[systemd-devel] Relationship between cgroup hierarchy and slice names

[Yeongjin Kwon]

Hi,

I'm trying to override the parent slice of a certain slice unit so I can
reorganize the cgroup hierarchy. I looked into overriding the "Slice"
property to set the containing slice of the unit, but it seems that
property does not do anything for slices because the location in the cgroup
hierarchy and parent are already set by the unit name. The documentation
says the unit name is composed of a series of words separated by dashes,
where each word represents a directory in the cgroup hierarchy. If the unit
name was independent from the location of the slice in the cgroup
hierarchy, then I would imagine that would make overriding the parent slice
possible. I tried to find the reason why the slice unit name must strictly
match the cgroup hierarchy, but I could not come up with an answer after
searching online and thinking about it myself.
One possibility for why the slice name must match the hierarchy could be
naming conflicts. If there are two slices with the same name but in
different cgroups, they could conflict with each other. But I decided this
could not be the case, since developers could just avoid the conflicts by
using the slice naming convention as a convention instead of following it
as a hard rule. Furthermore, there are several unit types like scopes and
services which can be organized in the cgroup hierarchy like slices, but
don't follow the same naming rule. If it was decided that naming conflicts
in relation to the cgroup hierarchy would not pose a problem for these
other units, then why was it decided for slices?
Another possibility that comes to mind is that naming slices this way makes
using them easier, since developers would not have to put an extra Slice
property on the unit. It would also be possible to tell the location of a
slice in the cgroup hierarchy just by looking at the name. But would people
sacrifice the capability for users to reorganize and override the cgroup
hierarchy, for some convenience?
Another possibility is aesthetic reasons, but then again that would be
enforcing a personal decision on a community, which I don't think would
happen.

Is there another way to override the location of a slice in the cgroup
hierarchy? And what is the reason why slice names have to correspond with
their location in the cgroup hierarchy?
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