我有一个学校项目,在这里我们应该使用并行编程来使某些算法更快地工作。例如,我选择了“图像阈值”。
因此,我创建了一个Java程序,该程序正常执行(加载图像,循环遍历所有像素,计数阈值,再次遍历所有像素并设置黑色或白色(如果大于或小于阈值)。
这需要我在笔记本上花费约5秒,大约4000x3000的图片,花费49秒,使用图像11500x11500。
然后,我创建了另一个程序,该程序应该使用线程,以使它们的循环更快地完成。
现在,我创建4个线程,每个线程处理图像的1/4。首先,他们将阈值vlaues添加到同步的arraylist中,并且在所有这些完成之后,我计算阈值。然后,我再创建4个线程,它们分别处理图像的1/4,并将黑色或白色设置为图片。
这用4000x3000的图像花了我12秒的时间,并抛出了java.lang.OutOfMemoryError:Java堆空间(在所有线程中)都使用了11500x11500。
public class PprPrahovaniParalelne{
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException {
final Threshold image = new Threshold(nactiObrazek("ryba.jpg"));
final int width = image.getImage().getWidth();
final int height = image.getImage().getHeight();
Thread t1 = new Thread(){
int threshold;
public void run(){
System.out.println("Thread 1 - Started");
for(int y = 0; y < height/4;y++){
for(int x = 0; x < width;x++){
Color color = new Color(image.getImage().getRGB(x,y));
threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
image.addThreshold(threshold);
}
}
System.out.println("Thread 1 - finished");
}
};
Thread t2 = new Thread(){
int threshold;
@Override
public void run(){
for(int y = height/4; y < height/4*2;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
image.addThreshold(threshold);
}
}
System.out.println("Thread 2 - finished");
}
};
Thread t3 = new Thread(){
int threshold;
@Override
public void run(){
for(int y = height/4*2; y < height/4*3;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
image.addThreshold(threshold);
}
}
System.out.println("Thread 3 - finished");
}
};
Thread t4 = new Thread(){
int threshold;
@Override
public void run(){
for(int y = height/4*3; y < height;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
image.addThreshold(threshold);
}
}
System.out.println("Thread 4 - finished");
}
};
t1.start();
t2.start();
t4.start();
t3.start();
try{
t1.join();
t2.join();
t3.join();
t4.join();
}catch(InterruptedException e){
e.printStackTrace();
}
image.countThreshold();
System.out.println("Threshold je: " + image.getThreshold());
Thread t5 = new Thread(){
Color cerna = new Color(255,255,255);
Color bila = new Color(0,0,0);
int threshold;
@Override
public void run(){
for(int y = 0; y < height/4;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
if(threshold > image.getThreshold()){
image.getImage().setRGB(x, y, cerna.getRGB());
}else{
image.getImage().setRGB(x, y, bila.getRGB());
}
}
}
System.out.println("Thread 5 - finished");
}
};
Thread t6 = new Thread(){
Color cerna = new Color(255,255,255);
Color bila = new Color(0,0,0);
int threshold;
@Override
public void run(){
for(int y = height/4; y < height/4*2;y++){
for(int x = 0; x < width;x++){
Color color = new Color(image.getImage().getRGB(x,y));
threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
if(threshold > image.getThreshold()){
image.getImage().setRGB(x, y, cerna.getRGB());
}else{
image.getImage().setRGB(x, y, bila.getRGB());
}
}
}
System.out.println("Thread 6 - finished");
}
};
Thread t7 = new Thread(){
Color cerna = new Color(255,255,255);
Color bila = new Color(0,0,0);
int threshold;
@Override
public void run(){
for(int y = height/4*2; y < height/4*3;y++){
for(int x = 0; x < width;x++){
Color color = new Color(image.getImage().getRGB(x,y));
threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
if(threshold > image.getThreshold()){
image.getImage().setRGB(x, y, cerna.getRGB());
}else{
image.getImage().setRGB(x, y, bila.getRGB());
}
}
}
System.out.println("Thread 7 - finished");
}
};
Thread t8 = new Thread(){
Color cerna = new Color(255,255,255);
Color bila = new Color(0,0,0);
int threshold;
@Override
public void run(){
for(int y = height/4*3; y < height;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
if(threshold > image.getThreshold()){
image.getImage().setRGB(x, y, cerna.getRGB());
}else{
image.getImage().setRGB(x, y, bila.getRGB());
}
}
}
System.out.println("Thread 8 - finished");
}
};
t5.start();
t6.start();
t7.start();
t8.start();
try{
t5.join();
t6.join();
t7.join();
t8.join();
}catch(InterruptedException e){
e.printStackTrace();
}
File hotovo = new File("ryba_prahovanej.jpg");
ImageIO.write(image.getImage(), "jpg", hotovo);
}
public static BufferedImage nactiObrazek(String nazev){
BufferedImage img = null;
try {
