DotNetty發送請求的最佳實踐

長鏈接發送request/response時, 絕大部分包都是小包, 而每個小包都要消耗一個IP包, 成本大約是20-30us, 普通千兆網卡的pps大約是60Wpps, 所以想要提高長鏈接密集IO的應用性能, 需要做包的合併, 也稱為了scatter/gather io或者vector io.

在linux下有readv/writev就是對應這個需求的, 減少系統調用, 減少pps, 提高網卡的吞吐量. 關於readv提高讀的速度, 可以看看陳碩muduo裡面對於readv的使用, 思路是就是在棧上面弄一個64KB的數組, 組成readv的第二塊buffer, 從而儘可能一次性把socket緩衝區的內容全部出來(參見5). 這裡不再贅述, 重點描述DotNetty下麵怎麼做Gathering Write.

首先得有一個Channel<IMessage>, 用來做寫的緩衝, 讓業務關心業務, 網路關心網路, 否則每個業務都WriteAndFlushAsync, 那是不太可能有合併發送的.

然後就是SendingLoop的主迴圈, 裡面不停的從Channel裡面TryRead包, 然後WriteAsync, 隔幾個包Flush一次. 類似的思想在Orleans Network裡面也存在.

 1 public void RunSendLoopAsync(IChannel channel)
 2 {
 3     var allocator = channel.Allocator;
 4     var reader = this.queue.Reader;
 5     Task.Run(async () => 
 6     {
 7         while (!this.stop) 
 8         {
 9             var more = await reader.WaitToReadAsync();
10             if (!more) 
11             {
12                 break;
13             }
14 
15             IOutboundMessage message = default;
16             var number = 0;
17             try 
18             {
19                 while (number < 4 && reader.TryRead(out message)) 
20                 {
21                     Interlocked.Decrement(ref this.queueCount);
22                     var msg = message.Inner as IMessage;
23                     var buffer = msg.ToByteBuffer(allocator);
24                     await channel.WriteAsync(buffer);
25                     number++;
26                 }
27                 channel.Flush();
28                 number = 0;
29             }
30             catch (Exception e)  when(message != default)
31             {
32                 logger.LogError("SendOutboundMessage Fail, SessionID:{0}, Exception:{1}",
33                     this.sessionID, e.Message);
34                 this.messageCenter.OnMessageFail(message);
35             }
36         }
37         this.logger.LogInformation("SessionID:{0}, SendingLoop Exit", this.sessionID);
38     });
39 }

第19-27行是關鍵, 這邊每4個包做一下flush, 然後flush會觸發DotNetty的DoWrite:

 1 protected override void DoWrite(ChannelOutboundBuffer input)
 2 {
 3     List<ArraySegment<byte>> sharedBufferList = null;
 4     try
 5     {
 6         while (true)
 7         {
 8             int size = input.Size;
 9             if (size == 0)
10             {
11                 // All written
12                 break;
13             }
14             long writtenBytes = 0;
15             bool done = false;
16 
17             // Ensure the pending writes are made of ByteBufs only.
18             int maxBytesPerGatheringWrite = ((TcpSocketChannelConfig)this.config).GetMaxBytesPerGatheringWrite();
19             sharedBufferList = input.GetSharedBufferList(1024, maxBytesPerGatheringWrite);
20             int nioBufferCnt = sharedBufferList.Count;
21             long expectedWrittenBytes = input.NioBufferSize;
22             Socket socket = this.Socket;
23 
24             List<ArraySegment<byte>> bufferList = sharedBufferList;
25             // Always us nioBuffers() to workaround data-corruption.
26             // See https://github.com/netty/netty/issues/2761
27             switch (nioBufferCnt)
28             {
29                 case 0:
30                     // We have something else beside ByteBuffers to write so fallback to normal writes.
31                     base.DoWrite(input);
32                     return;
33                 default:
34                     for (int i = this.Configuration.WriteSpinCount - 1; i >= 0; i--)
35                     {
36                         long localWrittenBytes = socket.Send(bufferList, SocketFlags.None, out SocketError errorCode);
37                         if (errorCode != SocketError.Success && errorCode != SocketError.WouldBlock)
38                         {
39                             throw new SocketException((int)errorCode);
40                         }

DotNetty TcpSocketChannel類的DoWrite函數, 19行獲取當前ChannelOutboundBuffer的Segment<byte>數組, 然後在36行調用Socket.Send一次性發出去, 這個是Gathering Write的關鍵. 有了這個, 就可以不在業務層用CompositeByteBuffer.

DotNetty Libuv Transport的實現可以看6, 思想是類似的.

實際上Orleans 3.x做的網路優化, 也有類似的思想:

 1 private async Task ProcessOutgoing()
 2 {
 3     await Task.Yield();
 4 
 5     Exception error = default;   
 6     PipeWriter output = default;
 7     var serializer = this.serviceProvider.GetRequiredService<IMessageSerializer>();
 8     try
 9     {
10         output = this.Context.Transport.Output;
11         var reader = this.outgoingMessages.Reader;
12         if (this.Log.IsEnabled(LogLevel.Information))
13         {
14             this.Log.LogInformation(
15                 "Starting to process messages from local endpoint {Local} to remote endpoint {Remote}",
16                 this.LocalEndPoint,
17                 this.RemoteEndPoint);
18         }
19 
20         while (true)
21         {
22             var more = await reader.WaitToReadAsync();
23             if (!more)
24             {
25                 break;
26             }
27 
28             Message message = default;
29             try
30             {
31                 while (inflight.Count < inflight.Capacity && reader.TryRead(out message) && this.PrepareMessageForSend(message))
32                 {
33                     inflight.Add(message);
34                     var (headerLength, bodyLength) = serializer.Write(ref output, message);
35                     MessagingStatisticsGroup.OnMessageSend(this.MessageSentCounter, message, headerLength + bodyLength, headerLength, this.ConnectionDirection);
36                 }
37             }
38             catch (Exception exception) when (message != default)
39             {
40                 this.OnMessageSerializationFailure(message, exception);
41             }
42 
43             var flushResult = await output.FlushAsync();
44             if (flushResult.IsCompleted || flushResult.IsCanceled)
45             {
46                 break;
47             }
48 
49             inflight.Clear();
50         }

核心在31行, 開始寫, 43行開始flush, 只不過Orleans用的pipelines io, DotNetty是傳統模型.

這樣做, 可以在有限的pps下, 支撐更高的吞吐量.

個人感覺DotNetty更好用一些.

參考:

1. https://github.com/Azure/DotNetty/blob/dev/src/DotNetty.Transport/Channels/Sockets/TcpSocketChannel.cs#L271-L288

2. https://github.com/dotnet/orleans/blob/master/src/Orleans.Core/Networking/Connection.cs#L282-L294

3. https://docs.microsoft.com/zh-cn/windows/win32/winsock/scatter-gather-i-o-2

4. https://linux.die.net/man/2/writev

5. https://github.com/chenshuo/muduo/blob/d980315dc054b122612f423ee2e1316cb14bd3b5/muduo/net/Buffer.cc#L28-L38

6. https://github.com/Azure/DotNetty/blob/dev/src/DotNetty.Transport.Libuv/Native/WriteRequest.cs#L106-L128