数据报套接字使UDP程序中的其他线程处于饥饿状态

Question

作为课程作业的一部分，我们的任务是在java提供的UDP层之上添加额外的可靠性层，以便发送大型图片文件。这将使用Go-Back-N协议来完成：http://en.wikipedia.org/wiki/Go_back_N

据我所知，这个问题的症结在于能够发送数据包，同时检查是否有任何确认进入旧的数据包，这将允许您移动您的窗口。

我目前通过两个线程来做到这一点:一个线程在窗口中有空间时发送下一个数据包；另一个线程不断地侦听任何传入的确认并做出适当的反应。

我的问题是，程序应该被线程化，就好像这两个线程同时运行一样，但实际上，似乎ACKReceiver线程获得了非常不成比例的时间。从线程转储来看，当发送线程到达DataSocket.receive()行时，它似乎会让发送线程“饥饿”一段时间，阻塞执行，同时不给另一个线程运行的机会。

我看了以下问题，似乎暗示问题与DatagramSocket.receive是synchronized...but没有提供可用的解决方案的事实有关：

Java Thread won't pause on I/O operation

这是我代码的发送者部分的代码，我相对确定另一端的接收者是完全正常的(首先，我不需要使用任何线程来让它工作！)：

import java.io.*;
import java.net.*;
import java.nio.ByteBuffer;

public class Sender3 {
    short base = 0;
    short nextSeqNum = 0;
    DatagramPacket[] packets;
    ByteBuffer bb;
    String endSys;
    int portNum;
    String fileName;
    int retryTime;
    int windowSize;
    DatagramSocket clientSocket;
    InetAddress IPAddress;
    boolean timedOut = false;

    public Sender3(String endSys, int portNum, String fileName, int retryTime, int windowSize){
        this.endSys = endSys;
        this.portNum = portNum;
        this.fileName = fileName;
        this.retryTime = retryTime;
        this.windowSize = windowSize;
    }

    public static void main(String args[]) throws Exception{
        //Check for current arguments and assign them
        if(args.length != 5){
            System.out.println("Invalid number of arguments. Please specify: <endSystem> <portNumber> <fileName> <retryTimeout><windowSize>");
            System.exit(1);
        }

        Sender3 sendy = new Sender3(args[0], Integer.parseInt(args[1]), args[2], Integer.parseInt(args[3]), Integer.parseInt(args[4]));

        sendy.go();
    }

    private void go() throws Exception{

        clientSocket = new DatagramSocket();



        bb = ByteBuffer.allocate(2);
        byte[] picData = new byte[1021];
        byte[] sendData = new byte[1024];

        Thread.yield()
        short seqNum = 0; 
        byte[] seqBytes = new byte[2];
        byte EOFFlag = 0;
        boolean acknowledged = false;
        int lastPacketRetrys = 0;
        int resends = 0;
        IPAddress = InetAddress.getByName(endSys);

        FileInputStream imReader = new FileInputStream(new File(fileName));
        double fileSizeKb = imReader.available() / 1021.0; //We add 3 bytes to every packet, so dividing by 1021 will give us total kb sent. 
        int packetsNeeded = (int) Math.ceil(fileSizeKb);
        packets = new DatagramPacket[packetsNeeded];
        long startTime = System.currentTimeMillis();
        long endTime;
        double throughput;

        //Create array of packets to send
        for(int i = 0; i < packets.length; i++){
            if(i == packets.length - 1){
                EOFFlag = 1;
                picData = new byte[imReader.available()];
                sendData = new byte[picData.length + 3];
            }
            imReader.read(picData);
            bb.putShort((short)i);
            bb.flip();
            seqBytes = bb.array();
            bb.clear();
            System.arraycopy(seqBytes, 0, sendData, 0, seqBytes.length);
            sendData[2] = EOFFlag;
            System.arraycopy(picData, 0, sendData, 3, picData.length);
            packets[i] = new DatagramPacket((byte[])sendData.clone(), sendData.length, IPAddress, portNum);
        }

        ACKGetter ackGet = new ACKGetter();
        Thread ackThread = new Thread(ackGet);
        ackThread.start();

        //System.out.println("timeout is: " + timedOut + " base is: " + base + " packet length is: " + packets.length + " nextSeqNum: " + nextSeqNum);

        while(base != packets.length){
            if(timedOut){
                //System.out.println("Timed out waiting for acknowledgement, resending all unACKed packets in window");
                clientSocket.setSoTimeout(retryTime);
                resends++;
                if(nextSeqNum == packets.length)
                    lastPacketRetrys++;
                //Resend all packets in window
                for (int i = base; i < nextSeqNum; i++){
                //  System.out.println("Resending packets with number: " + i);
                    clientSocket.send(packets[i]);
                }
                timedOut = false;
            }

            if(nextSeqNum - base < windowSize && nextSeqNum < packets.length){
                //System.out.println("sending packet with seqNum: " + nextSeqNum);
                clientSocket.send(packets[nextSeqNum]);
                if(base == nextSeqNum){
                    clientSocket.setSoTimeout(retryTime); 
                }
                nextSeqNum++;
            }
            else{
                //Thread.yield();
            }

        }




        if(lastPacketRetrys > 10){
            System.out.println("Last packet ACK was lost (we think). So we just gave up, number of retransmissions will probably be higher");
        }
        endTime = System.currentTimeMillis();
        throughput = 1000 * fileSizeKb / (endTime - startTime);
        clientSocket.close();
        imReader.close();
        System.out.println("Number of retransmissions: " + resends);
        System.out.println("Average throughput is: " + throughput + "Kb/s");

    }


    private class ACKGetter implements Runnable {
        //Listen out for ACKs and update pointers accordingly
        DatagramPacket ackPacket;
        byte[] ackData = new byte[2];
        public void run() {
            while(base != packets.length){
                if(base != nextSeqNum){
                    try{
                        ackPacket = new DatagramPacket(ackData, ackData.length);
                        clientSocket.receive(ackPacket);
                        ackData = ackPacket.getData();
                        bb.put(ackData[0]);
                        bb.put(ackData[1]);
                        bb.flip();
                        short ack = bb.getShort();
                        bb.clear();
                        if(base <= ack){
                            //System.out.println("acknowledgement for base num: " + base + "ack num:" + ack);
                            base = (short) (ack + 1);
                            //If theres nothing left in window, stop timing, otherwise restart the timer
                            if(base == nextSeqNum){
                                clientSocket.setSoTimeout(0);
                            }
                            else{
                                clientSocket.setSoTimeout(retryTime);
                            }
                        }
                        else{
                            //System.out.println("ACK didnt change anything: " + ack);
                        }
                    }
                    catch(Exception ex){
                        timedOut = true;
                        //System.out.println("Packet timed out...resending..");
                    }
                }

                Thread.yield();


            }
        }
    }
}

Answer 1

我认为您在这里遇到了死锁，因为当发送方调用clientSocket.setSoTimeout()时，读取器线程在clientSocket.receive()中。请参阅以下DatagramSocket方法定义：

public synchronized void setSoTimeout(int timeout) throws SocketException {
...
public synchronized void receive(DatagramPacket p) throws IOException {

如果接收时套接字超时为0，则receive将挂起等待数据包。如果您发出SIGQUIT，您的JVM将转储线程并显示死锁，您可以跟踪堆栈帧以查看发送方和接收方被卡住的位置。

要解决这个问题，您应该停止更改setSoTimeout值，这在我看来是非常糟糕的做法。我将切换到使用DatagramChannel，使套接字成为非阻塞的，并使用NIO接收来执行读取。有关如何使用通道Selector的详细信息，请参阅NIO docs。