img = ImageIO.read(new File(nazev));
} catch (IOException e) {
}
return img;
}
}
和阈值类:
public class Threshold {
private BufferedImage image;
final private List<Integer> list;
private int threshold;
public int getThreshold() {
return threshold;
}
public List<Integer> getList(){
return list;
}
public Threshold(BufferedImage obrazek) {
this.list = Collections.synchronizedList(new ArrayList<Integer>());
this.image = obrazek;
}
public void setObrazek(BufferedImage obrazek){
this.image = obrazek;
}
public BufferedImage getImage(){
return this.image;
}
public void addThreshold(int threshold){
list.add(threshold);
}
public void countThreshold(){
long sum = 0;
for (Iterator<Integer> it = list.iterator(); it.hasNext();) {
int item = it.next();
sum += item;
}
this.threshold = (int) (sum/list.size());
}
}
那么,为什么在多线程时速度较慢?除了列表之外,我这里没有同步任何东西,因为线程不应该在像素数组中使用相同的索引。
探查器图片在这里:
序列号:
并列:
最佳答案
在这种并行化情况下,需要考虑几件事。
您的代码仅执行图像的异步处理(计算阈值和创建阈值图像),但是在所有线程完成处理之前,将阻止图像IO(写入)。
其他更重要的因素是您如何提出4线程解决方案。选择4个线程是理想数量的线程的背后原因是什么?在像您这样的CPU和内存密集型多线程程序中,理想的线程数= Number of CPUs + 1
。拥有更多的线程并不能使您的程序执行得更快,实际上会降低性能。
图像处理当然会占用大量内存,因此在运行带有大图像的程序时需要增加堆空间。
请考虑上述内容。
编辑
您可以通过使代码更具可读性和减少代码重复来开始。您可以利用CyclicBarrier来实现并行任务的顺序执行。
import java.awt.image.*;
import java.io.*;
import java.awt.*;
import javax.imageio.*;
import java.util.concurrent.*;
public class PprPrahovaniParalelne {
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException {
final Threshold image = new Threshold(nactiObrazek("DSC03691.jpg"));
final int width = image.getImage().getWidth();
final int height = image.getImage().getHeight();
final int nCpu = Runtime.getRuntime().availableProcessors() + 1;
ExecutorService threadPool = Executors.newFixedThreadPool(nCpu);
System.out.println("Number of CPUs : "+nCpu);
CyclicBarrier cyclicBarrier = new CyclicBarrier(4, new Runnable() {
private int count = 1;
public void run() {
if(count == 1) {
image.countThreshold();
System.out.println("Threshold je: " + image.getThreshold());
} else {
try {
File hotovo = new File("ryba_prahovanej.jpg");
ImageIO.write(image.getImage(), "jpg", hotovo);
} catch(IOException e) {
System.err.println("Error while writing : " + e);
}
threadPool.shutdownNow();
}
count++;
}
});
threadPool.submit(new ImageProcessingTask(0, height/4, width, image, cyclicBarrier));
threadPool.submit(new ImageProcessingTask(height/4, height/4*2, width, image, cyclicBarrier));
threadPool.submit(new ImageProcessingTask(height/4*2, height/4*3, width, image, cyclicBarrier));
threadPool.submit(new ImageProcessingTask(height/4*3, height, width, image, cyclicBarrier));
}
public static BufferedImage nactiObrazek(String nazev){
BufferedImage img = null;
try {
img = ImageIO.read(new File(nazev));
} catch (IOException e) {
}
return img;
}
}
class ImageProcessingTask implements Runnable {
private int start;
private int height;
private int width;
private Threshold image;
private CyclicBarrier barrier;
public ImageProcessingTask(int start, int height, int width, Threshold image, CyclicBarrier barrier) {
this.start = start;
this.height = height;
this.width = width;
this.image = image;
this.barrier = barrier;
}
public void run(){
int threshold;
System.out.println(Thread.currentThread().getName()+" - Started");
for(int y = start; y < height;y++){
for(int x = 0; x < width;x++){
Color color = new Color(image.getImage().getRGB(x,y));
threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
image.addThreshold(threshold);
}
}
try {
int count = barrier.await();
if(count == 0) {
barrier.reset();
System.out.println("Resetting Cyclic Barrier");
}
} catch(InterruptedException e) {
Thread.currentThread().interrupt();
} catch(Exception e) {
e.printStackTrace();
}
Color cerna = new Color(255,255,255);
Color bila = new Color(0,0,0);
for(int y = start; y < height;y++){
for(int x = 0; x < width;x++){
Color barva = new Color(image.getImage().getRGB(x,y));
threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
if(threshold > image.getThreshold()){
image.getImage().setRGB(x, y, cerna.getRGB());
}else{
image.getImage().setRGB(x, y, bila.getRGB());
}
}
}
try {
barrier.await();
} catch(InterruptedException e) {
Thread.currentThread().interrupt();
} catch(Exception e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" - finished");
}
}