Pool SocketSenders (#30771)

- SocketAsyncEventArgs have lots of state on them and as a result are quite big (~350) bytes at runtime. We can pool these since sends are usually very fast and we can reduce the per connection overhead as a result.
- We also allocate one per IOQueue to reduce contention.
- Fixed buffer list management
- Disposed pool when the transport is disposed
- Added project to slnf so running tests in VS was possible
- Clear the buffer and buffer list before returning to the pool
- This cleans up dumps as the pooled senders don't see references to buffers
while pooled in the queue
- Keep track of items in the pool separately from the queue count.

src/Servers/Kestrel/Kestrel.slnf

@@ -35,7 +35,8 @@
       "src\\Servers\\Kestrel\\test\\Sockets.BindTests\\Sockets.BindTests.csproj",
       "src\\Servers\\Kestrel\\test\\Sockets.FunctionalTests\\Sockets.FunctionalTests.csproj",
       "src\\Servers\\Kestrel\\tools\\CodeGenerator\\CodeGenerator.csproj",
-      "src\\ObjectPool\\src\\Microsoft.Extensions.ObjectPool.csproj"
+      "src\\ObjectPool\\src\\Microsoft.Extensions.ObjectPool.csproj",
+      "src\\Testing\\src\\Microsoft.AspNetCore.Testing.csproj"
     ]
   }
 }

src/Servers/Kestrel/Transport.Sockets/src/Client/SocketConnectionFactory.cs

@@ -22,6 +22,7 @@ internal class SocketConnectionFactory : IConnectionFactory, IAsyncDisposable
         private readonly SocketsTrace _trace;
         private readonly PipeOptions _inputOptions;
         private readonly PipeOptions _outputOptions;
+        private readonly SocketSenderPool _socketSenderPool;
 
         public SocketConnectionFactory(IOptions<SocketTransportOptions> options, ILoggerFactory loggerFactory)
         {
@@ -46,9 +47,12 @@ public SocketConnectionFactory(IOptions<SocketTransportOptions> options, ILogger
             // These are the same, it's either the thread pool or inline
             var applicationScheduler = _options.UnsafePreferInlineScheduling ? PipeScheduler.Inline : PipeScheduler.ThreadPool;
             var transportScheduler = applicationScheduler;
+            // https://github.com/aspnet/KestrelHttpServer/issues/2573
+            var awaiterScheduler = OperatingSystem.IsWindows() ? transportScheduler : PipeScheduler.Inline;
 
             _inputOptions = new PipeOptions(_memoryPool, applicationScheduler, transportScheduler, maxReadBufferSize, maxReadBufferSize / 2, useSynchronizationContext: false);
             _outputOptions = new PipeOptions(_memoryPool, transportScheduler, applicationScheduler, maxWriteBufferSize, maxWriteBufferSize / 2, useSynchronizationContext: false);
+            _socketSenderPool = new SocketSenderPool(awaiterScheduler);
         }
 
         public async ValueTask<ConnectionContext> ConnectAsync(EndPoint endpoint, CancellationToken cancellationToken = default)
@@ -72,6 +76,7 @@ public async ValueTask<ConnectionContext> ConnectAsync(EndPoint endpoint, Cancel
                 _memoryPool,
                 _inputOptions.ReaderScheduler, // This is either threadpool or inline
                 _trace,
+                _socketSenderPool,
                 _inputOptions,
                 _outputOptions,
                 _options.WaitForDataBeforeAllocatingBuffer);

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketAwaitableEventArgs.cs

@@ -11,7 +11,7 @@
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
 {
-    internal sealed class SocketAwaitableEventArgs : SocketAsyncEventArgs, ICriticalNotifyCompletion
+    internal class SocketAwaitableEventArgs : SocketAsyncEventArgs, ICriticalNotifyCompletion
     {
         private static readonly Action _callbackCompleted = () => { };
 

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketConnection.cs

@@ -20,7 +20,8 @@ internal sealed class SocketConnection : TransportConnection
         private readonly Socket _socket;
         private readonly ISocketsTrace _trace;
         private readonly SocketReceiver _receiver;
-        private readonly SocketSender _sender;
+        private SocketSender? _sender;
+        private readonly SocketSenderPool _socketSenderPool;
         private readonly IDuplexPipe _originalTransport;
         private readonly CancellationTokenSource _connectionClosedTokenSource = new CancellationTokenSource();
 
