异步I/O处理

异步I/O处理过程

异步I/O处理的优点
    I/O密集型计算（进程所执行的I/O操作比执行的处理操作更多）的任务中，使用异步I/O的方式，可以提高CPU对应用程序处理的吞吐率，应用程序无需进行I/O阻塞。保证在I/O处理时，仍能进行应用程序的处理
异步I/O的缺点
    如果程序的I/O操作频繁并且短暂，则I/O线程切换代价较大，并且异步I/O库处理有一定的开销（磁盘磁头的寻址切换等），异步I/O对程序的写法有一定的门槛。

异步I/O的应用场景
    NodeJS的单进程，单线程模型。为了能同时处理更多的请求，I/O操作，必需使用异步处理的方式，NodeJS采用的是libeio的实现方式。

异步I/O的实现
glibc：
在文件目录下新建file.txt文件，file.txt内容：hello world!

#include <aio.h>  
#include <errno.h>  
#include <stdio.h>  
#include <stdlib.h>  
#include <string.h>  
char bufferAO[8192] __attribute__((aligned(4096)));  
  
void aio_completion_handler(int signo, siginfo_t *info, void *context)  
{  
    int ret;  
        write(1, "callback\n", 9);  
    struct aiocb *req;  
    // Ensure it's our signal  
    if (info->si_signo == SIGIO) {  
      
        req = (struct aiocb *)info->si_value.sival_ptr;  
        printf("data: %s\n" ,req->aio_buf);  
        // Did the request complete?  
        if (aio_error( req ) == 0) {  
            // Request completed successfully, get the return status  
            ret = aio_return( req );  
        }  
  
    }  
    return;  
}  
  
int  main()  
{  
    int fd, ret;  
    struct sigaction sig_act;  
    struct aiocb my_aiocb;  
    fd = open("file.txt", O_RDONLY);  
    if(fd < 0){  
        perror("open fail");  
    }  
  
    // Set up the signal handler  
    sigemptyset(&sig_act.sa_mask);  
    sig_act.sa_flags = SA_SIGINFO;  
    sig_act.sa_sigaction = aio_completion_handler;  
  
  
    // Set up the AIO request  
    bzero( (char *)&my_aiocb, sizeof(struct aiocb) );  
    my_aiocb.aio_fildes = fd;  
    my_aiocb.aio_buf = malloc(sizeof(bufferAO)+1);  
    my_aiocb.aio_nbytes = sizeof(bufferAO);  
    my_aiocb.aio_offset = 0;  
  
      
    // Link the AIO request with the Signal Handler  
    my_aiocb.aio_sigevent.sigev_notify = SIGEV_SIGNAL;  
    my_aiocb.aio_sigevent.sigev_signo = SIGIO;  
    my_aiocb.aio_sigevent.sigev_value.sival_ptr = &my_aiocb;  
  
    // Map the Signal to the Signal Handler  
    ret = sigaction( SIGIO, &sig_act, NULL );  
      
    ret = aio_read(&my_aiocb);  
    if (ret < 0) {  
        perror("error info");  
    }  
    write(1, "caller thread\n", 14);  
    sleep(5);  
}  

compile:  gcc aio_demo.c -o aio_demo.o -lrt
run: ./aio_demo.o
result:
caller thread
callback
data: hello world!
kernel native aio:
待完善

libeio:
libeio的源码下载：https://github.com/scunningham/libeio

libeio的编译及运行过程：

./autogen.sh

./configure

make

sudo make install

export LD_LIBRARY_PATH=".libs;$LD_LIBRARY_PATH"

#此时可以编译运行demo.c了

gcc demo.c -o demo -leio

./demo

运行结果：

具体的libeio源码及流程分析可以查看大牛已经做的文档：
NodeJS代码阅读笔记之libeio
libeio异步I/O初探

Java：
Server端：

package org.operamasks.nio;
import java.io.IOException;
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.nio.charset.Charset;
import java.util.Calendar;
import java.util.Iterator;


public class TestServer {
	public static void main(String[] args) {
		new Thread(new EchoServer(1982)).start();
	}
}

class EchoServer implements Runnable {
	//要监听的端口号
	private int port;
	//生成一个信号监视器
	private Selector s;
	//读缓冲区
	private ByteBuffer r_bBuf = ByteBuffer.allocate(1024);
	private ByteBuffer w_bBuf;
	
