我不生产知识,我只是知识的搬运工。努力通过实践与各位博友交流一些自己的见解。
引文:
由于cpu和磁盘等存储设备的处理速度的差异,巧妙的io设计能够极大的提升工作效率。从硬件设计角度包括 SPOOLING(假脱机)技术(实现独占设备的共享),DMA(通过中断的方式实现内存到磁盘的传输通道)大大降低了io传输到cpu的调用和阻塞,通道IO(有自己的指令和程序,相比DMA有更强的独立处理数据能力。并且可以控制多台同类或不同类的设备)。————来自王道考研操作系统
总结:硬件实现cpu与磁盘的尽可能独立运行,磁盘读取尽可能少的通过中断程序来获取cpu的执行权。
解决了单个IO的CPU和磁盘独立运行,我们来看下多个IO连接时,操作系统如何优化? 也就是多个io链接如何管理的问题。IO作为计算机的核心功能,用户只能通过系统调用实现用户态到内核态的切换来读写磁盘数据。传统io 每建立一个io链接就要新建一个线程阻塞在当前操作中,在IO密集型任务中会大大降低CPU的利用率。通过IO复用监听多个文件描述符来提升程序的性能。
注意:IO复用虽然能同时监听多个文件描述符,但它本身是阻塞的。并且当多个文件描述符同时就绪时,如果不采取额外的措施,程序就只能按顺序依次处理其中的每一个文件描述符,这使得程序看起来就像是串行工作。如果要实现并发,只能使用多线程和多进程等编程手段。————来自Linux高性能服务器编程
理论知识:
系统调用 | select | poll | epoll |
事件集合 | 用户通过3个参数分别传入感兴趣的可读,可写和异常等事件,内核通过对这些参数的在线修改来反馈其中的就绪事件。这使得用户每次调用select都要重置这3个参数 | 统一处理所有事件类型,因此只需一个事件集参数。用户通过pollfd.events传入感兴趣的事件,内核通过修改polld.revents反馈其中就绪的事件 | 内核通过一个时间表直接管理用户感兴趣的所有事件。因此每次调用epoll_wait时,无需反复传入用户感兴趣的事件。epoll_wait系统调用的参数events仅用来反馈就绪的事件 |
应用程序索引就绪文件描述符的事件复杂度 | O(n) | O(n) | O(1) |
最大支持文件描述符数 | 有最大值限制(内核默认值为1024) | 65535 | 65535 |
工作模式 | LT | LT | 支持ET高效模式 |
内核实现和工作效率 | 采用轮询方法来检测就绪事件,算法事件复杂度为O(n) | 采用轮询方法来检测就绪事件,算法事件复杂度为O(n) | 采用回调方法来检测就绪事件,算法事件复杂度为O(n) |
系统调用API的演进路线:select————》poll(事件处理统一,编程接口更简洁。不需要重置参数)————》epoll(使用回调替换轮询机制,降低事件复杂度为O(1))
注意:当活动链接比较多的时候,epoll_wait的效率未必比selelct和poll高,因为此时回调函数出发得过于频繁。所有epoll_wait适用于连接数量多,但活动链接少的情况。
实践代码:
聊天室程序:
服务端:
/** * @author: ljf * @date: 2020/12/29 19:25 * @description: 聊天室服务端 * 功能: * 1.数据转发broadcast * 功能推导类属性: * 1.userNames * 2.userThreads * 3.port 监听端口 * <p> * accept阻塞监听客户端的链接,将链接添加到userNames,userThreads并启动用户线程接收数据 * 服务端的userThread只用来转发数据,read and write * TODO:多个客户端同时退出的并发问题, 客户端正常段开时给服务器能够检测到。同时给其他客户端发信息 * socket全双工通信 * @modified By: * @version: $ 1.0 */ public class ChatServer { private final int port; private final HashSet<String> userNames = new HashSet<>(); private final HashSet<UserThread> userThreads = new HashSet<>(); public AtomicInteger connectedCount = new AtomicInteger(0); public ChatServer(int port) { this.port = port; } public void execute() { //监听端口 try (ServerSocket serverSocket = new ServerSocket(port)) { System.out.println("chat server listening on port:" + port); while (true) { //创建服务端进程,accept阻塞监听 Socket clientSocket = serverSocket.accept(); UserThread newUser = new UserThread(clientSocket, this); userThreads.add(newUser); newUser.start(); } } catch (IOException e) { e.printStackTrace(); } } public static void main(String[] args) { int port = 12345; ChatServer chatServer = new ChatServer(port); chatServer.execute(); } public void broadMessage(String serverMessage, UserThread userThread) { for (UserThread user : userThreads) { if (user != null && user != userThread) { //除当前用户 user.sendMessage(serverMessage); } } } public Set<String> getUserNames() { return this.userNames; } /** * 删除用户,删除userName和userThread */ public void removeUser(String user, UserThread userThread) { boolean removed = userNames.remove(user); if (removed) { userThreads.remove(userThread); System.out.println(user + " quit group chat"); } } public boolean hasUsers() { return !this.userNames.isEmpty(); } public void addUserName(String userName) { this.userNames.add(userName); } }View Code
服务端每创建一个socket链接,就创建一个用户线程:
import java.io.*; import java.net.Socket; /** * @author: ljf * @date: 2020/12/29 19:34 * @description: 服务端socket线程,用来向客户端转发消息 * @modified By: * @version: $ 1.0 */ public class UserThread extends Thread { private final Socket clientSocket; private PrintWriter printWriter; private String userName; //客户端创建时输入用户名,网络传输获取 private final ChatServer chatServer; public UserThread(Socket clientSocket, ChatServer chatServer) { this.clientSocket = clientSocket; this.chatServer = chatServer; } public String getUserName() { return this.userName; } @Override public void run() { String serverMessage; try { OutputStream outputStream = clientSocket.getOutputStream(); printWriter = new PrintWriter(outputStream,true); //阻塞监听客户端发来的消息,然后转发给其他客户端 InputStream inputStream = clientSocket.getInputStream(); BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream)); printUsers(); //首条消息是 客户端姓名 userName = bufferedReader.readLine(); chatServer.addUserName(userName); System.out.println("connectedCount: " + chatServer.connectedCount.getAndIncrement() + " new user connected: " + userName); serverMessage = "new user " + userName + " connected"; chatServer.broadMessage(serverMessage, this); //read 阻塞,直到客户端发来"bye"消息,断开连接 String clientMessage; while (!