從操作系統(tǒng)層面分析Java IO演進(jìn)之路

作者：阿里技術(shù) 2021-06-30 15:25:13

本文從操作系統(tǒng)實(shí)際調(diào)用角度(以CentOS Linux release 7.5操作系統(tǒng)為示例)，力求追根溯源看IO的每一步操作到底發(fā)生了什么。

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前言

本文從操作系統(tǒng)實(shí)際調(diào)用角度(以CentOS Linux release 7.5操作系統(tǒng)為示例)，力求追根溯源看IO的每一步操作到底發(fā)生了什么。

關(guān)于如何查看系統(tǒng)調(diào)用，Linux可以使用 strace 來查看任何軟件的系統(tǒng)調(diào)動(dòng)(這是個(gè)很好的分析學(xué)習(xí)方法)：strace -ff -o ./out java TestJava

一 BIO

/** 
  * Alipay.com Inc. Copyright (c) 2004-2020 All Rights Reserved. 
  */ 
 package io;  
  
 import java.io.*; 
 import java.net.ServerSocket; 
 import java.net.Socket; 
  
 /** 
  * @author xiangyong.ding 
  * @version $Id: TestSocket.java, v 0.1 2020年08月02日 20:56 xiangyong.ding Exp $ 
  */ 
 public class BIOSocket { 
     public static void main(String[] args) throws IOException { 
         ServerSocket serverSocket = new ServerSocket(8090); 
         System.out.println("step1: new ServerSocket "); 
         while (true) { 
             Socket client = serverSocket.accept(); 
             System.out.println("step2: client\t" + client.getPort()); 
             new Thread(() -> { 
                 try { 
                     InputStream in = client.getInputStream(); 
                     BufferedReader reader = new BufferedReader(new InputStreamReader(in)); 
                     while (true) { 
                         System.out.println(reader.readLine()); 
                     } 
                 } catch (IOException e) { 
                     e.printStackTrace(); 
                 } 
             }).start(); 
         } 
     } 
 }

1 發(fā)生的系統(tǒng)調(diào)用

啟動(dòng)時(shí)

socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 5 
 bind(5, {sa_family=AF_INET, sin_port=htons(8090), sin_addr=inet_addr("0.0.0.0")}, 16) = 0 
 listen(5, 50)                           = 0 
 poll([{fd=5, events=POLLIN|POLLERR}], 1, -1) = 1 ([{fd=5, revents=POLLIN}])

poll函數(shù)會(huì)阻塞直到其中任何一個(gè)fd發(fā)生事件。

有客戶端連接后

accept(5, {sa_family=AF_INET, sin_port=htons(10253), sin_addr=inet_addr("42.120.74.252")}, [16]) = 6 
 clone(child_stack=0x7f013e5c4fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f013e5c59d0,         tls=0x7f013e5c5700, child_tidptr=0x7f013e5c59d0) = 13168 
 poll([{fd=5, events=POLLIN|POLLERR}], 1, -1

拋出線程(即我們代碼里的 new Thread() )后，繼續(xù)poll阻塞等待連接。

clone出來的線程

recvfrom(6, "hello,bio\n", 8192, 0, NULL, NULL) =

關(guān)于對(duì)recvfrom函數(shù)的說明，其中第四個(gè)參數(shù)0 表示這是一個(gè)阻塞調(diào)用。

客戶端發(fā)送數(shù)據(jù)后

recvfrom(6, "hello,bio\n", 8192, 0, NULL, NULL) = 10

2 優(yōu)缺點(diǎn)

優(yōu)點(diǎn)

代碼簡(jiǎn)單，邏輯清晰。

缺點(diǎn)

由于stream的read操作是阻塞讀，面對(duì)多個(gè)連接時(shí) 每個(gè)連接需要每線程。無法處理大量連接(C10K問題)。
誤區(qū)：可見JDK1.8中對(duì)于最初的BIO，在Linux OS下仍然使用的poll，poll本身也是相對(duì)比較高效的多路復(fù)用函數(shù)(支持非阻塞、多個(gè)socket同時(shí)檢查event)，只是限于JDK最初的stream API限制，無法支持非阻塞讀取。

二 NIO(non block)

改進(jìn)：使用NIO API，將阻塞變?yōu)榉亲枞?不需要大量線程。

/** 
  * Alipay.com Inc. Copyright (c) 2004-2020 All Rights Reserved. 
  */ 
 package io; 
  
 import java.io.IOException; 
 import java.net.InetSocketAddress; 
 import java.nio.ByteBuffer; 
 import java.nio.channels.ServerSocketChannel; 
 import java.nio.channels.SocketChannel; 
 import java.util.LinkedList; 
  
 /** 
  * @author xiangyong.ding 
  * @version $Id: NioSocket.java, v 0.1 2020年08月09日 11:25 xiangyong.ding Exp $ 
  */ 
 public class NIOSocket { 
     private static LinkedList<SocketChannel> clients = new LinkedList<>(); 
  
     private static void startClientChannelHandleThread(){ 
         new Thread(() -> { 
             while (true){ 
                 ByteBuffer buffer = ByteBuffer.allocateDirect(4096); 
  