@@ -36,6 +37,7 @@ internal SocketConnection(Socket socket,
                                   MemoryPool<byte> memoryPool,
                                   PipeScheduler transportScheduler,
                                   ISocketsTrace trace,
+                                  SocketSenderPool socketSenderPool,
                                   PipeOptions inputOptions,
                                   PipeOptions outputOptions,
                                   bool waitForData = true)
@@ -48,6 +50,7 @@ internal SocketConnection(Socket socket,
             MemoryPool = memoryPool;
             _trace = trace;
             _waitForData = waitForData;
+            _socketSenderPool = socketSenderPool;
 
             LocalEndPoint = _socket.LocalEndPoint;
             RemoteEndPoint = _socket.RemoteEndPoint;
@@ -59,8 +62,7 @@ internal SocketConnection(Socket socket,
             // https://github.com/aspnet/KestrelHttpServer/issues/2573
             var awaiterScheduler = OperatingSystem.IsWindows() ? transportScheduler : PipeScheduler.Inline;
 
-            _receiver = new SocketReceiver(_socket, awaiterScheduler);
-            _sender = new SocketSender(_socket, awaiterScheduler);
+            _receiver = new SocketReceiver(awaiterScheduler);
 
             var pair = DuplexPipe.CreateConnectionPair(inputOptions, outputOptions);
 
@@ -93,7 +95,7 @@ private async Task StartAsync()
                 await sendTask;
 
                 _receiver.Dispose();
-                _sender.Dispose();
+                _sender?.Dispose();
             }
             catch (Exception ex)
             {
@@ -183,13 +185,13 @@ private async Task ProcessReceives()
                 if (_waitForData)
                 {
                     // Wait for data before allocating a buffer.
-                    await _receiver.WaitForDataAsync();
+                    await _receiver.WaitForDataAsync(_socket);
                 }
 
                 // Ensure we have some reasonable amount of buffer space
                 var buffer = input.GetMemory(MinAllocBufferSize);
 
-                var bytesReceived = await _receiver.ReceiveAsync(buffer);
+                var bytesReceived = await _receiver.ReceiveAsync(_socket, buffer);
 
                 if (bytesReceived == 0)
                 {
@@ -282,7 +284,12 @@ private async Task ProcessSends()
                 var isCompleted = result.IsCompleted;
                 if (!buffer.IsEmpty)
                 {
-                    await _sender.SendAsync(buffer);
+                    _sender = _socketSenderPool.Rent();
+                    await _sender.SendAsync(_socket, buffer);
+                    // We don't return to the pool if there was an exception, and
+                    // we keep the _sender assigned so that we can dispose it in StartAsync.
+                    _socketSenderPool.Return(_sender);
+                    _sender = null;
                 }
 
                 output.AdvanceTo(end);

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketReceiver.cs

@@ -7,34 +7,34 @@
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
 {
-    internal sealed class SocketReceiver : SocketSenderReceiverBase
+    internal sealed class SocketReceiver : SocketAwaitableEventArgs
     {
-        public SocketReceiver(Socket socket, PipeScheduler scheduler) : base(socket, scheduler)
+        public SocketReceiver(PipeScheduler ioScheduler) : base(ioScheduler)
         {
         }
 
-        public SocketAwaitableEventArgs WaitForDataAsync()
+        public SocketAwaitableEventArgs WaitForDataAsync(Socket socket)
         {
-            _awaitableEventArgs.SetBuffer(Memory<byte>.Empty);
+            SetBuffer(Memory<byte>.Empty);
 
-            if (!_socket.ReceiveAsync(_awaitableEventArgs))
+            if (!socket.ReceiveAsync(this))
             {
-                _awaitableEventArgs.Complete();
+                Complete();
             }
 
-            return _awaitableEventArgs;
+            return this;
         }
 
-        public SocketAwaitableEventArgs ReceiveAsync(Memory<byte> buffer)
+        public SocketAwaitableEventArgs ReceiveAsync(Socket socket, Memory<byte> buffer)
         {
-            _awaitableEventArgs.SetBuffer(buffer);
+            SetBuffer(buffer);
 
-            if (!_socket.ReceiveAsync(_awaitableEventArgs))
+            if (!socket.ReceiveAsync(this))
             {
-                _awaitableEventArgs.Complete();
+                Complete();
             }
 
-            return _awaitableEventArgs;
+            return this;
         }
     }
 }

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketSender.cs

@@ -11,55 +11,61 @@
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
 {
-    internal sealed class SocketSender : SocketSenderReceiverBase
+    internal sealed class SocketSender : SocketAwaitableEventArgs
     {
         private List<ArraySegment<byte>>? _bufferList;
 
-        public SocketSender(Socket socket, PipeScheduler scheduler) : base(socket, scheduler)
+        public SocketSender(PipeScheduler scheduler) : base(scheduler)
         {
         }
 
