Multiheading Linux Systems

We used Red Hat 7.2 as the baseline because it has XFree86 4, which supports multiple displays (and input devices). Commercial X servers have supported this for a while, but XFree86 4 comes with new Linux distributions and it's free. Version 4 threw in a new configuration file and format to allow this flexibility. The key section is ServerLayout, which positions one screen relative to another. Here's an excerpt:

Section "ServerLayout"
        Identifier   "XFree86 Configured"
        Screen   0 "Screen0" 0 0
        Screen   1 "Screen1" LeftOf "Screen0"
        InputDevice  "Mouse0" "CorePointer"
        InputDevice  "Keyboard0" "CoreKeyboard"
EndSection

As you undoubtedly realize from this straightforward format, this setup will configure Screen0 (the screen associated with your primary video card) and then configure Screen1 and place it to the left of Screen0. In other words, you would move your mouse across Screen0, hit the left edge and then enter Screen1 from the right side (pretty confusing if you have the monitors backward). You can choose Right, Top, Bottom or even Relative positioning so the screens are diagonal. Do a man XFree86 to get the details. The screens don't have to be the same resolution or even the same color depth. Make one or both virtual if you want, though we didn't find this easy to use. Each screen in ServerLayout is composed of a Monitor reference and Device (video card) reference as shown below:

Section "Screen"
        Identifier "Screen0"
        Device   "Card0"
        Monitor  "Monitor0"
        SubSection "Display"
                Depth   8
                #Virtual         1024 768
        EndSubSection
EndSection

Here's the monitor and device sections to support Screen0:

Section "Monitor"
        Identifier  "Monitor0"
        VendorName  "GWY"
        ModelName  "311"
EndSection
Section "Device"
        Identifier "Card0"
        Driver   "mga"
        VendorName "Matrox"
        BoardName  "MGA 2164W"
        BusID    "PCI:0:14:0"
EndSection

The first key thing is to make sure everything is consistent, i.e., Screen0 has Monitor0 and Card0. What's important is that the names match, not what they are. We have developed a habit of making one set end in 0 and the next set end in 1, instead of keeping track of the card and monitor names. It's just easier to avoid mistakes.

The next key thing is the device driver. That gets identified by the XFree86 -configure step for both cards. Finally, it's critical to get the BusIDs straight. It's optional with only one card but mandatory for two or more. An AGP card will start with a 1, not a 0, and probably will be 1:0:0.

Now you have the basics and need to fine-tune /etc/X11/XFConfig-4 to get optimal performance. Edit this file and interleave the data identified in /root/XFConfig-4.new. Because we're using KDE's graphical login, you can tweak the file, log out, restart the X server from the KDE login window, do the login and see what happens. If you're not happy, tweak and repeat. It's much faster than manually restarting X. See Listing 1 [available at ftp.linuxjournal.com/pub/lj/listings/issue99/5958.tgz] for a full XFConfig-4 file.

At this point, you have a working multiheaded system utilizing the KDE graphical login and probably the KDE environment (Figure 1). It turns out that there are four different configurations that can be set up: GNOME or KDE login, and then the GNOME or KDE environment. We'll go through the advantages and disadvantages of each approach now.

The file /etc/sysconfig/desktop determines which login manager is used. It's a one-line file saying DESKTOP="GNOME" or DESKTOP="KDE". Edit as root, according to your requirements. Here are the four configuration options:

Configurations three and four take a different approach and use the Xinerama extensions in XFree86 4. The authors are grateful to Dennis Baker for his splendid HOWTO. Xinerama makes one big screen, so it's probably the easiest and most natural way to work. You just move the windows from one screen to another. There are two key steps for these configurations. First, change the /etc/sysconfig/desktop file as above. Second, in the file /etc/X11/gdm/gdm.conf, edit the line 0=/usr/bin/X11/X toward the end of the file by making it read 0=/usr/bin/X11/X +xinerama.