {
-    private static readonly ConcurrentQueue<StringBuilder> _stringBuilderPool = new();
+	private static readonly ConcurrentFixedCapacityStack<StringBuilder> _stringBuilderPool = new(PerfectNumberConstants.DefaultPoolCapacity);
 
-    public static StringBuilder Rent()
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    {
+	public static StringBuilder Rent() => _stringBuilderPool.Pop() is { } sb ? sb : new(PerfectNumberConstants.DefaultStringBuilderCapacity);
-        // TODO: Ensure oversized builders remain eligible for reuse without trimming so call sites keep their full capacity.
-        return _stringBuilderPool.TryDequeue(out var sb) ? sb : new();
-    }
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public static void Return(StringBuilder sb)
 	{
 		_ = sb.Clear();
 		// TODO: Preserve the builder's capacity when returning it so the pool hands the same buffer back without shrinkage.
-        _stringBuilderPool.Enqueue(sb);
+		_stringBuilderPool.Push(sb);
 	}
 }
 

PerfectNumbers.Core/TaskThreadPool.cs

@@ -1,21 +1,17 @@
 using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
 
 namespace PerfectNumbers.Core;
 
 internal sealed class TaskThreadPool
 {
-    private readonly struct WorkItem
+	private readonly struct WorkItem(Task task)
 	{
-        public WorkItem(Task task)
+		public readonly Task Task = task;
-        {
-            Task = task;
 	}
 
-        public Task Task { get; }
+	private readonly ConcurrentFixedCapacityStack<WorkItem> _pendingTaskQueue = new(PerfectNumberConstants.DefaultThreadPoolCapacity);
-    }
+	private readonly HashSet<Thread> _runningThreads = [];
-
-    private readonly ConcurrentQueue<WorkItem> _pendingTaskQueue = new();
-    private readonly HashSet<Thread> _runningThreads = new();
 	private readonly object _lock = new();
 	private readonly Action<Task> _taskExecutor;
 	private readonly int _maximumConcurrency;
@@ -58,21 +54,15 @@ internal sealed class TaskThreadPool
 
 	public void Queue(Task task)
 	{
-        if (task is null)
+		ArgumentNullException.ThrowIfNull(task);
-        {
-            throw new ArgumentNullException(nameof(task));
-        }
 
 		List<Thread>? threadsToStart;
 
 		lock (_lock)
 		{
-            if (_disposeCancellation.IsCancellationRequested)
+			ObjectDisposedException.ThrowIf(_disposeCancellation.IsCancellationRequested, nameof(TaskThreadPool));
-            {
-                throw new ObjectDisposedException(nameof(TaskThreadPool));
-            }
 
-            _pendingTaskQueue.Enqueue(new WorkItem(task));
+			_pendingTaskQueue.Push(new WorkItem(task));
 			_pendingTaskCount++;
 
 			threadsToStart = EnsureWorkerCapacityLocked(null);
@@ -95,7 +85,7 @@ internal sealed class TaskThreadPool
 
 		if (snapshot.Length == 0)
 		{
-            return Array.Empty<Task>();
+			return [];
 		}
 
 		Task[] tasks = new Task[snapshot.Length];
@@ -105,6 +95,7 @@ internal sealed class TaskThreadPool
 			tasks[i] = snapshot[i].Task;
 		}
 
+		ConcurrentFixedCapacityStack<WorkItem>.Return(snapshot);
 		return tasks;
 	}
 
@@ -137,7 +128,7 @@ internal sealed class TaskThreadPool
 		{
 			_runningThreads.Clear();
 
-            while (_pendingTaskQueue.TryDequeue(out _))
+			while (_pendingTaskQueue.Pop() is { })
 			{
 			}
 
@@ -162,7 +153,8 @@ internal sealed class TaskThreadPool
 				return false;
 			}
 
-            if (_pendingTaskQueue.TryDequeue(out workItem))
+			workItem = _pendingTaskQueue.Pop();
+			if (workItem is { })
 			{
 				Interlocked.Decrement(ref _pendingTaskCount);
 				return true;
@@ -221,7 +213,7 @@ internal sealed class TaskThreadPool
 			_runningThreads.Add(thread);
 			_currentThreads++;
 
-            threadsToStart ??= new List<Thread>();
+			threadsToStart ??= [];
 			threadsToStart.Add(thread);
 		}
 

PerfectNumbers.Core/ThreadStaticPools.cs

@@ -13,29 +13,23 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<EInteger> EIntegerPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _eintegerPool ??= ArrayPool<EInteger>.Create();
-            {
-                return _eintegerPool ??= ArrayPool<EInteger>.Create();
-            }
 		}
 
 		[ThreadStatic]
-        private static Queue<List<KeyValuePair<UInt128, int>>>? _keyValuePairUInt128IntegerPool;
+		private static FixedCapacityStack<List<KeyValuePair<UInt128, int>>>? _keyValuePairUInt128IntegerPool;
 
-        public static Queue<List<KeyValuePair<UInt128, int>>> KeyValuePairUInt128IntegerPool
+		public static FixedCapacityStack<List<KeyValuePair<UInt128, int>>> KeyValuePairUInt128IntegerPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _keyValuePairUInt128IntegerPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
-            {
-                return _keyValuePairUInt128IntegerPool ??= [];
-            }
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static List<KeyValuePair<UInt128, int>> Rent(this Queue<List<KeyValuePair<UInt128, int>>> pool, int capacity)
+		public static List<KeyValuePair<UInt128, int>> Rent(this FixedCapacityStack<List<KeyValuePair<UInt128, int>>> pool, int capacity)
 		{
-			if (pool.TryDequeue(out var pooled))
+			if (pool.Pop() is { } pooled)
 			{
 				if (capacity > pooled.Capacity)
 				{
@@ -49,25 +43,22 @@ namespace PerfectNumbers.Core
 			return new(capacity);
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static void Return(this Queue<List<KeyValuePair<UInt128, int>>> pool, List<KeyValuePair<UInt128, int>> list) => pool.Enqueue(list);
+		public static void Return(this FixedCapacityStack<List<KeyValuePair<UInt128, int>>> pool, List<KeyValuePair<UInt128, int>> list) => pool.Push(list);
 
 		[ThreadStatic]
-        private static Queue<List<UInt128>>? _uint128ListPool;
+		private static FixedCapacityStack<List<UInt128>>? _uint128ListPool;
 
-        public static Queue<List<UInt128>> UInt128ListPool
+		public static FixedCapacityStack<List<UInt128>> UInt128ListPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _uint128ListPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
-            {
-                return _uint128ListPool ??= [];
-            }
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static List<UInt128> Rent(this Queue<List<UInt128>> pool, int capacity)
+		public static List<UInt128> Rent(this FixedCapacityStack<List<UInt128>> pool, int capacity)
 		{
-			if (pool.TryDequeue(out var pooled))
+			if (pool.Pop() is { } pooled)
 			{
 				if (capacity > pooled.Capacity)
 				{
@@ -81,25 +72,22 @@ namespace PerfectNumbers.Core
 			return new(capacity);
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static void Return(this Queue<List<UInt128>> pool, List<UInt128> list) => pool.Enqueue(list);
+		public static void Return(this FixedCapacityStack<List<UInt128>> pool, List<UInt128> list) => pool.Push(list);
 
 		[ThreadStatic]
-        private static Queue<Dictionary<UInt128, int>>? _uint128IntDictionaryPool;
+		private static FixedCapacityStack<Dictionary<UInt128, int>>? _uint128IntDictionaryPool;
 
-        public static Queue<Dictionary<UInt128, int>> UInt128IntDictionaryPool
+		public static FixedCapacityStack<Dictionary<UInt128, int>> UInt128IntDictionaryPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _uint128IntDictionaryPool ??= new(PerfectNumberConstants.DefaultPoolCapacity);
-            {
-                return _uint128IntDictionaryPool ??= [];
-            }
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static Dictionary<UInt128, int> Rent(this Queue<Dictionary<UInt128, int>> pool, int capacity)
+		public static Dictionary<UInt128, int> Rent(this FixedCapacityStack<Dictionary<UInt128, int>> pool, int capacity)
 		{
-			if (pool.TryDequeue(out var pooled))
+			if (pool.Pop() is { } pooled)
 			{
 				pooled.EnsureCapacity(capacity);
 				pooled.Clear();
@@ -109,19 +97,16 @@ namespace PerfectNumbers.Core
 			return new(capacity);
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public static void Return(this Queue<Dictionary<UInt128, int>> pool, Dictionary<UInt128, int> dictionary) => pool.Enqueue(dictionary);
+		public static void Return(this FixedCapacityStack<Dictionary<UInt128, int>> pool, Dictionary<UInt128, int> dictionary) => pool.Push(dictionary);
 