                 //處理客戶端連接 
                 for (SocketChannel c : clients) { 
                     // 非阻塞, >0 表示讀取到的字節(jié)數(shù)量, 0或-1表示未讀取到或讀取異常 
                     int num = 0; 
                     try { 
                         num = c.read(buffer); 
                     } catch (IOException e) { 
                         e.printStackTrace(); 
                     } 
  
                     if (num > 0) { 
                         buffer.flip(); 
                         byte[] clientBytes = new byte[buffer.limit()]; 
                         //從緩沖區(qū) 讀取到內(nèi)存中 
                         buffer.get(clientBytes); 
  
                         System.out.println(c.socket().getPort() + ":" + new String(clientBytes)); 
  
                         //清空緩沖區(qū) 
                         buffer.clear(); 
                     } 
                 } 
             } 
         }).start(); 
     } 
  
     public static void main(String[] args) throws IOException { 
         //new socket,開啟監(jiān)聽 
         ServerSocketChannel socketChannel = ServerSocketChannel.open(); 
         socketChannel.bind(new InetSocketAddress(9090)); 
         //設(shè)置阻塞接受客戶端連接 
         socketChannel.configureBlocking(true); 
  
         //開始client處理線程 
         startClientChannelHandleThread(); 
  
         while (true) { 
             //接受客戶端連接; 非阻塞，無客戶端返回null(操作系統(tǒng)返回-1) 
             SocketChannel client = socketChannel.accept(); 
  
             if (client == null) { 
                 //System.out.println("no client"); 
             } else { 
                 //設(shè)置讀非阻塞 
                 client.configureBlocking(false); 
  
                 int port = client.socket().getPort(); 
                 System.out.println("client port :" + port); 
  
                 clients.add(client); 
             } 
         } 
     } 
 }

1 發(fā)生的系統(tǒng)調(diào)用

主線程

socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 4 
 bind(4, {sa_family=AF_INET, sin_port=htons(9090), sin_addr=inet_addr("0.0.0.0")}, 16) = 0 
 listen(4, 50)                           = 0 
 fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK)    = 0 
 accept(4, 0x7fe26414e680, 0x7fe26c376710) = -1 EAGAIN (Resource temporarily unavailable)

有連接后，子線程

read(6, 0x7f3f415b1c50, 4096)           = -1 EAGAIN (Resource temporarily unavailable) 
 read(6, 0x7f3f415b1c50, 4096)           = -1 EAGAIN (Resource temporarily unavailable) 
 ...

資源使用情況：

2 優(yōu)缺點(diǎn)

優(yōu)點(diǎn)

線程數(shù)大大減少。

缺點(diǎn)

需要程序自己掃描每個(gè)連接read，需要 O(n)時(shí)間復(fù)雜度系統(tǒng)調(diào)用 (此時(shí)可能只有一個(gè)連接發(fā)送了數(shù)據(jù))，高頻系統(tǒng)調(diào)用(導(dǎo)致CPU 用戶態(tài)內(nèi)核態(tài)切換)高。導(dǎo)致CPU消耗很高。

三多路復(fù)用器(select、poll、epoll)

改進(jìn)：不需要用戶掃描所有連接，由kernel 給出哪些連接有數(shù)據(jù)，然后應(yīng)用從有數(shù)據(jù)的連接讀取數(shù)據(jù)。

1 epoll

import java.net.InetSocketAddress; 
 import java.nio.ByteBuffer; 
 import java.nio.channels.SelectionKey; 
 import java.nio.channels.Selector; 
 import java.nio.channels.ServerSocketChannel; 
 import java.nio.channels.SocketChannel; 
 import java.util.Iterator; 
 import java.util.LinkedList; 
 import java.util.Set; 
  
 /** 
  * 多路復(fù)用socket 
  * 
  * @author xiangyong.ding 
  * @version $Id: MultiplexingSocket.java, v 0.1 2020年08月09日 12:19 xiangyong.ding Exp $ 
  */ 
 public class MultiplexingSocket { 
  
     static ByteBuffer buffer = ByteBuffer.allocateDirect(4096); 
  
     public static void main(String[] args) throws Exception { 
  
         LinkedList<SocketChannel> clients = new LinkedList<>(); 
  
         //1.啟動(dòng)server 
         //new socket,開啟監(jiān)聽 
         ServerSocketChannel socketChannel = ServerSocketChannel.open(); 
         socketChannel.bind(new InetSocketAddress(9090)); 
         //設(shè)置非阻塞，接受客戶端 
         socketChannel.configureBlocking(false); 
  
         //多路復(fù)用器（JDK包裝的代理，select /poll/epoll/kqueue） 
         Selector selector = Selector.open(); //java自動(dòng)代理，默認(rèn)為epoll 
         //Selector selector = PollSelectorProvider.provider().openSelector();//指定為poll 
  
         //將服務(wù)端socket 注冊(cè)到 多路復(fù)用器 
         socketChannel.register(selector, SelectionKey.OP_ACCEPT); 
  
