X11 Pipeline: Extreme Thrashing

Question

I have a graphics intensive application that I am working on, and I am getting severe thrashing while running in X. Results are excellent from an e-machine running Windows Vista.
Here's the basic idea of the program. It uses AWT (no Swing is ever referenced). There is a collection of "sprites" which were created from GraphicsDevice.createCompatibleImage() to give them the best chance at acceleration. Then there is a backbuffer, to which the program draws. It attempts to deliver higher framerates by only updating portions of the screen which are changing. Originally, this backbuffer was also created with GraphicsDevice.createCompatibleImage(), however this leads to the thrashing I mentioned earlier.
After reading through:
[Troubleshooting Guide for Java SE 6 Desktop Technologies|http://java.sun.com/javase/6/webnotes/trouble/TSG-Desktop/html/toc.html]
I managed to isolate the problem to X11 offscreen pixmaps, and running with the flag -Dsun.java2d.pmoffscreen=false removes the thrashing.
The problem I have here is that I am now confused. It seems to me that I want my sprites accelerated, but shouldn't my backbuffer also be accelerated? I'm not using antialiasing, nor alpha compositing, nor translations. I do make heavy use of setting the clip on the graphics object. Is that where my problem resides? Am I confused on the meaning of alpha compositing... does that include 1-bit transparency?
Quote from the above referenced document:
"The use of pixmaps typically results in better performance. However, in certain cases, the opposite is true. Such cases typically involve the use of operations which cannot be performed using the X protocol, such as antialiasing, alpha compositing, and transforms that are more complex than simple translation transforms.
For these operations the X11 pipeline must do the rendering using the built-in software renderer. In most cases this includes reading the contents of the pixmap to a system memory (over the network in the case of remote X server), performing the rendering, and then sending the pixels back to the pixmap. Such operations could result in extremely poor performance, especially if the X server is remote."
Could someone enlighten me here? Why are my accelerated surfaces being copied back into system memory when all I am doing is drawing accelerated surfaces onto accelerated surfaces using drawImage() and setClip().

Answer 1

You're totally right. I was preparing a code example, during which I discovered my mistake.
I'm still curious as to why Swing is causing a problem with off screen pixmaps in X. I've seen this problem before but I was never concerned with rapid updating of the screen so I never investigated. I think it must be Swing's backbuffer causing it, but is this a problem with the Java2D X11 pipeline, or maybe a bug somewhere else in my OS?
Here is a code example which produces the problem, running it with sun.java2d.pmoffscreen=false runs fine, but without it, X begins hogging CPU time and starves Java out resulting in pauses in execution.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
class SwingFail implements ActionListener {
  private static JFrame frame;
  class RefreshPanel extends JPanel {
    public RefreshPanel() {
      setPreferredSize(new Dimension(512, 512));
    public void paintComponent(Graphics g) {
      g.setColor(Color.black);
      g.fillRect(0, 0, 512, 512);
      g.setColor(Color.white);
      g.drawString(new Date().toString(), 25, 250);
  public SwingFail() {
    frame = new JFrame("Rapid Refresh");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JPanel panel = (JPanel) frame.getContentPane();
    panel.add(new RefreshPanel());
    frame.pack();
    frame.setVisible(true);
  public void actionPerformed(ActionEvent e) {
    frame.repaint();
  public static void main(String[] args) {
    SwingFail sf = new SwingFail();
    javax.swing.Timer t = new javax.swing.Timer(1, sf);
    t.setCoalesce(false);
    t.start();
}PS This is with Java 5, and the underlying OS is in bad need of an update so I wouldn't be surprised if this issue has long since been resolved, but I'd still love to hear from someone else running X11 if this works well or not. Of course hardware performance may be a factor, but the difference in CPU time should still be evident between turning offscreen pixmaps on or off.
Edited by: DecadeOfJava on Jun 19, 2010 6:34 AM