 		[ThreadStatic]
 		private static ArrayPool<FactorEntry>? _factorEntryPool;
 
 		public static ArrayPool<FactorEntry> FactorEntryPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _factorEntryPool ??= ArrayPool<FactorEntry>.Create();
-            {
-                return _factorEntryPool ??= ArrayPool<FactorEntry>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -129,11 +114,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<FactorEntry128> FactorEntry128Pool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _factorEntry128Pool ??= ArrayPool<FactorEntry128>.Create();
-            {
-                return _factorEntry128Pool ??= ArrayPool<FactorEntry128>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -141,45 +123,36 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<GpuUInt128> GpuUInt128Pool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _gpuUInt128Pool ??= ArrayPool<GpuUInt128>.Create();
-            {
-                return _gpuUInt128Pool ??= ArrayPool<GpuUInt128>.Create();
-            }
 		}
 
 		[ThreadStatic]
 		private static MersenneCpuDivisorScanSession? _mersenneCpuDivisorSession;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-        internal static MersenneCpuDivisorScanSession? RentMersenneCpuDivisorSession()
+		internal static MersenneCpuDivisorScanSession RentMersenneCpuDivisorSession(Gpu.Accelerators.PrimeOrderCalculatorAccelerator gpu, ComputationDevice orderDevice)
 		{
-            MersenneCpuDivisorScanSession? session = _mersenneCpuDivisorSession;
+			if (_mersenneCpuDivisorSession is { } session)
-            if (session is null)
 			{
-                return null;
+				session.Configure(gpu, orderDevice);
-            }
-
 				_mersenneCpuDivisorSession = null;
 				return session;
 			}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			return new MersenneCpuDivisorScanSession(gpu, orderDevice);
-        internal static void ReturnMersenneCpuDivisorSession(MersenneCpuDivisorScanSession session)
-        {
-            _mersenneCpuDivisorSession = session;
 		}
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		internal static void ReturnMersenneCpuDivisorSession(MersenneCpuDivisorScanSession session) => _mersenneCpuDivisorSession = session;
+
 		[ThreadStatic]
 		private static ArrayPool<UInt128>? _uint128Pool;
 
 		public static ArrayPool<UInt128> UInt128Pool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _uint128Pool ??= ArrayPool<UInt128>.Create();
-            {
-                return _uint128Pool ??= ArrayPool<UInt128>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -187,11 +160,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<ulong> UlongPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _ulongPool ??= ArrayPool<ulong>.Create();
-            {
-                return _ulongPool ??= ArrayPool<ulong>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -199,11 +169,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<bool> BoolPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _boolPool ??= ArrayPool<bool>.Create();
-            {
-                return _boolPool ??= ArrayPool<bool>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -211,11 +178,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<byte> BytePool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _bytePool ??= ArrayPool<byte>.Create();
-            {
-                return _bytePool ??= ArrayPool<byte>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -223,11 +187,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<MontgomeryDivisorData> MontgomeryDivisorDataPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _montgomeryDivisorDataPool ??= ArrayPool<MontgomeryDivisorData>.Create();
-            {
-                return _montgomeryDivisorDataPool ??= ArrayPool<MontgomeryDivisorData>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -235,11 +196,8 @@ namespace PerfectNumbers.Core
 
 		internal static ArrayPool<GpuDivisorPartialData> GpuDivisorPartialDataPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _gpuDivisorPartialDataPool ??= ArrayPool<GpuDivisorPartialData>.Create();
-            {
-                return _gpuDivisorPartialDataPool ??= ArrayPool<GpuDivisorPartialData>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -247,11 +205,8 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<int> IntPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _intPool ??= ArrayPool<int>.Create();
-            {
-                return _intPool ??= ArrayPool<int>.Create();
-            }
 		}
 
 		[ThreadStatic]
@@ -259,28 +214,21 @@ namespace PerfectNumbers.Core
 
 		public static ArrayPool<char> CharPool
 		{
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-            get
+			get => _charPool ??= ArrayPool<char>.Create();
-            {
-                return _charPool ??= ArrayPool<char>.Create();
-            }
 		}
 