         //2. 輪訓(xùn)多路復(fù)用器 
         // 先詢問有沒有連接,如果有則返回?cái)?shù)量以及對(duì)應(yīng)的對(duì)象(fd) 
         while (selector.select() > 0) { 
             System.out.println(); 
             Set<SelectionKey> selectionKeys = selector.selectedKeys(); 
             Iterator<SelectionKey> iter = selectionKeys.iterator(); 
  
             while (iter.hasNext()) { 
                 SelectionKey key = iter.next(); 
                 iter.remove(); 
  
                 //2.1 處理新的連接 
                 if (key.isAcceptable()) { 
                     //接受客戶端連接; 非阻塞，無客戶端返回null(操作系統(tǒng)返回-1) 
                     SocketChannel client = socketChannel.accept(); 
                     //設(shè)置讀非阻塞 
                     client.configureBlocking(false); 
  
                     //同樣，把client也注冊(cè)到selector 
                     client.register(selector, SelectionKey.OP_READ); 
                     System.out.println("new client : " + client.getRemoteAddress()); 
                 } 
                 //2.2 處理讀取數(shù)據(jù) 
                 else if (key.isReadable()) { 
                     readDataFromSocket(key); 
                 } 
             } 
         } 
     } 
  
     protected static void readDataFromSocket(SelectionKey key) throws Exception { 
         SocketChannel socketChannel = (SocketChannel) key.channel(); 
         // 非阻塞, >0 表示讀取到的字節(jié)數(shù)量, 0或-1表示未讀取到或讀取異常 
         // 請(qǐng)注意：這個(gè)例子降低復(fù)雜度，不考慮報(bào)文大于buffer size的情況 
         int num = socketChannel.read(buffer); 
  
         if (num > 0) { 
             buffer.flip(); 
             byte[] clientBytes = new byte[buffer.limit()]; 
             //從緩沖區(qū) 讀取到內(nèi)存中 
             buffer.get(clientBytes); 
  
             System.out.println(socketChannel.socket().getPort() + ":" + new String(clientBytes)); 
  
             //清空緩沖區(qū) 
             buffer.clear(); 
         } 
     } 
 }

2 發(fā)生的系統(tǒng)調(diào)用

啟動(dòng)

socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 4 
 bind(4, {sa_family=AF_INET, sin_port=htons(9090), sin_addr=inet_addr("0.0.0.0")}, 16) = 0 
 listen(4, 50) 
 fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK)    = 0 
 epoll_create(256)                       = 7 
 epoll_ctl(7, EPOLL_CTL_ADD, 5, {EPOLLIN, {u32=5, u64=4324783852322029573}}) = 0 
 epoll_ctl(7, EPOLL_CTL_ADD, 4, {EPOLLIN, {u32=4, u64=158913789956}}) = 0 
 epoll_wait(7

關(guān)于對(duì)epoll_create(對(duì)應(yīng)著Java的 Selector selector = Selector.open()) 的說明，本質(zhì)上是在內(nèi)存的操作系統(tǒng)保留區(qū)，創(chuàng)建一個(gè)epoll數(shù)據(jù)結(jié)構(gòu)。用于后面當(dāng)有client連接時(shí)，向該epoll區(qū)中添加監(jiān)聽。

有連接

epoll_wait(7,[{EPOLLIN, {u32=4, u64=158913789956}}], 8192, -1) = 1 
 accept(4, {sa_family=AF_INET, sin_port=htons(29597), sin_addr=inet_addr("42.120.74.252")}, [16]) = 8 
 fcntl(8, F_SETFL, O_RDWR|O_NONBLOCK)    = 0 
 epoll_ctl(7, EPOLL_CTL_ADD, 8, {EPOLLIN, {u32=8, u64=3212844375897800712}}) = 0

關(guān)于epoll_ctl (對(duì)應(yīng)著Java的 client.register(selector, SelectionKey.OP_READ) )。其中 EPOLLIN 恰好對(duì)應(yīng)著Java的 SelectionKey.OP_READ 即監(jiān)聽數(shù)據(jù)到達(dá)讀取事件。

客戶端發(fā)送數(shù)據(jù)

epoll_wait(7,[{EPOLLIN, {u32=8, u64=3212844375897800712}}], 8192, -1) = 1 
 read(8, "hello,multiplex\n", 4096)      = 16 
 epoll_wait(7,

note：epoll_wait第四個(gè)參數(shù)-1表示block。

poll 和 epoll 對(duì)比

根據(jù)“1.BIO”中的poll函數(shù)調(diào)用和epoll函數(shù)對(duì)比如下：

poll和epoll本質(zhì)上都是同步IO，區(qū)別于BIO的是多路復(fù)用充分降低了 system call，而epoll更進(jìn)一步，再次降低了system call的時(shí)間復(fù)雜度。

3 優(yōu)缺點(diǎn)

優(yōu)點(diǎn)

