which is better gdm or lightdm?

This tutorial will help you to install multiple display managers on the Ubuntu desktop. Also provides you instructions to switch between display managers.

A Display Manager is a graphical login program and session manager that is responsible for user authentication. It is a totally separate application from the desktop environment.

Display Manager is also known as “login manager”. In this tutorial, we will discuss 3 popular display managers that are GDM, SDDM, and LightDM. Some of the display managers are installed by default with the desktop environment.

Although, multiple Display Managers are available for the Ubuntu desktop environment. But we would focus on three prominent ones – GDM, LightDM, XDM, and SDDM. GDM (GDM3) is the default Display Manager for Ubuntu 16.10 and newer versions. Still, some of the other Ubuntu flavors prefer to use LightDM as the default display manager.

Use one of the below commands to install Display Manager on the Ubuntu system.

During the installation of the display manager, the installer will prompt to change the default display manager.

After installation, you can also switch the display manager with the below-mentioned command. Open a terminal and type:

Use the UP + DOWN keys to select the display manager and hit the Ok button.

In case the above command fails, you can directly edit the /etc/X11/default-display-manager file and set the new display manager.

In Ubuntu 18.10, the nvidia packages activated modeset=1 by default, and the upgrade to 19.04 preserves this setting.

19.04 (new install) does not activate modeset=1, to avoid a big gdm3 bug.

Problem: gdm3 kills the nvidia card from displaying to external monitors when your use modeset=1. Bug still exists in Ubuntu 19.04. Speculation is that gdm3 sees the driver in modesetting operation, and decides wayland must be in use, and there is no way to get it to behave otherwise. It must be a hard bug to fix. An Ubuntu dev (Daniel van Vugt) says that is really a nvidia bug: something about the nvidia driver requires root access, which gdm3 denies to it (unlike all the other display managers).

Reference: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1716857

https://devtalk.nvidia.com/default/topic/1042774/nvidia-drivers-390-77-no-external-display-on-ubuntu-18-04-and-quadro-m1000m/?offset=24