java socket参数详解:TcpNoDelay

原文链接：http://blog.csdn.net/huang_xw/article/details/7340241

TcpNoDelay=false，为启用nagle算法，也是默认值。 Nagle算法的立意是良好的，避免网络中充塞小封包，提高网络的利用率。但是当Nagle算法遇到delayed ACK悲剧就发生了。Delayed ACK的本意也是为了提高TCP性能，跟应答数据捎带上ACK，同时避免糊涂窗口综合症，也可以一个ack确认多个段来节省开销。悲剧发生在这种情况，假设一端发送数据并等待另一端应答，协议上分为头部和数据，发送的时候不幸地选择了write-write，然后再read，也就是先发送头部，再发送数据，最后等待应答。

实验模型：

发送端（客户端）

write(head);

write(body);

read(response);

接收端（服务端）

read(request);

process(request);

write(response);

这里假设head和body都比较小，当默认启用nagle算法，并且是第一次发送的时候，根据nagle算法，第一个段head可以立即发送，因为没有等待确认的段；接收端（服务端）收到head，但是包不完整，继续等待body达到并延迟ACK；发送端（客户端）继续写入body，这时候nagle算法起作用了，因为head还没有被ACK，所以body要延迟发送。这就造成了发送端（客户端）和接收端（服务端）都在等待对方发送数据的现象：

发送端（客户端）等待接收端ACK head以便继续发送body；

接收端（服务端）在等待发送方发送body并延迟ACK，悲剧的无以言语。

这种时候只有等待一端超时并发送数据才能继续往下走。

代码：

发送端代码

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package socket.nagle;



import java.io.*;

import java.net.*;

import org.apache.log4j.Logger;



public class Client {

private static Logger logger = Logger.getLogger(Client.class);

public static void main(String[] args) throws Exception {

// 是否分开写head和body

boolean writeSplit = true;

String host = "localhost";

logger.debug("WriteSplit:" + writeSplit);



Socket socket = new Socket();

socket.setTcpNoDelay(false);

socket.connect(new InetSocketAddress(host, 10000));



InputStream in = socket.getInputStream();

OutputStream out = socket.getOutputStream();

BufferedReader reader = new BufferedReader(new InputStreamReader(in));



String head = "hello ";

String body = "world\r\n";

for (int i = 0; i < 10; i++) {

long label = System.currentTimeMillis();

if (writeSplit) {

out.write(head.getBytes());

out.write(body.getBytes());

} else {

out.write((head + body).getBytes());

}

String line = reader.readLine();

logger.debug("RTT:" + (System.currentTimeMillis() - label) + ", receive: " + line);

}

in.close();

out.close();

socket.close();

}

}

接收端代码

[java] view
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package socket.nagle;



import java.io.*;

import java.net.*;



import org.apache.log4j.Logger;



public class Server {

private static Logger logger = Logger.getLogger(Server.class);



public static void main(String[] args) throws Exception {

ServerSocket serverSocket = new ServerSocket();

serverSocket.bind(new InetSocketAddress(10000));

logger.debug(serverSocket);

logger.debug("Server startup at 10000");

while (true) {

Socket socket = serverSocket.accept();

InputStream in = socket.getInputStream();

OutputStream out = socket.getOutputStream();



while (true) {

try {

BufferedReader reader = new BufferedReader(new InputStreamReader(in));

String line = reader.readLine();

logger.debug(line);

out.write((line + "\r\n").getBytes());

} catch (Exception e) {

break;

}

}

}

}

}

实验结果：

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[test5@cent4 ~]$ java socket.nagle.Server

1 [main] DEBUG socket.nagle.Server - ServerSocket[addr=0.0.0.0/0.0.0.0,port=0,localport=10000]

