Série Netty: explicação detalhada do protocolo de transmissão epoll

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Introdução

Nos capítulos anteriores, explicamos o uso e o princípio do kqueue, então vamos ver o uso do epoll. Ambos são métodos de IO mais avançados, e ambos precisam ser implementados por métodos nativos.A diferença é que o Kqueue é usado no sistema mac e o epoll é usado no sistema liunx.

Uso detalhado de epoll

O uso do epoll também é muito simples, ainda usamos a sala de chat comumente usada como exemplo para explicar o uso do epoll.

Para o lado do servidor, você precisa criar bossGroup e workerGroup. No NIO, esses dois grupos são NIOEventLoopGroup, e no epoll, você precisa usar EpollEventLoopGroup:

        EventLoopGroup bossGroup = new EpollEventLoopGroup(1);
        EventLoopGroup workerGroup = new EpollEventLoopGroup();
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Então você precisa passar bossGroup e workerGroup para ServerBootstrap:

ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
             .channel(EpollServerSocketChannel.class)
             .handler(new LoggingHandler(LogLevel.INFO))
             .childHandler(new NativeChatServerInitializer());
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Observe que o canal passado aqui é EpollServerSocketChannel, que é usado especialmente para processar solicitações de epoll. Outras partes são iguais aos serviços NIO normais.

Em seguida, vamos dar uma olhada no cliente do epoll. Para o cliente, você precisa criar um EventLoopGroup. Aqui usamos o EpollEventLoopGroup:

EventLoopGroup group = new EpollEventLoopGroup();
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Em seguida, passe este grupo para o Bootstrap:

Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(EpollSocketChannel.class)
             .handler(new NativeChatClientInitializer());
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O canal usado aqui é o EpollSocketChannel, que é o canal do cliente correspondente ao EpollServerSocketChannel.

EpollEventLoopGroup

Primeiro, veja a definição de EpollEventLoopGroup:

public final class EpollEventLoopGroup extends MultithreadEventLoopGroup 
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Assim como KqueueEventLoopGroup, EpollEventLoopGroup também herda de MultithreadEventLoopGroup, o que significa que ele pode abrir vários threads.

Antes de usar o EpollEventLoopGroup, você precisa garantir que as interfaces JNI relacionadas ao epoll tenham sido preparadas:

Epoll.ensureAvailability();
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newChild方法用来生成EpollEventLoopGroup的子EventLoop:

    protected EventLoop newChild(Executor executor, Object... args) throws Exception {
        Integer maxEvents = (Integer) args[0];
        SelectStrategyFactory selectStrategyFactory = (SelectStrategyFactory) args[1];
        RejectedExecutionHandler rejectedExecutionHandler = (RejectedExecutionHandler) args[2];
        EventLoopTaskQueueFactory taskQueueFactory = null;
        EventLoopTaskQueueFactory tailTaskQueueFactory = null;

        int argsLength = args.length;
        if (argsLength > 3) {
            taskQueueFactory = (EventLoopTaskQueueFactory) args[3];
        }
        if (argsLength > 4) {
            tailTaskQueueFactory = (EventLoopTaskQueueFactory) args[4];
        }
        return new EpollEventLoop(this, executor, maxEvents,
                selectStrategyFactory.newSelectStrategy(),
                rejectedExecutionHandler, taskQueueFactory, tailTaskQueueFactory);
    }
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从方法中可以看到，newChild接受一个executor和多个额外的参数，这些参数分别是SelectStrategyFactory，RejectedExecutionHandler，taskQueueFactory和tailTaskQueueFactory，最终将这些参数传入EpollEventLoop中，返回一个新的EpollEventLoop对象。

EpollEventLoop

EpollEventLoop是由EpollEventLoopGroup通过使用new child方法来创建的。

对于EpollEventLoop本身来说，是一个SingleThreadEventLoop:

class EpollEventLoop extends SingleThreadEventLoop 
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借助于native epoll IO的强大功能，EpollEventLoop可以在单线程的情况下快速进行业务处理，十分优秀。

和EpollEventLoopGroup一样，EpollEventLoop在初始化的时候需要检测系统是否支持epoll:

    static {
        Epoll.ensureAvailability();
    }
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在EpollEventLoopGroup调用的EpollEventLoop的构造函数中，初始化了三个FileDescriptor,分别是epollFd,eventFd和timerFd,这三个FileDescriptor都是调用Native方法创建的：

this.epollFd = epollFd = Native.newEpollCreate();
this.eventFd = eventFd = Native.newEventFd();
this.timerFd = timerFd = Native.newTimerFd();
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然后调用Native.epollCtlAdd建立FileDescriptor之间的关联关系：

