DotNetty发送请求的最佳实践

长链接发送request/response时, 绝大部分包都是小包, 而每个小包都要消耗一个IP包, 成本大约是20-30us, 普通千兆网卡的pps大约是60Wpps, 所以想要提高长链接密集IO的应用性能, 需要做包的合并, 也称为了scatter/gather io或者vector io.

在linux下有readv/writev就是对应这个需求的, 减少系统调用, 减少pps, 提高网卡的吞吐量. 关于readv提高读的速度, 可以看看陈硕muduo里面对于readv的使用, 思路是就是在栈上面弄一个64KB的数组, 组成readv的第二块buffer, 从而尽可能一次性把socket缓冲区的内容全部出来(参见5). 这里不再赘述, 重点描述DotNetty下面怎么做Gathering Write.

首先得有一个Channel<IMessage>, 用来做写的缓冲, 让业务关心业务, 网络关心网络, 否则每个业务都WriteAndFlushAsync, 那是不太可能有合并发送的.

然后就是SendingLoop的主循环, 里面不停的从Channel里面TryRead包, 然后WriteAsync, 隔几个包Flush一次. 类似的思想在Orleans Network里面也存在.

public void RunSendLoopAsync(IChannel channel)
{
    var allocator = channel.Allocator;
    var reader = this.queue.Reader;
    Task.Run(async () => 
    {
        while (!this.stop) 
        {
            var more = await reader.WaitToReadAsync();
            if (!more) 
            {
                break;
            }

            IOutboundMessage message = default;
            var number = 0;
            try 
            {
                while (number < 4 && reader.TryRead(out message) && message != null) 
                {
                    Interlocked.Decrement(ref this.queueCount);
                    var msg = message.Inner as IMessage;
                    var buffer = msg.ToByteBuffer(allocator);
                    channel.WriteAsync(buffer);
                    number++;
                }
                channel.Flush();
                number = 0;
            }
            catch (Exception e)  when(message != default)
            {
                logger.LogError("SendOutboundMessage Fail, SessionID:{0}, Exception:{1}",
                    this.sessionID, e.Message);
                this.messageCenter.OnMessageFail(message);
            }
        }
        this.logger.LogInformation("SessionID:{0}, SendingLoop Exit", this.sessionID);
    });
}

第19-27行是关键, 这边每4个包做一下flush, 然后flush会触发DotNetty的DoWrite:

protected override void DoWrite(ChannelOutboundBuffer input)
{
    List<ArraySegment<byte>> sharedBufferList = null;
    try
    {
        while (true)
        {
            int size = input.Size;
            if (size == 0)
            {
                // All written
                break;
            }
            long writtenBytes = 0;
            bool done = false;

            // Ensure the pending writes are made of ByteBufs only.
            int maxBytesPerGatheringWrite = ((TcpSocketChannelConfig)this.config).GetMaxBytesPerGatheringWrite();
            sharedBufferList = input.GetSharedBufferList(1024, maxBytesPerGatheringWrite);
            int nioBufferCnt = sharedBufferList.Count;
            long expectedWrittenBytes = input.NioBufferSize;
            Socket socket = this.Socket;

            List<ArraySegment<byte>> bufferList = sharedBufferList;
            // Always us nioBuffers() to workaround data-corruption.
            // See https://github.com/netty/netty/issues/2761
            switch (nioBufferCnt)
            {
                case 0:
                    // We have something else beside ByteBuffers to write so fallback to normal writes.
                    base.DoWrite(input);
                    return;
                default:
                    for (int i = this.Configuration.WriteSpinCount - 1; i >= 0; i--)
                    {
                        long localWrittenBytes = socket.Send(bufferList, SocketFlags.None, out SocketError errorCode);
                        if (errorCode != SocketError.Success && errorCode != SocketError.WouldBlock)
                        {
                            throw new SocketException((int)errorCode);
                        }

DotNetty TcpSocketChannel类的DoWrite函数, 19行获取当前ChannelOutboundBuffer的Segment<byte>数组, 然后在36行调用Socket.Send一次性发出去, 这个是Gathering Write的关键. 有了这个, 就可以不在业务层用CompositeByteBuffer.

DotNetty Libuv Transport的实现可以看6, 思想是类似的.

实际上Orleans 3.x做的网络优化, 也有类似的思想:

private async Task ProcessOutgoing()
{
    await Task.Yield();

    Exception error = default;   
    PipeWriter output = default;
    var serializer = this.serviceProvider.GetRequiredService<IMessageSerializer>();
    try
    {
        output = this.Context.Transport.Output;
        var reader = this.outgoingMessages.Reader;
        if (this.Log.IsEnabled(LogLevel.Information))
        {
            this.Log.LogInformation(
                "Starting to process messages from local endpoint {Local} to remote endpoint {Remote}",
                this.LocalEndPoint,
                this.RemoteEndPoint);
        }

        while (true)
        {
            var more = await reader.WaitToReadAsync();
            if (!more)
            {
                break;
            }

            Message message = default;
            try
            {
                while (inflight.Count < inflight.Capacity && reader.TryRead(out message) && this.PrepareMessageForSend(message))
                {
                    inflight.Add(message);
                    var (headerLength, bodyLength) = serializer.Write(ref output, message);
                    MessagingStatisticsGroup.OnMessageSend(this.MessageSentCounter, message, headerLength + bodyLength, headerLength, this.ConnectionDirection);
                }
            }
            catch (Exception exception) when (message != default)
            {
                this.OnMessageSerializationFailure(message, exception);
            }

            var flushResult = await output.FlushAsync();
            if (flushResult.IsCompleted || flushResult.IsCanceled)
            {
                break;
            }

            inflight.Clear();
        }

核心在31行, 开始写, 43行开始flush, 只不过Orleans用的pipelines io, DotNetty是传统模型.

这样做, 可以在有限的pps下, 支撑更高的吞吐量.

个人感觉DotNetty更好用一些.

参考:

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