 		[ThreadStatic]
-        private static List<List<ulong>>? _ulongListPool;
+		private static FixedCapacityStack<List<ulong>>? _ulongListPool;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public static List<ulong> RentUlongList(int capacityHint)
 		{
-            List<List<ulong>>? pool = _ulongListPool;
+			FixedCapacityStack<List<ulong>>? pool = _ulongListPool;
 			if (pool is not null)
 			{
-                int poolCount = pool.Count;
+				if (pool.Pop() is { } list)
-                if (poolCount > 0)
 				{
-                    int lastIndex = poolCount - 1;
-                    List<ulong> list = pool[lastIndex];
-                    pool.RemoveAt(lastIndex);
 					if (list.Capacity < capacityHint)
 					{
 						list.Capacity = capacityHint;
@@ -289,27 +237,21 @@ namespace PerfectNumbers.Core
 					return list;
 				}
 			}
+			else
+			{
+				_ulongListPool = new(capacityHint);
+			}
 
 			return new List<ulong>(capacityHint);
 		}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-        public static void ReturnUlongList(List<ulong> list)
+		public static void ReturnUlongList(List<ulong> list) => _ulongListPool!.Push(list);
-        {
-            List<List<ulong>>? pool = _ulongListPool;
-            if (pool is null)
-            {
-                pool = new List<List<ulong>>(4);
-                _ulongListPool = pool;
-            }
-
-            pool.Add(list);
-        }
 
 		[ThreadStatic]
 		private static ArrayPool<(ulong, ulong)>? _ulongUlongTupleArrayPool;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public static (ulong, ulong)[] RentUlongUlongArray(int capacityHint)
 		{
 			_ulongUlongTupleArrayPool ??= ArrayPool<(ulong, ulong)>.Create();
@@ -317,24 +259,20 @@ namespace PerfectNumbers.Core
 			return pool.Rent(capacityHint);
 		}
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public static void Return((ulong, ulong)[] array) => _ulongUlongTupleArrayPool!.Return(array, clearArray: false);
 
 		[ThreadStatic]
-        private static List<List<PrimeOrderCalculator.PendingEntry>>? _primeOrderPendingEntryListPool;
+		private static FixedCapacityStack<List<PrimeOrderCalculator.PendingEntry>>? _primeOrderPendingEntryListPool;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		internal static List<PrimeOrderCalculator.PendingEntry> RentPrimeOrderPendingEntryList(int capacityHint)
 		{
-            List<List<PrimeOrderCalculator.PendingEntry>>? pool = _primeOrderPendingEntryListPool;
+			FixedCapacityStack<List<PrimeOrderCalculator.PendingEntry>>? pool = _primeOrderPendingEntryListPool;
 			if (pool is not null)
 			{
-                int poolCount = pool.Count;
+				if (pool.Pop() is { } list)
-                if (poolCount > 0)
 				{
-                    int lastIndex = poolCount - 1;
-                    List<PrimeOrderCalculator.PendingEntry> list = pool[lastIndex];
-                    pool.RemoveAt(lastIndex);
 					if (list.Capacity < capacityHint)
 					{
 						list.Capacity = capacityHint;
@@ -343,68 +281,47 @@ namespace PerfectNumbers.Core
 					return list;
 				}
 			}
+			else
+			{
+				_primeOrderPendingEntryListPool = new(PerfectNumberConstants.DefaultPoolCapacity);
+			}
 
 			return new List<PrimeOrderCalculator.PendingEntry>(capacityHint);
 		}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-        internal static void ReturnPrimeOrderPendingEntryList(List<PrimeOrderCalculator.PendingEntry> list)
+		internal static void ReturnPrimeOrderPendingEntryList(List<PrimeOrderCalculator.PendingEntry> list) => _primeOrderPendingEntryListPool!.Push(list);
-        {
-            List<List<PrimeOrderCalculator.PendingEntry>>? pool = _primeOrderPendingEntryListPool;
-            if (pool is null)
-            {
-                pool = new List<List<PrimeOrderCalculator.PendingEntry>>(4);
-                _primeOrderPendingEntryListPool = pool;
-            }
-
-            pool.Add(list);
-        }
 
