How to debug why a unit is started when?

[arvidjaar@xxxxxxxxx (Andrei Borzenkov)]

16.03.2018 18:49, Paul Menzel пиÑ?еÑ?:
> Dear systemd folks,
> 
> 
> I am trying to get the GDM login screen started earlier on a Dell XPS 13
> 9370 with Debian Sid/unstable system with systemd 238. Currently, after
> selecting the Linux kernel in GRUB itâ??s only displayed after roughly
> eight to ten seconds while Linux takes around two seconds [1].
> 
> Using systemd-bootchart I see that GDM is started quite late [1], and I
> wondering if there is an option to find out why.
> 
> GDMâ??s service unit [2] has the â??dependenciesâ?? below.
> 
> Â Â Â  After=rc-local.service plymouth-start.service
> systemd-user-sessions.service
> 
> Is there a debug option, where systemd says, why a certain unit is
> started? For example, reached target X and therefore starting Y.
> 

systemctl --after (--recursive) --list-dependencies gdm.service

may be the first step. Or use systemd-analyze chart to see full picture
of what was started when.