(clientMessage = bufferedReader.readLine()).equals("bye")) { //拼接上当前socket的用户,转发给其他用户 serverMessage = "[" + userName + "]: " + clientMessage; chatServer.broadMessage(serverMessage, this); } } catch (IOException e) { // e.printStackTrace(); System.err.println(e.getMessage()); } finally { //与客户端socket断开连接 chatServer.removeUser(userName, this); if(clientSocket!=null && clientSocket.isConnected()){ try { clientSocket.shutdownOutput();//立即关闭输出流 clientSocket.close(); } catch (IOException e) { e.printStackTrace(); } } //转发我离开的消息 serverMessage = userName + " has quited"; chatServer.broadMessage(serverMessage, this); } } /** * 向新链接的客户端发送当前服务器的用户列表 */ public void printUsers() { if (chatServer.hasUsers()) { printWriter.println("connected users: " + chatServer.getUserNames()); } else { printWriter.println("no other users connected"); } } /** * 向客户端发送消息 * * @param message:消息内容 */ public void sendMessage(String message) { printWriter.println(message); } }View Code
客户端:
public class ChatClient { private volatile String userName; private final String hostName; private final int port; private volatile boolean closed = false; public ChatClient(String hostName,int port){ this.hostName = hostName; this.port = port; } public static void main(String[] args) { String hostName = "localhost"; int port = 12345; ChatClient chatClient = new ChatClient(hostName, port); chatClient.execute(); } /** * 与服务端建立连接 */ public void execute(){ try { //必须输入用户名字后才能创建socket Scanner scanner = new Scanner(System.in); System.out.print("\nEnter your name: "); userName = scanner.nextLine(); Socket clientSocket = new Socket(hostName, port); System.out.println("connected to chat server"); new ReadThread(clientSocket,this).start(); new WriteThread(clientSocket,this).start(); } catch (IOException e) { e.printStackTrace(); } } public void setClosed(){ closed = true; } public boolean isClosed(){ return closed; } public String getUserName() { return userName; } }
客户端socket读线程
import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.net.Socket; import java.net.SocketException; /** * @author: ljf * @date: 2020/12/29 20:55 * @description: * @modified By: * @version: $ 1.0 */ public class ReadThread extends Thread{ private final ChatClient chatClient; private final Socket clientSocket; public ReadThread(Socket clientSocket,ChatClient chatClient){ this.chatClient = chatClient; this.clientSocket = clientSocket; } @Override public void run() { while (!chatClient.isClosed()) { try { InputStream inputStream = clientSocket.getInputStream(); BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream)); String response = bufferedReader.readLine(); System.out.println("\n" + response); // prints the username after displaying the server's message if (chatClient.getUserName() != null) { System.out.print("[" + chatClient.getUserName() + "]: "); } } catch (SocketException se){ //TODO:正常退出替代这里 System.out.println("quit"); System.err.println(se.getMessage()); break; } catch (IOException ex) { System.out.println("Error reading from server: " + ex.getMessage()); ex.printStackTrace(); break; } } } }
客户端socket写线程
import java.io.IOException; import java.io.OutputStream; import java.io.PrintWriter; import java.net.Socket; import java.util.Scanner; /** * @author: ljf * @date: 2020/12/29 20:59 * @description: * @modified By: * @version: $ 1.0 */ public class WriteThread extends Thread { private final ChatClient chatClient; private final Socket clientSocket; private PrintWriter printWriter; public WriteThread(Socket clientSocket, ChatClient chatClient) { this.chatClient = chatClient; this.clientSocket = clientSocket; try { OutputStream outputStream = clientSocket.getOutputStream(); printWriter = new PrintWriter(outputStream,true); } catch (IOException e) { e.printStackTrace(); } } @Override public void run() { Scanner scanner = new Scanner(System.in); String userName = chatClient.getUserName(); printWriter.println(userName); String text; // System.out.print("[" + userName + "]: "); while (!(text = scanner.nextLine()).equals("bye")) { printWriter.println(text); System.out.print("[" + userName + "]: "); } try { printWriter.println(text); clientSocket.close(); } catch (IOException ex) { System.out.println("Error writing to server: " + ex.getMessage()); } } }
TODO:当前是用传统io流实现,后期加入nio,零拷贝。