 		[ThreadStatic]
-        private static List<Stack<ulong>>? _ulongStackPool;
+		private static FixedCapacityStack<FixedCapacityStack<ulong>>? _ulongStackPool;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-        public static Stack<ulong> RentUlongStack(int capacityHint)
+		public static FixedCapacityStack<ulong> RentUlongStack(int capacityHint)
 		{
-            List<Stack<ulong>>? pool = _ulongStackPool;
+			FixedCapacityStack<FixedCapacityStack<ulong>>? pool = _ulongStackPool;
 			if (pool is not null)
 			{
-                int poolCount = pool.Count;
+				if (pool.Pop() is { } stack)
-                if (poolCount > 0)
 				{
-                    int lastIndex = poolCount - 1;
+					stack.Capacity = capacityHint;
-                    Stack<ulong> stack = pool[lastIndex];
-                    pool.RemoveAt(lastIndex);
-
-                    if (stack.Count < capacityHint)
-                    {
-                        stack.EnsureCapacity(capacityHint);
-                    }
-
 					return stack;
 				}
 			}
-
+			else
-            return new Stack<ulong>(capacityHint);
-        }
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void ReturnUlongStack(Stack<ulong> stack)
 			{
-            List<Stack<ulong>>? pool = _ulongStackPool;
+				_ulongStackPool = new(PrimeOrderConstants.GpuSmallPrimeFactorSlots);
-            if (pool is null)
-            {
-                pool = new List<Stack<ulong>>(4);
-                _ulongStackPool = pool;
 			}
 
-            pool.Add(stack);
+			return new FixedCapacityStack<ulong>(capacityHint);
 		}
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		public static void ReturnUlongStack(FixedCapacityStack<ulong> stack) => _ulongStackPool!.Push(stack);
+
 		[ThreadStatic]
 		private static Dictionary<ulong, int>? _factorCountDictionary;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public static Dictionary<ulong, int> RentFactorCountDictionary()
 		{
 			Dictionary<ulong, int>? dictionary = _factorCountDictionary;
@@ -417,41 +334,34 @@ namespace PerfectNumbers.Core
 			return dictionary;
 		}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-        public static void ReturnFactorCountDictionary(Dictionary<ulong, int> dictionary)
+		public static void ReturnFactorCountDictionary(Dictionary<ulong, int> dictionary) => _factorCountDictionary = dictionary;
-        {
-            _factorCountDictionary = dictionary;
-        }
 
 		[ThreadStatic]
 		private static Dictionary<ulong, int>? _factorScratchDictionary;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public static Dictionary<ulong, int> RentFactorScratchDictionary()
 		{
-            Dictionary<ulong, int>? dictionary = _factorScratchDictionary;
+			if (_factorScratchDictionary is { } dictionary)
-            if (dictionary is null)
 			{
-                return new Dictionary<ulong, int>(8);
-            }
-
 				_factorScratchDictionary = null;
 				return dictionary;
 			}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			return new Dictionary<ulong, int>(8);
-        public static void ReturnFactorScratchDictionary(Dictionary<ulong, int> dictionary)
-        {
-            _factorScratchDictionary = dictionary;
 		}
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		public static void ReturnFactorScratchDictionary(Dictionary<ulong, int> dictionary) => _factorScratchDictionary = dictionary;
+
 		[ThreadStatic]
 		private static ExponentRemainderStepperCpu _exponentRemainderStepperCpu;
 
 		[ThreadStatic]
 		private static bool _hasExponentRemainderStepperCpu;
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		internal static ExponentRemainderStepperCpu RentExponentStepperCpu(in MontgomeryDivisorData divisorData, ulong cycleLength = 0UL)
 		{
 			if (_hasExponentRemainderStepperCpu && _exponentRemainderStepperCpu.MatchesDivisor(divisorData, cycleLength))
@@ -465,10 +375,9 @@ namespace PerfectNumbers.Core
 			return new ExponentRemainderStepperCpu(divisorData, cycleLength);
 		}
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		internal static void ReturnExponentStepperCpu(ExponentRemainderStepperCpu stepper)
 		{
-            stepper.Reset();
 			_exponentRemainderStepperCpu = stepper;
 			_hasExponentRemainderStepperCpu = true;
 		}

