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X11 Xdummy

Xdummy is used with seamless servers on posix platforms.

Xdummy was originally developed by Karl Runge as a script to allow a standard X11 server to be used by non-root users with the dummy video driver

Since then, the X11 server gained the ability to run without those LD_SO_PRELOAD hacks and this is now available for most distributions.

Why use Xdummy instead of Xvfb?

The only feature that Xvfb currently lacks is the ability to simulate arbitrary DPI values, this is accomplished using custom patches which are not available with the default dummy driver builds from any distribution.

This is only an issue with applications that bypass the various ways they're supposed to retrieve the DPI preferences.

The downside of using Xdummy is its incomplete RandR support: the resolutions have to be pre-defined before starting the virtual display, whereas Xvfb now supports adding new resolutions at runtime. Therefore, Xvfb provides better screen resolution matching.

Usage

Xdummy standlone

You can start a new display using the dummy driver without needing any special privileges (no root, no suid), you should specify your own log and config files:

Xorg -noreset +extension GLX +extension RANDR +extension RENDER \
     -logfile ./10.log -config /etc/xpra/xorg.conf :10

You can find a sample configuration file for dummy here: xorg.conf. It contains many of the most common resolutions you are likely to need, including those found on phones and tablets. However if your client uses unusual resolutions, for instance multiple screens of differing sizes, you may want to add new Modelines to match your specific resolution.

Xdummy with Xpra

With Xpra, this may have been configured automatically for you when installing (on some distributions only).
You choose at build time whether or not to use Xdummy using the --with[out]-Xdummy build switch.
If your packages do not enable Xdummy by default, you may be able to switch to it by modifying the xvfb value in /etc/xpra/conf.d/55_server_x11.conf, something like:

xvfb=Xorg -dpi 96 -noreset -nolisten tcp \
          +extension GLX +extension RANDR +extension RENDER \
          -logfile ${HOME}/.xpra/Xvfb-10.log -config ${HOME}/xorg.conf

The -noreset option is only needed if the window manager is not the first application started on the display, for example if you use the --start-child= option, or if you want the display to survive once the window manager exits - generally, this is a good idea since xpra could crash and when it exits cleanly via xpra stop it already takes care of shutting down the X11 server.

Configuration

Defaults

By default the configuration file shipped with xpra allocates 768MB of memory and defines a large number of common screen resolutions, including common sizes for double and triple display setups.

Modelines

Since it is impossible to pre-define all the combinations possible, if your client resolution does not match one of the pre-defined values, you may want to add this resolution to the configuration file.

adding new modelines

Use a modeline calculator like xtiming.sf.net or using a command line utility like gtf or cvt and add the new modeline to the X11 server config (usually located in /etc/xpra/xorg.conf with xpra)

The only restriction on modelines is the pixel clock defined for the dummy driver and monitor: at higher resolution, you may need to lower the vertical refresh rate to ensure the mode remains valid. If your new resolution does not get used, check the X11 server log file (usually in ~/.xpra/Xorg.$DISPLAY.log with xpra)

Large Screens

If you have an unusually large display configuration (multiple monitors), you may also need to increase the memory and/or increase the "virtual size".

Packaging

versions required

Most recent distributions now ship compatible packages: Xorg version 1.12 or later, dummy driver version 0.3.5 or later; though some may have issues with non world-readable binaries

patches

The dummy packages in the xpra repositories include those patches.

libGL Driver Conflicts

With older distributions that do not use libglvnd, proprietary drivers usually install their own copy of libGL which conflicts with the use of software OpenGL rendering. You cannot use this GL library to render directly on Xdummy (or Xvfb).

The best way to deal with this is to use VirtualGL to take advantage of the OpenGL acceleration provided by the graphics card, just run: vglrun yourapplication.

To make vglrun work properly with Nvidia proprietary drivers make sure to create /etc/X11/xorg.conf using sudo nvidia-xconfig.

The alternative is often to disable OpenGL altogether. (more information here: #580

Ubuntu

Ubuntu does weird things with their Xorg server which prevents it from running Xdummy (tty permission issues).

Status: at time of writing, Xdummy can be used with 18.04 but not with earlier versions.

Then there are also ABI problems with their HWE releases, which is why Xdummy is not used by default on Ubuntu.

non-suid binary

If you distribution ships the newer version but only installs a suid Xorg binary, Xpra should have installed the xpra_Xdummy wrapper script and configured xpra.conf to use it instead of the regular Xorg binary.

This script executes Xorg via ld-linux.so, which takes care of stripping the suid bit.