java socket keep alive is slow, reopening a socket is faster

Question

I am trying to come up with a java implementation of a simple HTTP client that keeps a socket open and reuses it to query other (or same) URLs on the same host.

I have a simple implementation that uses java.net.Socket but somehow the performance when I keep the socket open is worse than when I keep creating a new one.

Results first, full executable code below:

With KeepAlive: slower starting at iteration #2

> java -server -Xms100M -Xmx100M -cp . KeepAlive 10 true
--- Warm up ---
18
61
60
60
78
62
59
60
59
60
Total exec time: 626
--- Run ---
26
59
60
61
60
59
60
60
62
58
Total exec time: 576

Recreating the socket every time gives better results:

> java -server -Xms100M -Xmx100M -cp . KeepAlive 10 false
--- Warm up ---
188
34
39
33
33
33
33
33
34
33
Total exec time: 494
--- Run ---
33
35
33
34
44
34
33
34
32
34
Total exec time: 346

KeepAlive.java (standalone, no dependencies)

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.InputStreamReader;
import java.net.InetSocketAddress;
import java.net.Socket;

public class KeepAlive {

    private static final String NL = "\r\n";
    private static final int READ_SIZE = 1000;
    private Socket socket;
    private DataOutputStream writer;
    private BufferedReader reader;

    public static void main(String[] args) throws Exception {
        if (args.length == 2) {
            KeepAlive ka = new KeepAlive();
            System.out.println("--- Warm up ---");
            ka.query(Integer.parseInt(args[0]), args[1].equals("true"));
            System.out.println("--- Run ---");
            ka.query(Integer.parseInt(args[0]), args[1].equals("true"));
        } else {
            System.out.println("Usage: keepAlive <n queries> <reuse socket>");
        }
    }

    private void query(int n, boolean reuseConnection) throws Exception {
        long t0 = System.currentTimeMillis();
        if (reuseConnection) {
            open();
            for (int i = 0; i < n; i++) {
                long tq0 = System.currentTimeMillis();
                query();
                System.out.println(System.currentTimeMillis() - tq0);
            }
            close();
        } else {
            for (int i = 0; i < n; i++) {
                long tq0 = System.currentTimeMillis();
                open();
                query();
                close();
                System.out.println(System.currentTimeMillis() - tq0);
            }
        }
        System.out.println("Total exec time: " + (System.currentTimeMillis() - t0));
    }

    private void open() throws Exception {
        socket = new Socket();
        socket.setKeepAlive(false);
        socket.connect(new InetSocketAddress("example.org", 80));
        writer = new DataOutputStream(socket.getOutputStream());
        reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    }

    private void query() throws Exception {
        StringBuilder req = new StringBuilder();
        req.append("GET / HTTP/1.1").append(NL);
        req.append("Host: example.org").append(NL);
        req.append("Connection: Keep-Alive").append(NL);
        req.append(NL);
        String reqStr = req.toString();

        long t0 = System.currentTimeMillis();
        writer.writeBytes(reqStr);
        writer.flush();

        String line;
        int contentLength = 0;
        while ((line = reader.readLine()) != null) {
            if (line.startsWith("Content-Length: ")) {
                contentLength = Integer.parseInt(line.substring(16));
            }
            if (line.equals("")) {
                char[] buf = new char[contentLength];
                int offset = 0;
                while (offset < contentLength) {
                  int len = contentLength - offset;
                  if (len > READ_SIZE) {
                    len = READ_SIZE;
                  }
                  int ret = reader.read(buf, offset, len);
                  if (ret == -1) {
                    System.out.println("End of stream. Exiting");
                    System.exit(1);
                  }
                  offset += ret;
                }

                break;
            }
        }
    }

    private void close() throws Exception {
        writer.close();
        reader.close();
        socket.close();
    }
}

Now, I'm pretty sure that either:

1. the web server sucks at handling the new requests fast (HTTP Keep Alive and TCP keep alive)

2. something is wrong with the way I use the buffered reader because that's where all the time is lost but looking at the other methods available (and I tried a few), I can't find what I need to do to fix this...

Any idea how I could make this work faster? Maybe a config to change on the server itself?...

---

Solution

As explained by apangin below, the slower perf is caused by Nagle's algorithm, which is enabled by default. Using setTcpNoDelay(true), I get the updated following perfs:

Without keep-alive:

java -server -Xms100M -Xmx100M -cp . KeepAlive 10 false
--- Warm up ---
49
22
25
23
23
22
23
23
28
28
Total exec time: 267
--- Run ---
31
23
23
24
25
22
23
25
33
23
Total exec time: 252

With keep-alive:

java -server -Xms100M -Xmx100M -cp . KeepAlive 10 true
--- Warm up ---
13
12
12
14
11
12
13
12
11
12
Total exec time: 168
--- Run ---
14
12
11
12
11
12
13
11
21
28
Total exec time: 158

So here, we can see the keep-alive version performing far better than the non keep-alive one for each iteration and also if comparing total execution times. :)

Answer 1

That's the effect of Nagle's algorithm. It delays sending TCP packets in anticipation of more outgoing data.

Nagle's algorithm interacts badly with TCP delayed acknowledgment in write-write-read scenarios. This is exactly your case, because writer.writeBytes(reqStr) sends a string byte-by-byte.

Now you have two options to fix the behavior:

1. use socket.setTcpNoDelay(true) to disable Nagle's algorithm;
2. send the complete request in one operation: writer.write(reqStr.getBytes());

In both cases the reused connection will expectedly work faster.

Answer 2

reader.read(buf);

Your test is invalid. You aren't necessarily reading the entire response. You need to change this to a loop that counts the data returned. And if you don't read the entire response, you will be getting out of sync in the keep-alive case.