According to the mailing list, Ubuntu 21.10 plans to release its systemd package using a unified cgroup hierarchy (cgroups v2) by default.
Cgroups (control groups) is a mechanism provided by the Linux kernel that can limit the resources used by a single process or multiple processes and can achieve fine-grained control over resources such as cpu and memory. Developers can also use the refined control capabilities provided by cgroups to limit the resource usage of a certain process or group of processes.
In the email, the Ubuntu developers admitted that the plan has been “delayed for a long time”. The upstream systemd already uses the Cgroups v2 hierarchy by default. Other Linux distributions, such as Debian, will switch to this structure in 2019. Although upstream Snap is not currently supported, there are already related patches that have been merged in this cycle. Therefore, Ubuntu will also use systemd, which is supported by the unified cgroupsv2 hierarchy.
In addition, if for some reason, users need to keep the traditional cgroup v1 hierarchy, they can select it through the kernel parameter at startup: systemd.unified_cgroup_hierarchy=0.