java socket參數詳解:TcpNoDelay降低通信延遲

TcpNoDelay=false，爲啓用nagle算法，也是默認值。 Nagle算法的立意是良好的，避免網絡中充塞小封包，提高網絡的利用率。但是當Nagle算法遇到delayed ACK悲劇就發生了。Delayed ACK的本意也是爲了提高TCP性能，跟應答數據捎帶上ACK，同時避免糊塗窗口綜合症，也可以一個ack確認多個段來節省開銷。悲劇發生在這種情況，假設一端發送數據並等待另一端應答，協議上分爲頭部和數據，發送的時候不幸地選擇了write-write，然後再read，也就是先發送頭部，再發送數據，最後等待應答。
實驗模型：
發送端（客戶端）
write(head);
write(body);
read(response);
接收端（服務端）
read(request);  
process(request);  
write(response);
這裏假設head和body都比較小，當默認啓用nagle算法，並且是第一次發送的時候，根據nagle算法，第一個段head可以立即發送，因爲沒有等待確認的段；接收端（服務端）收到head，但是包不完整，繼續等待body達到並延遲ACK；發送端（客戶端）繼續寫入body，這時候nagle算法起作用了，因爲head還沒有被ACK，所以body要延遲發送。這就造成了發送端（客戶端）和接收端（服務端）都在等待對方發送數據的現象：
發送端（客戶端）等待接收端ACK head以便繼續發送body；
接收端（服務端）在等待發送方發送body並延遲ACK，悲劇的無以言語。
這種時候只有等待一端超時併發送數據才能繼續往下走。
代碼：
發送端代碼

package socket.nagle;

import java.io.*;
import java.net.*;
import org.apache.log4j.Logger;

publicclass Client {
privatestatic Logger logger = Logger.getLogger(Client.class);
publicstaticvoid main(String[] args) throws Exception {
// 是否分開寫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();
    }
}

接收端代碼

package socket.nagle;

import java.io.*;
import java.net.*;

import org.apache.log4j.Logger;

publicclass Server {
privatestatic Logger logger = Logger.getLogger(Server.class);

publicstaticvoid 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;
                }
            }
        }
    }
}

實驗結果：

[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 -

實驗1：當WriteSplit=true and TcpNoDelay=false 啓用nagle算法

[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，每次請求到應答的時間間隔都在40ms，除了第一次。linux的delayed ack是40ms，而不是原來以爲的200ms。第一次立即ACK，似乎跟linux的quickack mode有關，這裏我不是特別清楚，
其實問題不是出在nagle算法身上的，問題是出在write-write-read這種應用編程上。禁用nagle算法可以暫時解決問題，但是禁用 nagle算法也帶來很大壞處，網絡中充塞着小封包，網絡的利用率上不去，在極端情況下，大量小封包導致網絡擁塞甚至崩潰。在這種情況下，其實你只要避免write-write-read形式的調用就可以避免延遲現象，如下面這種情況發送的數據不要再分割成兩部分。
實驗2：當WriteSplit=false and TcpNoDelay=false 啓用nagle算法

[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

實驗3：當WriteSplit=true and TcpNoDelay=true 禁用nagle算法

[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

實驗4：當WriteSplit=false and TcpNoDelay=true 禁用nagle算法

[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

實驗2到4，都沒有出現延時的情況。
注意：以上實驗在windows上測試下面的代碼，客戶端和服務器必須分在兩臺機器上，似乎winsock對loopback連接的處理不一樣。下面的我的做法是：服務端與客戶端都在一臺Linux機上。

轉載自：http://blog.csdn.net/huang_xw/article/details/7340241