Changes coming for systemd and control groups
Lennart Poettering posted a message to the systemd-devel mailing list that outlines some changes coming. It refers to an earlier message that started describing differences coming in the kernel's control group (cgroup) implementation and their effects on systemd.
Those cgroup changes were discussed in February 2012 and cgroup maintainer Tejun Heo decided to eventually drop multiple cgroup hierarchies in March 2012. The basic idea is that the single hierarchy will also have a single manager on the system and all changes to the cgroups will need to go through that manager.