perf: pool SubList match builders and cleanup scans

src/NATS.Server/Subscriptions/SubList.cs

@@ -12,6 +12,10 @@ public sealed class SubList : IDisposable
 {
     private const int CacheMax = 1024;
     private const int CacheSweep = 256;
+    [ThreadStatic]
+    private static MatchBuilder? s_matchBuilder;
+    [ThreadStatic]
+    private static List<RoutedSubKey>? s_remoteSubRemovalKeys;
 
     private readonly ReaderWriterLockSlim _lock = new();
     private readonly TrieLevel _root = new();
@@ -243,23 +247,31 @@ public sealed class SubList : IDisposable
         _lock.EnterWriteLock();
         try
         {
+            var removalKeys = RentRemoteSubRemovalKeys();
             var removed = 0;
-            foreach (var kvp in _remoteSubs.ToArray())
+            foreach (var (key, _) in _remoteSubs)
             {
-                if (!string.Equals(kvp.Key.RouteId, routeId, StringComparison.Ordinal))
+                if (!string.Equals(key.RouteId, routeId, StringComparison.Ordinal))
                     continue;
 
-                if (_remoteSubs.Remove(kvp.Key))
+                removalKeys.Add(key);
+            }
+
+            foreach (var key in removalKeys)
+            {
+                if (_remoteSubs.Remove(key, out var removedSub))
                 {
                     removed++;
                     InterestChanged?.Invoke(new InterestChange(
                         InterestChangeKind.RemoteRemoved,
-                        kvp.Value.Subject,
-                        kvp.Value.Queue,
-                        kvp.Value.Account));
+                        removedSub.Subject,
+                        removedSub.Queue,
+                        removedSub.Account));
                 }
             }
 
+            removalKeys.Clear();
+
             if (removed > 0)
                 Interlocked.Increment(ref _generation);
 
@@ -276,26 +288,34 @@ public sealed class SubList : IDisposable
         _lock.EnterWriteLock();
         try
         {
+            var removalKeys = RentRemoteSubRemovalKeys();
             var removed = 0;
-            foreach (var kvp in _remoteSubs.ToArray())
+            foreach (var (key, _) in _remoteSubs)
             {
-                if (!string.Equals(kvp.Key.RouteId, routeId, StringComparison.Ordinal)
-                    || !string.Equals(kvp.Key.Account, account, StringComparison.Ordinal))
+                if (!string.Equals(key.RouteId, routeId, StringComparison.Ordinal)
+                    || !string.Equals(key.Account, account, StringComparison.Ordinal))
                 {
                     continue;
                 }
 
-                if (_remoteSubs.Remove(kvp.Key))
+                removalKeys.Add(key);
+            }
+
+            foreach (var key in removalKeys)
+            {
+                if (_remoteSubs.Remove(key, out var removedSub))
                 {
                     removed++;
                     InterestChanged?.Invoke(new InterestChange(
                         InterestChangeKind.RemoteRemoved,
-                        kvp.Value.Subject,
-                        kvp.Value.Queue,
-                        kvp.Value.Account));
+                        removedSub.Subject,
+                        removedSub.Queue,
+                        removedSub.Account));
                 }
             }
 
+            removalKeys.Clear();
+
             if (removed > 0)
                 Interlocked.Increment(ref _generation);
 
@@ -565,22 +585,9 @@ public sealed class SubList : IDisposable
                 return cached.Result;
             }
 
-            var plainSubs = new List<Subscription>();
-            var queueSubs = new List<List<Subscription>>();
-            MatchLevel(_root, tokens, 0, plainSubs, queueSubs);
-
-            SubListResult result;
-            if (plainSubs.Count == 0 && queueSubs.Count == 0)
-            {
-                result = SubListResult.Empty;
-            }
-            else
-            {
-                var queueSubsArr = new Subscription[queueSubs.Count][];
-                for (int i = 0; i < queueSubs.Count; i++)
-                    queueSubsArr[i] = queueSubs[i].ToArray();
-                result = new SubListResult(plainSubs.ToArray(), queueSubsArr);
-            }
+            var builder = RentMatchBuilder();
+            MatchLevel(_root, tokens, 0, builder);
+            var result = builder.ToResult();
 
             if (_cache != null)
             {
@@ -676,6 +683,20 @@ public sealed class SubList : IDisposable
         return false;
     }
 
