Can Xorg-Server and Wayland-Compositor be run simultaneously in Linux?

Question

At graphical login with lightdm I can set which session type to start and am offered:

• Gnome
• Gnome on Xorg
• Xfce session

The first Gnome is the Gnome DE running with the standard Wayland backend, and as I understand it means Gnome acts as a wayland protocol compositor, i.e talk to the kernel for KMS and evdev (as displayed on Wayland Architecture | freedesktop.org), the second option Gnome on Xorg when chosen creates also a Gnome session, yet there is a process Xorg, i.e. as I can tell an Xorg server running.

I was courious, since both Xorg-Server, as the wayland protocol's compositor might be using similar/same resources (i.e. the graphic adapter) if it is possible to run both at the same time.

I have tested therefore to login, once into a Gnome wayland session and then secondly with another account account in to a Gnome on Xorg session. The result has been that this seemingly works, yet there are processes named /usr/bin/Xwayland running, and I am unsure if the displayed Xorg session, is eventually only "fake", hence displaying via the XWayland "Xorg" server.

Being confused I wanted to pose the question here, whether "Xorg-Server and Wayland-Compositor [can] be run simultanously in Linux?"

Answer 1

They run simultaneously, but they don't control the display simultaneously. Typically, the login screen ends up on the first VT, and your desktop on the second; you can try switching back and forth with CtrlAltF1 and CtrlAltF2. You'll find hand-off messages in the logs; for example, in ~/.local/share/xorg/Xorg.1.log, I see

AIGLX: Suspending AIGLX clients for VT switch
systemd-logind: got pause for ...

when switching away from the VT running that X server, and

systemd-logind: got resume for ...
AIGLX: Resuming AIGLX clients after VT switch

when switching to the VT.