Databases
StackOverflow2013
Postsdbo
POJava ForkJoin Multi-threaded is slower than single thread
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POJava ForkJoin Multi-threaded is slower than single thread
    primarykey
    data
    text
    <p>I was trying out the Java ForkJoin framework and wrote a simple test program that sets the pixels of an image to random colors. E.g. it generates pseudo-noise. </p> <p>But while testing performance I found that it's actually faster to run single threaded than to run it with multiple threads. I make it run single threaded by passing a high threshold. </p> <p>This is the class worker class:</p> <pre><code>public class Noise extends RecursiveAction { private BufferedImage image; private int xMin; private int yMin; private int xMax; private int yMax; private int threshold = 2000000; // max pixels per thread public Noise(BufferedImage image, int xMin, int yMin, int xMax, int yMax, int threshold) { this.image = image; this.xMin = xMin; this.yMin = yMin; this.xMax = xMax; this.yMax = yMax; this.threshold = threshold; } public Noise(BufferedImage image, int xMin, int yMin, int xMax, int yMax) { this.image = image; this.xMin = xMin; this.yMin = yMin; this.xMax = xMax; this.yMax = yMax; } @Override protected void compute() { int ppt = (xMax - xMin) * (yMax - yMin); // pixels pet thread if(ppt &gt; threshold) { // split int verdeling = ((xMax - xMin) / 2) + xMin; invokeAll(new Noise(image, xMin, yMin, verdeling, yMax), new Noise(image, verdeling+1, yMin, xMax, yMax)); } else { // execute! computeDirectly(xMin, yMin, xMax, yMax); } } private void computeDirectly(int xMin, int yMin, int xMax, int yMax) { Random generator = new Random(); for (int x = xMin; x &lt; xMax; x++) { for (int y = yMin; y &lt; yMax; y++) { //image.setPaint(new Color(generator.nextInt())); int rgb = generator.nextInt(); int red = (rgb &gt;&gt; 16) &amp; 0xFF; int green = (rgb &gt;&gt; 8) &amp; 0xFF; int blue = rgb &amp; 0xFF; red = (int) Math.round((Math.log(255L) / Math.log((double) red)) * 255); green = (int) Math.round((Math.log(255L) / Math.log((double) green)) * 255); blue = (int) Math.round((Math.log(255L) / Math.log((double) blue)) * 255); int rgbSat = red; rgbSat = (rgbSat &lt;&lt; 8) + green; rgbSat = (rgbSat &lt;&lt; 8) + blue; image.setRGB(x, y, rgbSat); } } Graphics2D g2D = image.createGraphics(); g2D.setPaint(Color.RED); g2D.drawRect(xMin, yMin, xMax-xMin, yMax-yMin); } } </code></pre> <p>When generating a 6000 * 6000 image the results are:<br> Single thread: 9.4sec @ 25% CPU load<br> Multi thread: 16.5sec @ 80%-90% CPU load<br> (Core2quad Q9450)</p> <p>Why is the multi-threaded version slower?<br> How do I fix this?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USPaul
    1. USPaul
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POJava ForkJoin Multi-threaded is slower than single thread
      1. This table or related slice is empty.
      1. VTUpMod
    1. COhave you tried splitting by y instead of x? I think BufferedImage is likely to be stored row-major in memory, and this might make a difference.
      singulars
      1. POJava ForkJoin Multi-threaded is slower than single thread
      1. USyiding
    2. COA bigger problem could be that BufferedImage and Graphics objects are not thread safe. You might want to generate the noise in arrays, and the creating a BufferedImage at the end directly from the data (which will also be faster than using setRGB).
      singulars
      1. POJava ForkJoin Multi-threaded is slower than single thread
      1. USyiding
    3. COAs to your first comment: I've switched to splitting by Y, but it makes no difference.
      singulars
      1. POJava ForkJoin Multi-threaded is slower than single thread
      1. USPaul
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload