Is gdm3 preferred to lightdm?

I have hard a hard time finding good information on the performance difference between display managers on Ubuntu (specifically between lightdm and gdm3).

With Ubuntu switching to Gnome, I would assume that it would also be recommended to switch to gdm3 instead of lightdm but I can't find any information one way or the other.

So:

• Is this the case? Will lightdm still be developed?
• Is lightdm faster than gdm3? (Answers like this say lightdm is "lightweight and fast" but do not say whether or not gdm3 is slow....)

GDM is the default DM in Ubuntu as of 17.10. LightDM is still the default for some other flavours, like Xubuntu or Lubuntu, and I doubt either of these projects will move to GDM, so LightDM should continue to be supported in Ubuntu.

See this mailing list post from June 2017:

As you might be aware the Ubuntu desktop has decided to switch to using GNOME and this means we will be using GDM instead of LightDM Unfortunately this means there is likely to be a reduction in development effort from Canonical.

However, I want to assure you that we are not abandoning LightDM:

• We continue to ship LightDM in our existing Ubuntu desktop releases and we will support those for many years to come (see below).
• Many Ubuntu derivatives use LightDM and we continue to support them in doing that.
• Ultimately we think that LightDM is the right cross-desktop solution for display management, it's just not something we can make use of in Ubuntu desktop at this time. Who knows how the future will pan out :)

I will continue to do release management for LightDM and review and merge branches as I can. If anyone else is capable and interested in helping out with these jobs I'd love to hear from you - I don't want to be a blocker on development just because my focus is elsewhere.

The LightDM greeter is configurable, and a barebones greeter might well be faster than GDM, but a complex one might be slower.

gdm3 has an annoying bug for Nvidia Optimus users not using wayland. If you run your nvidia driver kernel modeset (modeset=1), you enable "prime sync" which means you no longer have tearing on your laptop screen. prime sync only works with modeset=1, so most Optimus users will want to enable this, unless they don't use their laptop's display, or don't care about tearing.

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

lightdm does not have this problem (sddm from KDE also does not have this problem).

EDIT: Apart from using lightdm, there is a workaround for gdm3 (which Pop!OS uses out of the box. System76 sells Optimus hardware so a good Nvidia experience is mission-critical). The fix is a one-liner, see the 'Jeremy Soller' comment on the relevant Ubuntu bug report.

https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1716857/comments/26

Note that this means Pop!OS decided to keep gdm3 and use this workaround, instead of using lightdm, which I suppose is an authoritative vote on which is the best approach.

1. Ubuntu GNOME uses the gdm3, which is the default GNOME 3.x desktop environment greeter.

2. As its name suggests LightDM is more lightweight than gdm3 and it's also faster.

3. LightDM will continue to be developed. Ubuntu MATE 17.10's default Slick Greeter (slick-greeter) uses LightDM under the hood, and as its name suggests it is described as a slick-looking LightDM greeter. The default Slick Greeter in Ubuntu MATE 18.04 also uses LightDM under the hood. Ubuntu MATE 20.04 switched from Slick Greeter to LightDM Arctica Greeter, so LightDM will not be going away any time in the near future.