+    private static MatchBuilder RentMatchBuilder()
+    {
+        var builder = s_matchBuilder ??= new MatchBuilder();
+        builder.Reset();
+        return builder;
+    }
+
+    private static List<RoutedSubKey> RentRemoteSubRemovalKeys()
+    {
+        var keys = s_remoteSubRemovalKeys ??= [];
+        keys.Clear();
+        return keys;
+    }
+
     private bool HasExactQueueInterestNoLock(string subject, string queue)
     {
         var subs = new List<Subscription>();
@@ -822,6 +843,42 @@ public sealed class SubList : IDisposable
             AddNodeToResults(pwc, plainSubs, queueSubs);
     }
 
+    private static void MatchLevel(TrieLevel? level, string[] tokens, int tokenIndex, MatchBuilder builder)
+    {
+        TrieNode? pwc = null;
+        TrieNode? node = null;
+
+        for (int i = tokenIndex; i < tokens.Length; i++)
+        {
+            if (level == null)
+                return;
+
+            if (level.Fwc != null)
+                AddNodeToResults(level.Fwc, builder);
+
+            pwc = level.Pwc;
+            if (pwc != null)
+                MatchLevel(pwc.Next, tokens, i + 1, builder);
+
+            node = null;
+            if (level.Nodes.TryGetValue(tokens[i], out var found))
+            {
+                node = found;
+                level = node.Next;
+            }
+            else
+            {
+                level = null;
+            }
+        }
+
+        if (node != null)
+            AddNodeToResults(node, builder);
+
+        if (pwc != null)
+            AddNodeToResults(pwc, builder);
+    }
+
     private static void AddNodeToResults(TrieNode node,
         List<Subscription> plainSubs, List<List<Subscription>> queueSubs)
     {
@@ -853,6 +910,19 @@ public sealed class SubList : IDisposable
         }
     }
 
+    private static void AddNodeToResults(TrieNode node, MatchBuilder builder)
+    {
+        builder.PlainSubs.AddRange(node.PlainSubs);
+
+        foreach (var (queueName, subs) in node.QueueSubs)
+        {
+            if (subs.Count == 0)
+                continue;
+
+            builder.AddQueueGroup(queueName, subs);
+        }
+    }
+
     public SubListStats Stats()
     {
         _lock.EnterReadLock();
@@ -1368,4 +1438,47 @@ public sealed class SubList : IDisposable
         public bool IsEmpty => PlainSubs.Count == 0 && QueueSubs.Count == 0 &&
                                (Next == null || (Next.Nodes.Count == 0 && Next.Pwc == null && Next.Fwc == null));
     }
+
+    private sealed class MatchBuilder
+    {
+        private readonly Dictionary<string, int> _queueIndexes = new(StringComparer.Ordinal);
+        private readonly List<List<Subscription>> _queueGroups = [];
+        private int _queueGroupCount;
+
+        public List<Subscription> PlainSubs { get; } = [];
+
+        public void Reset()
+        {
+            PlainSubs.Clear();
+            _queueIndexes.Clear();
+            for (var i = 0; i < _queueGroupCount; i++)
+                _queueGroups[i].Clear();
+            _queueGroupCount = 0;
+        }
+
+        public void AddQueueGroup(string queueName, HashSet<Subscription> subs)
+        {
+            if (!_queueIndexes.TryGetValue(queueName, out var index))
+            {
+                index = _queueGroupCount++;
+                _queueIndexes[queueName] = index;
+                if (index == _queueGroups.Count)
+                    _queueGroups.Add([]);
+            }
+
+            _queueGroups[index].AddRange(subs);
+        }
+
+        public SubListResult ToResult()
+        {
+            if (PlainSubs.Count == 0 && _queueGroupCount == 0)
+                return SubListResult.Empty;
+
+            var queueSubsArr = new Subscription[_queueGroupCount][];
+            for (var i = 0; i < _queueGroupCount; i++)
+                queueSubsArr[i] = _queueGroups[i].ToArray();
+
+            return new SubListResult(PlainSubs.ToArray(), queueSubsArr);
+        }
+    }
 }