To keep track of the processes in a running system, systemd uses <emphasis>control groups</emphasis> (cgroups). A control group is a set of processes used to allocate resources such as CPU, memory or I/O bandwidth. There can be multiple control group hierarchies, allowing each kind of resource to be managed independently.
The command <command>systemd-cgls</command> lists all control groups in the <literal>systemd</literal> hierarchy, which is what systemd uses to keep track of the processes belonging to each service or user session:
Similarly, <command>systemd-cgls cpu</command> shows the cgroups in the CPU hierarchy, which allows per-cgroup CPU scheduling priorities. By default, every systemd service gets its own CPU cgroup, while all user sessions are in the top-level CPU cgroup. This ensures, for instance, that a thousand run-away processes in the <literal>httpd.service</literal> cgroup cannot starve the CPU for one process in the <literal>postgresql.service</literal> cgroup. (By contrast, it they were in the same cgroup, then the PostgreSQL process would get 1/1001 of the cgroup’s CPU time.) You can limit a service’s CPU share in <filename>configuration.nix</filename>:
There also is a <literal>memory</literal> hierarchy that controls memory allocation limits; by default, all processes are in the top-level cgroup, so any service or session can exhaust all available memory. Per-cgroup memory limits can be specified in <filename>configuration.nix</filename>; for instance, to limit <literal>httpd.service</literal> to 512 MiB of RAM (excluding swap):