線程數(shù)同樣很少，甚至可以把a(bǔ)cceptor線程和worker線程使用同一個(gè)。
時(shí)間復(fù)雜度低，Java實(shí)現(xiàn)的Selector(在Linux OS下使用的epoll函數(shù))支持多個(gè)clientChannel事件的一次性獲取，且時(shí)間復(fù)雜度維持在O(1)。
CPU使用低：得益于Selector，我們不用向 “2.NIO”中需要自己一個(gè)個(gè)ClientChannel手動(dòng)去檢查事件，因此使得CPU使用率大大降低。

缺點(diǎn)

數(shù)據(jù)處理麻煩：目前socketChannel.read 讀取數(shù)據(jù)完全是基于字節(jié)的，當(dāng)我們需要需要作為HTTP服務(wù)網(wǎng)關(guān)時(shí)，對(duì)于HTTP協(xié)議的處理完全需要自己解析，這是個(gè)龐大、煩雜、容易出錯(cuò)的工作。
性能

現(xiàn)有socket數(shù)據(jù)的讀取(socketChannel.read(buffer))全部通過一個(gè)buffer 緩沖區(qū)來接受，一旦連接多起來，這無疑是一個(gè)單線程讀取，性能無疑是個(gè)問題。
那么此時(shí)buffer我們每次讀取都重新new出來呢?如果每次都new出來，這樣的內(nèi)存碎片對(duì)于GC無疑是一場(chǎng)災(zāi)難。如何平衡地協(xié)調(diào)好buffer的共享，既保證性能，又保證線程安全，這是個(gè)難題。

四 Netty

1 研究的目標(biāo)源碼(netty提供的入門example)

TelnetServer

package telnet; 
  
 import io.netty.bootstrap.ServerBootstrap; 
 import io.netty.channel.EventLoopGroup; 
 import io.netty.channel.nio.NioEventLoopGroup; 
 import io.netty.channel.socket.nio.NioServerSocketChannel; 
 import io.netty.handler.logging.LogLevel; 
 import io.netty.handler.logging.LoggingHandler; 
 import io.netty.handler.ssl.SslContext; 
 import io.netty.handler.ssl.SslContextBuilder; 
 import io.netty.handler.ssl.util.SelfSignedCertificate; 
  
 /** 
  * Simplistic telnet server. 
  */ 
 public final class TelnetServer { 
  
     static final boolean SSL = System.getProperty("ssl") != null; 
     static final int PORT = Integer.parseInt(System.getProperty("port", SSL? "8992" : "8023")); 
  
     public static void main(String[] args) throws Exception { 
         // Configure SSL. 
         final SslContext sslCtx; 
         if (SSL) { 
             SelfSignedCertificate ssc = new SelfSignedCertificate(); 
             sslCtx = SslContextBuilder.forServer(ssc.certificate(), ssc.privateKey()).build(); 
         } else { 
             sslCtx = null; 
         } 
  
         EventLoopGroup bossGroup = new NioEventLoopGroup(1); 
         EventLoopGroup workerGroup = new NioEventLoopGroup(); 
         try { 
             ServerBootstrap b = new ServerBootstrap(); 
             b.group(bossGroup, workerGroup) 
              .channel(NioServerSocketChannel.class) 
              .handler(new LoggingHandler(LogLevel.INFO)) 
              .childHandler(new TelnetServerInitializer(sslCtx)); 
  
             b.bind(PORT).sync().channel().closeFuture().sync(); 
         } finally { 
             bossGroup.shutdownGracefully(); 
             workerGroup.shutdownGracefully(); 
         } 
     } 
 }

TelnetServerHandler

package telnet; 
  
 import io.netty.channel.ChannelFuture; 
 import io.netty.channel.ChannelFutureListener; 
 import io.netty.channel.ChannelHandler.Sharable; 
 import io.netty.channel.ChannelHandlerContext; 
 import io.netty.channel.SimpleChannelInboundHandler; 
  
 import java.net.InetAddress; 
 import java.util.Date; 
  
 /** 
  * Handles a server-side channel. 
  */ 
 @Sharable 
 public class TelnetServerHandler extends SimpleChannelInboundHandler<String> { 
  
     @Override 
     public void channelActive(ChannelHandlerContext ctx) throws Exception { 
         // Send greeting for a new connection. 
         ctx.write("Welcome to " + InetAddress.getLocalHost().getHostName() + "!\r\n"); 
         ctx.write("It is " + new Date() + " now.\r\n"); 
         ctx.flush(); 
     } 
  
     @Override 
     public void channelRead0(ChannelHandlerContext ctx, String request) throws Exception { 
         // Generate and write a response. 
         String response; 
         boolean close = false; 
         if (request.isEmpty()) { 
             response = "Please type something.\r\n"; 
         } else if ("bye".equals(request.toLowerCase())) { 
             response = "Have a good day!\r\n"; 
             close = true; 
         } else { 
             response = "Did you say '" + request + "'?\r\n"; 
         } 
  
         // We do not need to write a ChannelBuffer here. 
         // We know the encoder inserted at TelnetPipelineFactory will do the conversion. 
         ChannelFuture future = ctx.write(response); 
  
