As of version 4.0, XFree86, the free X server for Linux, supports multiple monitors. You can mix AGP and PCI cards, and you're not limited to using multiple identical cards.

Test system

Mandrake Linux 7.0 with SMP kernel, dual P2-400 with 384 MB RAM, 3 monitors:

1. AGP Nvidia TNT2 Ultra, primary in BIOS
2. PCI Nvidia TNT, to the right of primary
3. PCI 3Dlabs Permedia 2, to the left of primary

Installing XFree86 4.0.1

Installation was a breeze. First, download the appropriate binary distribution from xfree86.org. For my system, I had to download the Linux-ix86-glibc21 distribution. Read the Install file to see what components are required and how to install everything (it is no more complicated than running a single shell script).

Configuring X

Run XFree86 -configure, this creates a basic XF86Config file. You will need to customize this file, see my own XF86Config for a sample, and complete documentation is also available.
There is also a graphical configuration utility, xf86cfg. Unfortunately, this didn't work on my system.

Multi-monitor modes

By default, multimon is configured to work as multiple independent displays: in this mode, the mouse can move freely between monitors, but you can't move a window to another monitor or stretch it across two or more monitors. You will need to start a window manager for each monitor, like this:

exec twm -display :0.0
exec twm -display :0.1
exec twm -display :0.2

The second mode is called Xinerama, and works similar to Windows 98/2000 multimon: you have one large desktop that spans all monitors, windows can be moved from one monitor to the other, and you can stretch windows across two or more monitors. To enable this mode, add +xinerama to the X server command line, for example in .xserverrc:

X +xinerama

Multimon desktop

Here is a picture of KDE running in Xinerama mode on two monitors.

2 monitors, each at 1024x768

The problem with KDE is that it isn't Xinerama-aware. This means that from KDE's point of view, you are using a single large monitor instead of multiple individual ones. This causes the following problems:

• windows are maximized to the desktop instead of the monitor they're on
• dialogs are split across monitors
• you can't use different resolutions due to problems with taskbars, windows opening in invisible areas of the desktop and menus extending outside of the visible area of a monitor

One window manager that is Xinerama-aware is Enlightenment version 0.16.4-2 or later. It handles dialogs and maximized windows correctly, and you can use different resolutions (I did have some problems though with menus not properly scrolling into view on the lower-resolution monitor).

I'm using the following command to start X:

xinit -- /usr/X11R6/bin/X -layout "2 Monitors"

Problems

After quitting X, secondary monitors aren't uninitialized, which can cause 'Out of Scan Range' and similar problems. This can be annoying if you don't stay in X all the time.

Another problem is my 3rd monitor: with XFree86 4.0.0, it worked okay in multihead mode, but in Xinerama mode, apps would sometimes open outside the visible desktop, and menus would popup on the wrong monitor. This was caused by the 3rd monitor being to the left of the primary, a configuration not supported by 4.0.0. Now after upgrading to 4.0.1, my 3rd monitor (Permedia 2) doesn't work at all: it starts up but then goes back to standby immediately. I have the same problem with Win98, but not with Win2000. But having a monitor to the left of the primary works okay in 4.0.1, I have verified this by setting monitor 2 to be 'LeftOf' the primary.
Thanks to Mandrake for the information.

Recommended setup

• XFree86 4.0.1 or later
• enable Xinerama mode
• use Enlightenment 0.16.4-2 or later

Conclusion

When using the latest versions of XFree and Enlightenment, multimon works very well. If your video cards are supported, give it a try, you'll never go back to a single-monitor system. Hopefully other window managers and desktop environments will be updated to support Xinerama as well.

If you have questions or comments, please post them to the forum. If you are already running a multimon Linux system, please share your experiences.

More information

• Xinerama How-to
• XFree86
• Enlightenment window manager