-        public SocketAwaitableEventArgs SendAsync(in ReadOnlySequence<byte> buffers)
+        public SocketAwaitableEventArgs SendAsync(Socket socket, in ReadOnlySequence<byte> buffers)
         {
             if (buffers.IsSingleSegment)
             {
-                return SendAsync(buffers.First);
+                return SendAsync(socket, buffers.First);
             }
 
-            if (!_awaitableEventArgs.MemoryBuffer.Equals(Memory<byte>.Empty))
-            {
-                _awaitableEventArgs.SetBuffer(null, 0, 0);
-            }
-
-            _awaitableEventArgs.BufferList = GetBufferList(buffers);
+            SetBufferList(buffers);
 
-            if (!_socket.SendAsync(_awaitableEventArgs))
+            if (!socket.SendAsync(this))
             {
-                _awaitableEventArgs.Complete();
+                Complete();
             }
 
-            return _awaitableEventArgs;
+            return this;
         }
 
-        private SocketAwaitableEventArgs SendAsync(ReadOnlyMemory<byte> memory)
+        public void Reset()
         {
-            // The BufferList getter is much less expensive then the setter.
-            if (_awaitableEventArgs.BufferList != null)
+            // We clear the buffer and buffer list before we put it back into the pool
+            // it's a small performance hit but it removes the confusion when looking at dumps to see this still
+            // holds onto the buffer when it's back in the pool
+            if (BufferList != null)
+            {
+                BufferList = null;
+
+                _bufferList?.Clear();
+            }
+            else
             {
-                _awaitableEventArgs.BufferList = null;
+                SetBuffer(null, 0, 0);
             }
+        }
 
-            _awaitableEventArgs.SetBuffer(MemoryMarshal.AsMemory(memory));
+        private SocketAwaitableEventArgs SendAsync(Socket socket, ReadOnlyMemory<byte> memory)
+        {
+            SetBuffer(MemoryMarshal.AsMemory(memory));
 
-            if (!_socket.SendAsync(_awaitableEventArgs))
+            if (!socket.SendAsync(this))
             {
-                _awaitableEventArgs.Complete();
+                Complete();
             }
 
-            return _awaitableEventArgs;
+            return this;
         }
 
-        private List<ArraySegment<byte>> GetBufferList(in ReadOnlySequence<byte> buffer)
+        private void SetBufferList(in ReadOnlySequence<byte> buffer)
         {
             Debug.Assert(!buffer.IsEmpty);
             Debug.Assert(!buffer.IsSingleSegment);
@@ -68,18 +74,14 @@ private List<ArraySegment<byte>> GetBufferList(in ReadOnlySequence<byte> buffer)
             {
                 _bufferList = new List<ArraySegment<byte>>();
             }
-            else
-            {
-                // Buffers are pooled, so it's OK to root them until the next multi-buffer write.
-                _bufferList.Clear();
-            }
 
             foreach (var b in buffer)
             {
                 _bufferList.Add(b.GetArray());
             }
 
-            return _bufferList;
+            // The act of setting this list, sets the buffers in the internal buffer list
+            BufferList = _bufferList;
         }
     }
 }

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketSenderPool.cs

@@ -0,0 +1,61 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Concurrent;
+using System.IO.Pipelines;
+using System.Threading;
+
+namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
+{
+    internal class SocketSenderPool : IDisposable
+    {
+        private const int MaxQueueSize = 1024; // REVIEW: Is this good enough?
+
+        private readonly ConcurrentQueue<SocketSender> _queue = new();
+        private int _count;
+        private readonly PipeScheduler _scheduler;
+        private bool _disposed;
+
+        public SocketSenderPool(PipeScheduler scheduler)
+        {
+            _scheduler = scheduler;
+        }
+
+        public SocketSender Rent()
+        {
+            if (_queue.TryDequeue(out var sender))
+            {
+                Interlocked.Decrement(ref _count);
+                return sender;
+            }
+            return new SocketSender(_scheduler);
+        }
+
+        public void Return(SocketSender sender)
+        {
+            // This counting isn't accurate, but it's good enough for what we need to avoid using _queue.Count which could be expensive
+            if (_disposed || Interlocked.Increment(ref _count) > MaxQueueSize)
+            {
+                Interlocked.Decrement(ref _count);
+                sender.Dispose();
+                return;
+            }
+
+            sender.Reset();
+            _queue.Enqueue(sender);
+        }
+
+        public void Dispose()
+        {
+            if (!_disposed)
+            {
+                _disposed = true;
+                while (_queue.TryDequeue(out var sender))
+                {
+                    sender.Dispose();
+                }
+            }
+        }
+    }
+}