         // Close the connection after sending 'Have a good day!' 
         // if the client has sent 'bye'. 
         if (close) { 
             future.addListener(ChannelFutureListener.CLOSE); 
         } 
     } 
  
     @Override 
     public void channelReadComplete(ChannelHandlerContext ctx) { 
         ctx.flush(); 
     } 
  
     @Override 
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { 
         cause.printStackTrace(); 
         ctx.close(); 
     } 
 }

TelnetServerInitializer

package telnet; 
  
 import io.netty.channel.ChannelInitializer; 
 import io.netty.channel.ChannelPipeline; 
 import io.netty.channel.socket.SocketChannel; 
 import io.netty.handler.codec.DelimiterBasedFrameDecoder; 
 import io.netty.handler.codec.Delimiters; 
 import io.netty.handler.codec.string.StringDecoder; 
 import io.netty.handler.codec.string.StringEncoder; 
 import io.netty.handler.ssl.SslContext; 
  
 /** 
  * Creates a newly configured {@link ChannelPipeline} for a new channel. 
  */ 
 public class TelnetServerInitializer extends ChannelInitializer<SocketChannel> { 
  
     private static final StringDecoder DECODER = new StringDecoder(); 
     private static final StringEncoder ENCODER = new StringEncoder(); 
  
     private static final TelnetServerHandler SERVER_HANDLER = new TelnetServerHandler(); 
  
     private final SslContext sslCtx; 
  
     public TelnetServerInitializer(SslContext sslCtx) { 
         this.sslCtx = sslCtx; 
     } 
  
     @Override 
     public void initChannel(SocketChannel ch) throws Exception { 
         ChannelPipeline pipeline = ch.pipeline(); 
  
         if (sslCtx != null) { 
             pipeline.addLast(sslCtx.newHandler(ch.alloc())); 
         } 
  
         // Add the text line codec combination first, 
         pipeline.addLast(new DelimiterBasedFrameDecoder(8192, Delimiters.lineDelimiter())); 
         // the encoder and decoder are static as these are sharable 
         pipeline.addLast(DECODER); 
         pipeline.addLast(ENCODER); 
  
         // and then business logic. 
         pipeline.addLast(SERVER_HANDLER); 
     } 
 }

2 啟動(dòng)后的系統(tǒng)調(diào)用

主線程(23109)

## 256無實(shí)際作用，這里只為了兼容舊版kernel api 
 epoll_create(256)                       = 7epoll_ctl(7, EPOLL_CTL_ADD, 5, {EPOLLIN, {u32=5, u64=5477705356928876549}}) = 0 
  
 epoll_create(256)                       = 10epoll_ctl(10, EPOLL_CTL_ADD, 8, {EPOLLIN, {u32=8, u64=17041805914081853448}}) = 0 
  
 epoll_create(256)                       = 13 
 epoll_ctl(13, EPOLL_CTL_ADD, 11, {EPOLLIN, {u32=11, u64=17042151607409573899}}) = 0 
  
 epoll_create(256)                       = 16 
 epoll_ctl(16, EPOLL_CTL_ADD, 14, {EPOLLIN, {u32=14, u64=17042497300737294350}}) = 0 
  
 epoll_create(256)                       = 19 
 epoll_ctl(19, EPOLL_CTL_ADD, 17, {EPOLLIN, {u32=17, u64=17042561450368827409}}) = 0 
  
 epoll_create(256)                       = 10 
 socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 20 
 clone(child_stack=0x7fc3c509afb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7fc3c509b9d0, tls=0x7fc3c509b700, child_tidptr=0x7fc3c509b9d0) = 23130

概括為：

向OS新建socket，并開啟clone boss線程23130。
為BOSS創(chuàng)建了一個(gè)epoll(論證參見下面“boss”)，每個(gè)worker創(chuàng)建一個(gè)epoll數(shù)據(jù)結(jié)構(gòu)(本質(zhì)上是在kernel內(nèi)存區(qū)創(chuàng)建了一個(gè)數(shù)據(jù)結(jié)構(gòu)，用于后續(xù)監(jiān)聽)。
創(chuàng)建boss線程監(jiān)聽的socket(本質(zhì)上在kernel中創(chuàng)建一個(gè)數(shù)據(jù)結(jié)構(gòu))。

boss(23130)

bind(20, {sa_family=AF_INET, sin_port=htons(8023), sin_addr=inet_addr("0.0.0.0")}, 16) = 0 
 listen(20, 128)                         = 0 
 getsockname(20, {sa_family=AF_INET, sin_port=htons(8023), sin_addr=inet_addr("0.0.0.0")}, [16]) = 0 
 getsockname(20, {sa_family=AF_INET, sin_port=htons(8023), sin_addr=inet_addr("0.0.0.0")}, [16]) = 0  
  
 ##將fd為7號(hào)epoll和fd為20號(hào)的socket綁定，事件：epoll_ctl_add和epoll_ctl_mod 
 epoll_ctl(7, EPOLL_CTL_ADD, 20, {EPOLLIN, {u32=20, u64=14198059139132817428}}) = 0 
 epoll_ctl(7, EPOLL_CTL_MOD, 20, {EPOLLIN, {u32=20, u64=20}}) = 0 
 epoll_wait(7, [{EPOLLIN, {u32=5, u64=17295150779149058053}}], 8192, 1000) = 1 
 epoll_wait(7, [], 8192, 1000)           = 0(不斷輪訓(xùn)，1S超時(shí)一次)

概括為：

將上一步中main線程創(chuàng)建的fd：20綁定端口8023，并開啟監(jiān)聽(網(wǎng)卡負(fù)責(zé)監(jiān)聽和接受連接和數(shù)據(jù)，kernel則負(fù)責(zé)路由到具體進(jìn)程，具體參見：關(guān)于socket和bind和listen，TODO )。
將7號(hào)socket對(duì)應(yīng)的fd綁定到20號(hào)對(duì)應(yīng)的epoll數(shù)據(jù)結(jié)構(gòu)上去(都是操作kernel中的內(nèi)存)。
開始1S中一次阻塞等待epoll有任何連接或數(shù)據(jù)到達(dá)。

3 客戶端連接

boss (23130)

accept(20, {sa_family=AF_INET, sin_port=htons(11144), sin_addr=inet_addr("42.120.74.122")}, [16]) = 24 
 getsockname(24, {sa_family=AF_INET, sin_port=htons(8023), sin_addr=inet_addr("192.168.0.120")}, [16]) = 0 
 getsockname(24, {sa_family=AF_INET, sin_port=htons(8023), sin_addr=inet_addr("192.168.0.120")}, [16]) = 0 
 setsockopt(24, SOL_TCP, TCP_NODELAY, [1], 4) = 0 
 getsockopt(24, SOL_SOCKET, SO_SNDBUF, [87040], [4]) = 0 
 getsockopt(24, SOL_SOCKET, SO_SNDBUF, [87040], [4]) = 0 
 ##拋出 work線程 
 clone(child_stack=0x7fc3c4c98fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7fc3c4c999d0, tls=0x7fc3c4c99700, child_tidptr=0x7fc3c4c999d0) = 2301

worker (2301)

writev(24, [{"Welcome to iZbp14e1g9ztpshfrla9m"..., 37}, {"It is Sun Aug 23 15:44:14 CST 20"..., 41}], 2) = 78 
 epoll_ctl(13, EPOLL_CTL_ADD, 24, {EPOLLIN, {u32=24, u64=24}}) = 0 
 epoll_ctl(13, EPOLL_CTL_MOD, 24, {EPOLLIN, {u32=24, u64=14180008216221450264}}) = 0 
 epoll_wait(13, [{EPOLLIN, {u32=11, u64=17042151607409573899}}], 8192, 1000) = 1  
 read(11, "\1", 128)                     = 1 
 ##開始無限loop 
 epoll_wait(13, [], 8192, 1000)          = 0 
 epoll_wait(13, [{EPOLLIN, {u32=24, u64=24}}], 8192, 1000) = 1

概括：

當(dāng)BOSS輪訓(xùn)epoll_wait等到了連接后，首先accept得到該socket對(duì)應(yīng)的fd。
連接建立后 BOSS立馬拋出一個(gè)線程(clone函數(shù))。
worker(即新建的線程)寫入了一段數(shù)據(jù)(這里是業(yè)務(wù)邏輯)。
worker將該client對(duì)應(yīng)的fd綁定到了13號(hào)epoll上。
worker繼續(xù)輪訓(xùn)監(jiān)聽13號(hào)epoll。

4 客戶端主動(dòng)發(fā)送數(shù)據(jù)

worker(2301)

read(24, "i am daojian\r\n", 1024)      = 14 
 write(24, "Did you say 'i am daojian'?\r\n", 29) = 29 
 ##繼續(xù)無限loop 
 epoll_wait(13, [], 8192, 1000)          = 0

概括為：

wait到數(shù)據(jù)后，立即read到用戶控件內(nèi)存中(讀取1024個(gè)字節(jié)到用戶控件某個(gè)buff中)。
寫入數(shù)據(jù)(業(yè)務(wù)邏輯，不必太關(guān)注)。
繼續(xù)輪訓(xùn)等待13號(hào)epoll。

5 客戶端發(fā)送bye報(bào)文，服務(wù)器斷開TCP連接

worker(2301)

read(24, "bye\r\n", 1024)               = 5 
 write(24, "Have a good day!\r\n", 18)   = 18 
 getsockopt(24, SOL_SOCKET, SO_LINGER, {onoff=0, linger=0}, [8]) = 0 
 dup2(25, 24)                            = 24 
 ##從epoll數(shù)據(jù)結(jié)構(gòu)中（OS）中刪除fd為24的socket 
 epoll_ctl(13, EPOLL_CTL_DEL, 24, 0x7f702dd531e0) = -1 ENOENT 
 ##關(guān)閉24 socket 
 close(24)                               = 0 
 ##繼續(xù)等待13 epoll數(shù)據(jù) 
 epoll_wait(13, [], 8192, 1000)          = 0

