Xvfb

Xvfb is:

an X server that can run on machines with no display hardware and no physical input devices. It emulates a dumb framebuffer using virtual memory.

Some applications can require a display to run properly, even if no interaction with the user is actually required. This can cause problems for those wanting to run such applications from within batch jobs.

One approach to this problem is to use an X Virtual Frame Buffer (Xvfb): this is a mechanism that provides what appears to applications be a real display device but is just a buffer in memory. An Xvfb can be created by an unprivileged user on-demand then killed when no longer needed. One can also take screenshots of Xvfb displays.

Usage

After connecting to Stanage (see Establishing a SSH connection), start an interactive session with the following command:

srun --pty bash -i

xvfb can be loaded with one of the following:

module load Xvfb/1.20.9-GCCcore-9.3.0
module load Xvfb/1.20.9-GCCcore-10.2.0

For example, to

  1. start an application that requires a display,

  2. take a screenshot of it an hour later

  3. then kill the associated Xvfb

you could add something similar to the following to your batch job submission script:

# Start an 'X virtual frame buffer' (a dummy display)
export DISPLAY=:1
Xvfb $DISPLAY &
xvfb_process_id=$!

# Start an application that requires a display
./myprogram arg1 arg2 &
myprogram_process_id=$!

# Wait an hour then take a screenshot
sleep 3600
import -window root myprogram_screenshot.png

# Kill your program
kill $myprogram_process_id

# Kill your 'X virtual frame buffer'
kill -9 $xvfb_process_id

Installation notes

xvfb was installed using Easybuild 4.7.0, build details can be found in folder $EBROOTXVFB/easybuild with the module loaded.