Optimize object pools by replacing ThreadLocal<T> with ThreadStatic

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ReubenBond

src/Orleans.Serialization/Invocation/Pools/ConcurrentObjectPool.cs

@@ -1,4 +1,6 @@
+using System;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using Microsoft.Extensions.ObjectPool;
 
@@ -14,17 +16,36 @@ public ConcurrentObjectPool() : base(new())
 
     internal class ConcurrentObjectPool<T, TPoolPolicy> : ObjectPool<T> where T : class where TPoolPolicy : IPooledObjectPolicy<T>
     {
-        private readonly ThreadLocal<Stack<T>> _objects = new(() => new());
+        private static int NextPoolId = -1;
 
+        private readonly int _poolId = Interlocked.Increment(ref NextPoolId);
         private readonly TPoolPolicy _policy;
 
         public ConcurrentObjectPool(TPoolPolicy policy) => _policy = policy;
 
         public int MaxPoolSize { get; set; } = int.MaxValue;
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private Stack<T> GetStack()
+        {
+            var poolId = _poolId;
+            var stacks = PerThreadStack.Stacks;
+            if (stacks is null)
+            {
+                stacks = PerThreadStack.Stacks = new Stack<T>[poolId + 1];
+            }
+            else if ((uint)poolId >= (uint)stacks.Length)
+            {
+                Array.Resize(ref stacks, Math.Max(poolId + 1, stacks.Length * 2));
+                PerThreadStack.Stacks = stacks;
+            }
+
+            return stacks[poolId] ??= new();
+        }
+
         public override T Get()
         {
-            var stack = _objects.Value;
+            var stack = GetStack();
             if (stack.TryPop(out var result))
             {
                 return result;
@@ -37,12 +58,19 @@ public override void Return(T obj)
         {
             if (_policy.Return(obj))
             {
-                var stack = _objects.Value;
+                var stack = GetStack();
                 if (stack.Count < MaxPoolSize)
                 {
                     stack.Push(obj);
                 }
             }
         }
+
+        // Thread-static stacks are indexed by pool instance to avoid sharing state between pools with different policies.
+        private static class PerThreadStack
+        {
+            [ThreadStatic]
+            internal static Stack<T>[] Stacks;
+        }
     }
 }