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I see this in the output of systemd-analyze critical-chain.

The example output in systemd-analyze(1) includes it, but fails to explain it.

       Example 3. systemd-analyze critical-chain

           $ systemd-analyze critical-chain
           multi-user.target @47.820s
           └─pmie.service @35.968s +548ms
             └─pmcd.service @33.715s +2.247s
               └─network-online.target @33.712s
                 └─systemd-networkd-wait-online.service @12.804s +20.905s
                   └─systemd-networkd.service @11.109s +1.690s
                     └─systemd-udevd.service @9.201s +1.904s
                       └─systemd-tmpfiles-setup-dev.service @7.306s +1.776s
                         └─kmod-static-nodes.service @6.976s +177ms
                           └─systemd-journald.socket
                             └─system.slice
                               └─-.slice

What is it -.slice?

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If you run systemctl list-units --type=slice, you'll see that the -.slice is described as the Root Slice.

man systemd.slice says:

A slice unit is a concept for hierarchically managing resources of a group of processes.

Slices are organized hierarchically in a tree. The name of the slice encodes the location in the tree. The name consists of a dash-separated series of names, which describes the path to the slice from the root slice. The root slice is named -.slice.

Example: foo-bar.slice is a slice that is located within foo.slice, which in turn is located in the root slice -.slice.

Metaphorically, the root slice would be the "main pie" from which all the other slices are ultimately carved out of. So if some resource has not been more specifically allocated to a more particular slice, it would count as part of the root slice, according to any slice-aware tools.

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    You could think of it as representing the / slice, as the naming syntax is the same as for mount units, where dashes are the result of converting a slash-separated path (such as -.mount which represents /, the root of the filesystem). systemd-cgls will show it graphically. Sep 1, 2023 at 9:14

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