PerfectNumbers.Core/ULongExtensions.cs

@@ -647,9 +647,8 @@ public static partial class ULongExtensions
 	{
 		ulong tLow = unchecked(a * b);
 		ulong m = unchecked(tLow * nPrime);
-		ulong mTimesModulusLow = unchecked(m * modulus);
+		ulong result = unchecked(m * modulus);
-
+		result = unchecked(a.MulHighCpu(b) + m.MulHighCpu(modulus) + (unchecked(tLow + result) < tLow ? 1UL : 0UL));
-		ulong result = unchecked(a.MulHighCpu(b) + m.MulHighCpu(modulus) + (unchecked(tLow + mTimesModulusLow) < tLow ? 1UL : 0UL));
 		if (result >= modulus)
 		{
 			result -= modulus;

PerfectNumbers.Core/UlongArrayPool.cs

@@ -0,0 +1,40 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core
+{
+	public struct UlongArrayPool(int poolCapacity)
+	{
+        private readonly ulong[][] _arrays = new ulong[poolCapacity][];
+        private int _count = 0;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public ulong[] Rent(int length)
+        {
+            int available = _count;
+            if (available == 0)
+            {
+                return new ulong[length];
+            }
+
+            ulong[][] arrays = _arrays;
+            available--;
+            _count = available;
+
+            ulong[] candidate = arrays[available];
+			return candidate.Length >= length ? candidate : new ulong[length];
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public void Return(ulong[] array)
+        {
+            ulong[][] arrays = _arrays;
+            int available = _count;
+
+            if (available < arrays.Length)
+            {
+				arrays[available] = array;
+				_count = available + 1;
+            }
+        }
+    }
+}

PerfectNumbers.Core/UnboundedTaskScheduler.cs

@@ -1,3 +1,5 @@
+using System.Runtime.CompilerServices;
+
 namespace PerfectNumbers.Core;
 
 public sealed class UnboundedTaskScheduler : TaskScheduler
@@ -5,17 +7,24 @@ public sealed class UnboundedTaskScheduler : TaskScheduler
 	private static UnboundedTaskScheduler? _instance;
 	private static int _configuredThreadCount = Environment.ProcessorCount;
 
-    public static UnboundedTaskScheduler Instance => _instance ??= new UnboundedTaskScheduler(_configuredThreadCount);
+	public static UnboundedTaskScheduler Instance
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _instance ??= new UnboundedTaskScheduler(_configuredThreadCount);
+	}
 
-    public static int ConfiguredThreadCount => _configuredThreadCount;
+	public static int ConfiguredThreadCount
+	{
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => _configuredThreadCount;
+	}
 
 	private TaskThreadPool _threadPool;
 
-    private UnboundedTaskScheduler(int threadCount)
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    {
+	private UnboundedTaskScheduler(int threadCount) => _threadPool = new TaskThreadPool(threadCount, ExecuteTask);
-        _threadPool = new TaskThreadPool(threadCount, ExecuteTask);
-    }
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 	public static void ConfigureThreadCount(int threadCount)
 	{
 		_configuredThreadCount = threadCount;
@@ -29,13 +38,15 @@ public sealed class UnboundedTaskScheduler : TaskScheduler
 		// }
 	}
 
-    public override int MaximumConcurrencyLevel => int.MaxValue;
+	public override int MaximumConcurrencyLevel
-
-    protected override void QueueTask(Task task)
 	{
-        _threadPool.Queue(task);
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+		get => int.MaxValue;
 	}
 
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+	protected override void QueueTask(Task task) => _threadPool.Queue(task);
+
 	private void UpdateThreadPool(int threadCount)
 	{
 		if (_threadPool.ThreadCount == threadCount)
@@ -49,19 +60,13 @@ public sealed class UnboundedTaskScheduler : TaskScheduler
 		oldPool.Dispose();
 	}
 
-    private void ExecuteTask(Task task)
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    {
+	private void ExecuteTask(Task task) => TryExecuteTask(task);
-        TryExecuteTask(task);
-    }
 
-    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    {
+	protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => TryExecuteTask(task);
-        return TryExecuteTask(task);
-    }
 
-    protected override IEnumerable<Task>? GetScheduledTasks()
+	[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-    {
+	protected override IEnumerable<Task>? GetScheduledTasks() => _threadPool.GetScheduledTasks();
-        return _threadPool.GetScheduledTasks();
-    }
 
 }