斷開客戶端連接概括為：

從epoll中刪除該客戶端對(duì)應(yīng)的fd(這里觸發(fā)源頭沒找到，可能是boss)。
close關(guān)閉客戶端24號(hào)fd。
繼續(xù)輪訓(xùn)epoll。

6 五個(gè)客戶端同時(shí)連接

boss線程(23130)

accept(20, {sa_family=AF_INET, sin_port=htons(1846), sin_addr=inet_addr("42.120.74.122")}, [16]) = 24 
 clone(child_stack=0x7f702cc51fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f702cc529d0, tls=0x7f702cc52700, child_tidptr=0x7f702cc529d0) = 10035 
  
 accept(20, {sa_family=AF_INET, sin_port=htons(42067), sin_addr=inet_addr("42.120.74.122")}, [16]) = 26 
 clone(child_stack=0x7f702cb50fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f702cb519d0, tls=0x7f702cb51700, child_tidptr=0x7f702cb519d0) = 10067 
  
 ...

woker線程(10035，第一個(gè)連接)

epoll_ctl(13, EPOLL_CTL_ADD, 24, {EPOLLIN, {u32=24, u64=24}}) = 0 
 epoll_ctl(13, EPOLL_CTL_MOD, 24, {EPOLLIN, {u32=24, u64=3226004877247250456}}) = 0 
 epoll_wait(13, [{EPOLLIN, {u32=11, u64=17042151607409573899}}], 8192, 1000) = 1                  = 1 
 epoll_wait(13, [], 8192, 1000)          = 0

worker線程(10067，第二個(gè)連接)

epoll_ctl(16, EPOLL_CTL_ADD, 26, {EPOLLIN, {u32=26, u64=26}}) = 0 
 epoll_ctl(16, EPOLL_CTL_MOD, 26, {EPOLLIN, {u32=26, u64=3221483685433835546}}) = 0 
 epoll_wait(16, [{EPOLLIN, {u32=14, u64=17042497300737294350}}], 8192, 1000) = 1 
 epoll_wait(16, [], 8192, 1000)          = 0 
 epoll_wait(16, [], 8192, 1000)          = 0

worker線程(10067，第二個(gè)連接)

epoll_ctl(19, EPOLL_CTL_ADD, 27, {EPOLLIN, {u32=27, u64=27}}) = 0 
 epoll_ctl(19, EPOLL_CTL_MOD, 27, {EPOLLIN, {u32=27, u64=3216966479350071323}}) = 0

worker線程(8055，第四個(gè)連接)

epoll_ctl(10, EPOLL_CTL_ADD, 28, {EPOLLIN, {u32=28, u64=28}}) = 0 
 epoll_ctl(10, EPOLL_CTL_MOD, 28, {EPOLLIN, {u32=28, u64=3302604828697427996}}) = 0

worker線程(10035，第五個(gè)連接，不在clone線程，而是復(fù)用了第一個(gè)epoll對(duì)應(yīng)的worker)

epoll_ctl(13, EPOLL_CTL_ADD, 29, {EPOLLIN, {u32=29, u64=29}}) = 0 
 epoll_ctl(13, EPOLL_CTL_MOD, 29, {EPOLLIN, {u32=29, u64=29}}) = 0

概括為：

epoll和boss、worker之間的關(guān)系：一共有4個(gè)worker對(duì)應(yīng)著4個(gè)epoll對(duì)象，boss和每個(gè)worker都有對(duì)應(yīng)自己的epoll。
boss根據(jù)epoll數(shù)量，平衡分配連接到每個(gè)worker對(duì)應(yīng)的epoll中。

7 總結(jié)

下圖通過對(duì)系統(tǒng)調(diào)用的調(diào)查得出 netty 和 kernel 交互圖：

初始化直接創(chuàng)建5個(gè)epoll，其中7號(hào)為boss使用，專門用于處理和客戶端連接;其余4個(gè)用來給worker使用，用戶處理和客戶端的數(shù)據(jù)交互。

work的線程數(shù)量，取決于初始化時(shí)創(chuàng)建了幾個(gè)epoll，worker的復(fù)用本質(zhì)上是epoll的復(fù)用。

work之間為什么要獨(dú)立使用epoll?為什么不共享?

為了避免各個(gè)worker之間發(fā)生爭(zhēng)搶連接處理，netty直接做了物理隔離，避免競(jìng)爭(zhēng)。各個(gè)worker只負(fù)責(zé)處理自己管理的連接，并且后續(xù)該worker中的每個(gè)client的讀寫操作完全由該線程單獨(dú)處理，天然避免了資源競(jìng)爭(zhēng)，避免了鎖。
worker單線程，性能考慮：worker不僅僅要epoll_wait，還是處理read、write邏輯，加入worker處理了過多的連接，勢(shì)必造成這部分消耗時(shí)間片過多，來不及處理更多連接，性能下降。

8 優(yōu)缺點(diǎn)

優(yōu)點(diǎn)