Native.epollCtlAdd(epollFd.intValue(), eventFd.intValue(), Native.EPOLLIN | Native.EPOLLET);
Native.epollCtlAdd(epollFd.intValue(), timerFd.intValue(), Native.EPOLLIN | Native.EPOLLET);
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在EpollEventLoop的run方法中，首先会调用selectStrategy.calculateStrategy方法，拿到当前的select状态，默认情况下有三个状态，分别是：

    int SELECT = -1;

    int CONTINUE = -2;

    int BUSY_WAIT = -3;
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这三个状态我们在kqueue中已经介绍过了，不同的是epoll支持BUSY_WAIT状态，在BUSY_WAIT状态下，会去调用Native.epollBusyWait(epollFd, events)方法返回busy wait的event个数。

如果是在select状态下，则会去调用Native.epollWait(epollFd, events, 1000)方法返回wait状态下的event个数。

接下来会分别调用processReady(events, strategy)和runAllTasks方法，进行event的ready状态回调处理和最终的任务执行。

EpollServerSocketChannel

先看下EpollServerSocketChannel的定义：

public final class EpollServerSocketChannel extends AbstractEpollServerChannel implements ServerSocketChannel
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EpollServerSocketChannel继承自AbstractEpollServerChannel并且实现了ServerSocketChannel接口。

EpollServerSocketChannel的构造函数需要传入一个LinuxSocket：

    EpollServerSocketChannel(LinuxSocket fd) {
        super(fd);
        config = new EpollServerSocketChannelConfig(this);
    }
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LinuxSocket是一个特殊的socket,用来处理和linux的native socket连接。

EpollServerSocketChannelConfig是构建EpollServerSocketChannel的配置，这里用到了4个配置选项，分别是SO_REUSEPORT,IP_FREEBIND,IP_TRANSPARENT,TCP_DEFER_ACCEPT和TCP_MD5SIG。每个配置项都对应着网络协议的特定含义。

我们再看一下EpollServerSocketChannel的newChildChannel方法：

    protected Channel newChildChannel(int fd, byte[] address, int offset, int len) throws Exception {
        return new EpollSocketChannel(this, new LinuxSocket(fd), address(address, offset, len));
    }
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newChildChannel和KqueueServerSocketChannel方法一样，也是返回一个EpollSocketChannel，并且将传入的fd构造成为LinuxSocket。

EpollSocketChannel

EpollSocketChannel是由EpollServerSocketChannel创建返回的，先来看下EpollSocketChannel的定义：

public final class EpollSocketChannel extends AbstractEpollStreamChannel implements SocketChannel {
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可以看到EpollSocketChannel继承自AbstractEpollStreamChannel，并且实现了SocketChannel接口。

回到之前EpollServerSocketChannel创建EpollSocketChannel时调用的newChildChannel方法，这个方法会调用EpollSocketChannel的构造函数如下所示：

    EpollSocketChannel(Channel parent, LinuxSocket fd, InetSocketAddress remoteAddress) {
        super(parent, fd, remoteAddress);
        config = new EpollSocketChannelConfig(this);

        if (parent instanceof EpollServerSocketChannel) {
            tcpMd5SigAddresses = ((EpollServerSocketChannel) parent).tcpMd5SigAddresses();
        }
    }
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从代码的逻辑可以看到，如果EpollSocketChannel是从EpollServerSocketChannel创建出来的话，那么默认会开启tcpMd5Sig的特性。

什么是tcpMd5Sig呢？

简单点说，tcpMd5Sig就是在TCP的数据报文中添加了MD5 sig,用来进行数据的校验，从而提示数据传输的安全性。

TCP MD5是在RFC 2385中提出的，并且只在linux内核中才能开启，也就是说如果你想使用tcpMd5Sig，那么必须使用EpollServerSocketChannel和EpollSocketChannel。

所以如果是追求性能或者特殊使用场景的朋友，需要接触这种native transport的时候还是很多的,可以仔细研究其中的配置选项。

再看一下EpollSocketChannel中非常重要的doConnect0方法：

    boolean doConnect0(SocketAddress remote) throws Exception {
        if (IS_SUPPORTING_TCP_FASTOPEN_CLIENT && config.isTcpFastOpenConnect()) {
            ChannelOutboundBuffer outbound = unsafe().outboundBuffer();
            outbound.addFlush();
            Object curr;
            if ((curr = outbound.current()) instanceof ByteBuf) {
                ByteBuf initialData = (ByteBuf) curr;
                long localFlushedAmount = doWriteOrSendBytes(
                        initialData, (InetSocketAddress) remote, true);
                if (localFlushedAmount > 0) {
                    outbound.removeBytes(localFlushedAmount);
                    return true;
                }
            }
        }
        return super.doConnect0(remote);
    }
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在这个方法中会首先判断是否开启了TcpFastOpen选项，如果开启了该选项，那么最终会调用LinuxSocket的write或者sendTo方法，这些方法可以添加初始数据，可以在建立连接的同时传递数据，从而达到Tcp fast open的效果。

如果不是tcp fast open,那么需要调用Socket的connect方法去建立传统的连接。

总结

epoll在netty中的实现和kqueue很类似，他们的不同在于运行的平台和具体的功能参数，如果追求高性能的朋友可以深入研究。

本文的代码，大家可以参考：

learn-netty4

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