How to debug why a unit is started when?

[pmenzel+systemd-devel@xxxxxxxxxxxxx (Paul Menzel)]

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.

     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.


Kind regards,

Paul


[1] https://bugzilla.gnome.org/show_bug.cgi?id=794336
[2] https://git.gnome.org/browse/gdm/tree/data/gdm.service.in

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