數(shù)據(jù)處理：netty提供了大量成熟的數(shù)據(jù)處理組件(ENCODER、DECODER)，HTTP、POP3拿來即用。
編碼復(fù)雜度、可維護(hù)性：netty充分使得業(yè)務(wù)邏輯與網(wǎng)絡(luò)處理解耦，只需要少量的BootStrap配置即可，更多的集中在業(yè)務(wù)邏輯處理上。
性能：netty提供了的ByteBuf(底層Java原生的ByteBuffer)，提供了池化的ByteBuf，兼顧讀取性能和ByteBuf內(nèi)存分配(在后續(xù)文檔中會(huì)再做詳解)。

缺點(diǎn)

入門有一定難度。

五 AIO

1 啟動(dòng)

main線程

epoll_create(256)                       = 5 
 epoll_ctl(5, EPOLL_CTL_ADD, 6, {EPOLLIN, {u32=6, u64=11590018039084482566}}) = 0 
  
 ##創(chuàng)建BOSS 線程(Proactor) 
 clone(child_stack=0x7f340ac06fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f340ac079d0, tls=0x7f340ac07700, child_tidptr=0x7f340ac079d0) = 22704 
  
 socket(AF_INET6, SOCK_STREAM, IPPROTO_IP) = 8 
 setsockopt(8, SOL_IPV6, IPV6_V6ONLY, [0], 4) = 0 
 setsockopt(8, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 
 bind(8, {sa_family=AF_INET6, sin6_port=htons(9090), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0 
 listen(8, 50) 
  
 accept(8, 0x7f67d01b3120, 0x7f67d9246690) = -1 
 epoll_ctl(5, EPOLL_CTL_MOD, 8, {EPOLLIN|EPOLLONESHOT, {u32=8, u64=15380749440025362440}}) = -1 ENOENT (No such file or directory) 
 epoll_ctl(5, EPOLL_CTL_ADD, 8, {EPOLLIN|EPOLLONESHOT, {u32=8, u64=15380749440025362440}}) = 0 
 read(0,

22704(BOSS 線程(Proactor))

epoll_wait(5,  <unfinished ...>

2 請(qǐng)求連接

22704(BOSS 線程(Proactor))處理連接

epoll_wait(5,[{EPOLLIN, {u32=9, u64=4294967305}}], 512, -1) = 1 
 accept(8, {sa_family=AF_INET6, sin6_port=htons(55320), inet_pton(AF_INET6, "::ffff:36.24.32.140", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 9 
 clone(child_stack=0x7ff35c99ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7ff35c9a09d0, tls=0x7ff35c9a0700, child_tidptr=0x7ff35c9a09d0) = 26241 
 epoll_wait(5,  <unfinished ...>

26241

#將client 連接的FD加入到BOSS的epoll中，以便BOSS線程監(jiān)聽網(wǎng)絡(luò)事件 
 epoll_ctl(5, EPOLL_CTL_MOD, 9, {EPOLLIN|EPOLLONESHOT, {u32=9, u64=4398046511113}}) = -1 ENOENT (No such file or directory) 
 epoll_ctl(5, EPOLL_CTL_ADD, 9, {EPOLLIN|EPOLLONESHOT, {u32=9, u64=4398046511113}}) = 0 
 accept(8, 0x7ff3440008c0, 0x7ff35c99f4d0) = -1 EAGAIN (Resource temporarily unavailable) 
 epoll_ctl(5, EPOLL_CTL_MOD, 8, {EPOLLIN|EPOLLONESHOT, {u32=8, u64=8}}) = 0

3 客戶端發(fā)送數(shù)據(jù)

22704(BOSS 線程(Proactor))處理連接

epoll_wait(5,[{EPOLLIN, {u32=9, u64=4294967305}}], 512, -1) = 1 
 ##數(shù)據(jù)讀出 
 read(9, "daojian111\r\n", 1024)         = 12 
 ##數(shù)據(jù)處理交給其他線程，這里由于線程池為空，需要先clone線程 
 clone(child_stack=0x7ff35c99ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7ff35c9a09d0, tls=0x7ff35c9a0700, child_tidptr=0x7ff35c9a09d0) = 26532

復(fù)制線程處理，線程號(hào)26532

write(1, "pool-1-thread-2-10received : dao"..., 41) = 41 
 write(1, "\n", 1) 
 accept(8, 0x7f11c400b5f0, 0x7f11f42fd4d0) = -1 EAGAIN (Resource temporarily unavailable) 
 epoll_ctl(5, EPOLL_CTL_MOD, 8, {EPOLLIN|EPOLLONESHOT, {u32=8, u64=8}}) = 0

4 總結(jié)

從系統(tǒng)調(diào)用角度，Java的AIO事實(shí)上是以多路復(fù)用(Linux上為epoll)等同步IO為基礎(chǔ)，自行實(shí)現(xiàn)了異步事件分發(fā)。
BOSS Thread負(fù)責(zé)處理連接，并分發(fā)事件。
WORKER Thread只負(fù)責(zé)從BOSS接收的事件執(zhí)行，不負(fù)責(zé)任何網(wǎng)絡(luò)事件監(jiān)聽。