6 [main] DEBUG socket.nagle.Server - Server startup at 10000

4012 [main] DEBUG socket.nagle.Server - hello world

4062 [main] DEBUG socket.nagle.Server - hello world

4105 [main] DEBUG socket.nagle.Server - hello world

4146 [main] DEBUG socket.nagle.Server - hello world

4187 [main] DEBUG socket.nagle.Server - hello world

4228 [main] DEBUG socket.nagle.Server - hello world

4269 [main] DEBUG socket.nagle.Server - hello world

4310 [main] DEBUG socket.nagle.Server - hello world

4350 [main] DEBUG socket.nagle.Server - hello world

4390 [main] DEBUG socket.nagle.Server - hello world

4392 [main] DEBUG socket.nagle.Server -

4392 [main] DEBUG socket.nagle.Server -

实验1：当WriteSplit=true and TcpNoDelay=false 启用nagle算法

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[test5@cent4 ~]$ java socket.nagle.Client

0 [main] DEBUG socket.nagle.Client - WriteSplit:true

52 [main] DEBUG socket.nagle.Client - RTT:12, receive: hello world

95 [main] DEBUG socket.nagle.Client - RTT:42, receive: hello world

137 [main] DEBUG socket.nagle.Client - RTT:41, receive: hello world

178 [main] DEBUG socket.nagle.Client - RTT:41, receive: hello world

218 [main] DEBUG socket.nagle.Client - RTT:40, receive: hello world

259 [main] DEBUG socket.nagle.Client - RTT:40, receive: hello world

300 [main] DEBUG socket.nagle.Client - RTT:41, receive: hello world

341 [main] DEBUG socket.nagle.Client - RTT:41, receive: hello world

382 [main] DEBUG socket.nagle.Client - RTT:41, receive: hello world

422 [main] DEBUG socket.nagle.Client - RTT:40, receive: hello world

可以看到，每次请求到应答的时间间隔都在40ms，除了第一次。linux的delayed ack是40ms，而不是原来以为的200ms。第一次立即ACK，似乎跟linux的quickack mode有关，这里我不是特别清楚，

其实问题不是出在nagle算法身上的，问题是出在write-write-read这种应用编程上。禁用nagle算法可以暂时解决问题，但是禁用 nagle算法也带来很大坏处，网络中充塞着小封包，网络的利用率上不去，在极端情况下，大量小封包导致网络拥塞甚至崩溃。在这种情况下，其实你只要避免write-write-read形式的调用就可以避免延迟现象，如下面这种情况发送的数据不要再分割成两部分。

实验2：当WriteSplit=false and TcpNoDelay=false 启用nagle算法

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[test5@cent4 ~]$ java socket.nagle.Client

0 [main] DEBUG socket.nagle.Client - WriteSplit:false

27 [main] DEBUG socket.nagle.Client - RTT:4, receive: hello world

31 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

34 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

38 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

42 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

44 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

47 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

50 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

53 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

54 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

实验3：当WriteSplit=true and TcpNoDelay=true 禁用nagle算法

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[test5@cent4 ~]$ java socket.nagle.Client

0 [main] DEBUG socket.nagle.Client - WriteSplit:true

25 [main] DEBUG socket.nagle.Client - RTT:6, receive: hello world

28 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

31 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

33 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

35 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

41 [main] DEBUG socket.nagle.Client - RTT:6, receive: hello world

49 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

52 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

56 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

59 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

实验4：当WriteSplit=false and TcpNoDelay=true 禁用nagle算法

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[test5@cent4 ~]$ java socket.nagle.Client

0 [main] DEBUG socket.nagle.Client - WriteSplit:false

21 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

23 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

27 [main] DEBUG socket.nagle.Client - RTT:3, receive: hello world

30 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

32 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

35 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

38 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

41 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

43 [main] DEBUG socket.nagle.Client - RTT:1, receive: hello world

46 [main] DEBUG socket.nagle.Client - RTT:2, receive: hello world

实验2到4，都没有出现延时的情况。

注意：以上实验在windows上测试下面的代码，客户端和服务器必须分在两台机器上，似乎winsock对loopback连接的处理不一样。下面的我的做法是：服务端与客户端都在一台Linux机上。