	public EchoServer(int port) {
		this.port = port;
		try {
			s = Selector.open();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	public void run() {
		try {
			//生成一个ServerScoket通道的实例对象，用于侦听可能发生的IO事件
			ServerSocketChannel ssc = ServerSocketChannel.open();
			//将该通道设置为异步方式
			ssc.configureBlocking(false);
			//绑定到一个指定的端口
			ssc.socket().bind(new InetSocketAddress(port));
			//注册特定类型的事件到信号监视器上
			ssc.register(s, SelectionKey.OP_ACCEPT);
			System.out.println("The server has been launched...");
			while(true) {
				//将会阻塞执行，直到有事件发生
				s.select();
				Iterator it = s.selectedKeys().iterator();
				while(it.hasNext()) {
					SelectionKey key = it.next();
					//key定义了四种不同形式的操作
					switch(key.readyOps()) {
					case SelectionKey.OP_ACCEPT :
						dealwithAccept(key);
						break;
					case SelectionKey.OP_CONNECT :
						break;
					case SelectionKey.OP_READ :
						dealwithRead(key);
						break;
					case SelectionKey.OP_WRITE :
						break;
					}
					//处理结束后移除当前事件，以免重复处理
					it.remove();
				}
			}
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	//处理接收连接的事件
	private void dealwithAccept(SelectionKey key) {
		try {
			System.out.println("新的客户端请求连接...");
			ServerSocketChannel server = (ServerSocketChannel)key.channel();
			SocketChannel sc = server.accept();
			sc.configureBlocking(false);
			//注册读事件
			sc.register(s, SelectionKey.OP_READ);
			System.out.println("客户端连接成功...");
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	//处理客户端发来的消息，处理读事件
	private void dealwithRead(SelectionKey key) {
		try {
			SocketChannel sc = (SocketChannel)key.channel();
			System.out.println("读入数据");
			r_bBuf.clear();
			//将字节序列从此通道中读入给定的缓冲区r_bBuf
			sc.read(r_bBuf);
			r_bBuf.flip();
			String msg = Charset.forName("UTF-8").decode(r_bBuf).toString();
			if(msg.equalsIgnoreCase("time")) {
				w_bBuf = ByteBuffer.wrap(getCurrentTime().getBytes("UTF-8"));
				sc.write(w_bBuf);
				w_bBuf.clear();
			} else if(msg.equalsIgnoreCase("bye")) {
				sc.write(ByteBuffer.wrap("已经与服务器断开连接".getBytes("UTF-8")));
				sc.socket().close();
			} else {
				sc.write(ByteBuffer.wrap(msg.getBytes("UTF-8")));
			}
			System.out.println(msg);
			System.out.println("处理完毕...");
			r_bBuf.clear();
			try {
				Thread.currentThread();
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	private String getCurrentTime() {
		Calendar date = Calendar.getInstance();
		String time = "服务器当前时间：" +
					  date.get(Calendar.YEAR) + "-" +
					  date.get(Calendar.MONTH)+1 + "-" +
					  date.get(Calendar.DATE) + " " +
					  date.get(Calendar.HOUR) + ":" +
					  date.get(Calendar.MINUTE) + ":" +
					  date.get(Calendar.SECOND);
		return time;
	}
}

Client:

package org.operamasks.nio;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;

public class TestClient {
	public static void main(String[] args) {
		new MiniClient("localhost", 1982);
	}
}
class MiniClient {
	private SocketChannel sc;
	private ByteBuffer w_bBuf;
	private ByteBuffer r_bBuf = ByteBuffer.allocate(1024);
	public MiniClient(String host, int port) {
		try {
			InetSocketAddress remote = new InetSocketAddress(host, port);
			sc = SocketChannel.open();
			sc.connect(remote);
			if(sc.finishConnect()) {
				System.out.println("已经与服务器成功建立连接...");
			}
			while(true) {
				if(!sc.socket().isConnected()) {
					System.out.println("已经与服务器失去了连接...");
					return ;
				}
				BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
				String str = br.readLine();
				System.out.println("读入一行数据，开始发送...");
				w_bBuf = ByteBuffer.wrap(str.getBytes("UTF-8"));
				//将缓冲区中数据写入通道
				sc.write(w_bBuf);
				System.out.println("数据发送成功...");
				w_bBuf.clear();
				System.out.println("接收服务器端响应消息...");
				try {
					Thread.currentThread();
					Thread.sleep(100);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				r_bBuf.clear();
				//将字节序列从此通道中读入给定的缓冲区r_bBuf
				sc.read(r_bBuf);
				r_bBuf.flip();
				String msg = Charset.forName("UTF-8").decode(r_bBuf).toString();
				System.out.println(msg);				
			}
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

参考文章：

1.使用异步I/O大大提高应用程序的性能

2.Linux AIO（异步IO）那些事儿

3.NodeJS代码阅读笔记之libeio