Synchronized threads for multiplying matrices

java, matrix-multiplication, multithreading, synchronization

Solution

- You are not synchronizing on a ressource, you need to share a lock object (in a static context or by constructor)

- I am not really able to figure out what in your programm should be synchronized when you don't even let them work synchronously.... You start a thread and directly wait for him to stop. I think you have to start them all firstly and then in another loop call the join on every thread.

Also, I am not quite sure what your Threads should work out seperately, I think they all work out the whole product matrix. You need to share a variable used to identify the already processed rows, which you access synchronized.

I could fix your code but I would appreciate that you do that work yourself, since it's a task to understand thread concurrency.

EDIT: Explanation of synchronized: Synchronized takes a object as a lock, which only one thread can hold the monitor for it. When having the monitor for the lock, the thread can process the block, if not, he has to wait to get the monitor. In your case, you could use `private static final Object lock = new Object();` as lock, you will synchronize on.

EDIT 2: I completely constructed your code I'm not that proud to get all of your work done, but doesn't matter, here it is.

package anything.synchronize_stackoverflow_post;

/**
 * @date 21.11.2012
 * @author Thomas Jahoda
 */
public class ConcurrentMatrixMultiplyingTask implements Runnable {

    private int[][] matrixA;
    private int[][] matrixB;
    private int[][] matrixProduct;
    //
    private final ConcurrencyContext context;

    public ConcurrentMatrixMultiplyingTask(ConcurrencyContext context, int[][] A, int[][] B, int[][] C) {
        if (context == null) {
            throw new IllegalArgumentException("context can not be null");
        }
        this.context = context;
        this.matrixA = A;
        this.matrixB = B;
        this.matrixProduct = C;
    }

    @Override
    public void run() {
        while (true) {
            int row;
            synchronized (context) {
                if (context.isFullyProcessed()) {
                    break;
                }
                row = context.nextRowNum();
            }
            System.out.println(Thread.currentThread().getName() + " is going to process row " + row);
            // i'm not really sure if this matrix algorithm here is right, idk..
            for (int j = 0; j < matrixB[0].length; j++) {
                for (int k = 0; k < matrixA[0].length; k++) {
                    matrixProduct[row][j] += matrixA[row][k] * matrixB[k][j];
                }
            }
        }
    }

    public static class ConcurrencyContext {

        private final int rowCount;
        private int nextRow = 0;

        public ConcurrencyContext(int rowCount) {
            this.rowCount = rowCount;
        }

        public synchronized int nextRowNum() {
            if (isFullyProcessed()) {
                throw new IllegalStateException("Already fully processed");
            }
            return nextRow++;
        }

        public synchronized boolean isFullyProcessed() {
            return nextRow == rowCount;
        }
    }
}

And the ProcessingTask

package anything.synchronize_stackoverflow_post;

import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;
import java.util.Scanner;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * @date 21.11.2012
 * @author Thomas Jahoda
 */
public class MatrixMulti {

    public static void main(String[] args) {
        int[][] matrixA;
        int[][] matrixB;
        int colA = 0;
        int rowA = 0;
        int colB = 0;
        int rowB = 0;
        Scanner userInput = new Scanner(System.in);
        System.out.println("Please enter the dimensions of matrix A");

        do {
            System.out.print("column for matrix A: ");
            colA = userInput.nextInt();
            System.out.println();
        } while (!validDimension(colA));

        rowB = colA;

        do {
            System.out.print("row for matrix A: ");
            rowA = userInput.nextInt();
            System.out.println();
        } while (!validDimension(rowA));

        matrixA = new int[rowA][colA];

        System.out.println("Please enter the dimensions of matrix B:");
        do {
            System.out.print("column for matrix B: ");
            colB = userInput.nextInt();
            System.out.println();
        } while (!validDimension(colB));

        matrixB = new int[rowB][colB];


        fillMatrix(matrixA);
        fillMatrix(matrixB);

        System.out.println("Would you like to print out matrix A and B? (y/n)");
        String userResponse = userInput.next();
        if (userResponse.equalsIgnoreCase("y")) {
            System.out.println("Matrix A:");
            printBackMatrix(matrixA);
            System.out.println();
            System.out.println("Matrix B:");
            printBackMatrix(matrixB);
            System.out.println();
        }


        int[][] matrixProduct3 = multMatrixWithThreadsSync(matrixA, matrixB);

        String fileName = "test.txt";
        System.out.println("Matrix product is being written to " + fileName);
        try {
            printMatrixToFile(matrixProduct3, fileName);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    private static int[][] multMatrixWithThreadsSync(int[][] matrixA, int[][] matrixB) {

        int[][] matrixProduct = new int[matrixA.length][matrixB[0].length];
        int[] matrixProductColumn = new int[matrixA.length];
        //
        ConcurrentMatrixMultiplyingTask.ConcurrencyContext context = new ConcurrentMatrixMultiplyingTask.ConcurrencyContext(matrixProduct.length);
        //
        Runnable task = new ConcurrentMatrixMultiplyingTask(context, matrixA, matrixB, matrixProduct);
        Thread[] workers = new Thread[5];
        for (int i = 0; i < workers.length; i++) {
            workers[i] = new Thread(task, "Worker-"+i);
        }
        for (int i = 0; i < workers.length; i++) {
            Thread worker = workers[i];
            worker.start();
        }
        for (int i = 0; i < workers.length; i++) {
            Thread worker = workers[i];
            try {
                worker.join();
            } catch (InterruptedException ex) {
                Logger.getLogger(MatrixMulti.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        return matrixProduct;
    }

    private static void printMatrixToFile(int[][] matrix, String fileName) throws IOException {
        PrintWriter userOutput = new PrintWriter(new FileWriter(fileName));
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[0].length; j++) {
                userOutput.print(matrix[i][j] + " ");
            }
            userOutput.println();
        }
        userOutput.close();

    }

    private static void printBackMatrix(int[][] matrix) {
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[0].length; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }

    private static void fillMatrix(int[][] matrix) {
        Random rand = new Random();

        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[0].length; j++) {
                matrix[i][j] = rand.nextInt(100) + 1;
            }
        }

    }

    public static boolean validDimension(int dim) {
        if (dim <= 0 || dim > 1000) {
            System.err.println("Dimension value entered is not valid");
            return false;
        }
        return true;

    }
}

Problem

Original source

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