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1# Control Groups {#sec-cgroups}
2
3To keep track of the processes in a running system, systemd uses
4*control groups* (cgroups). A control group is a set of processes used
5to allocate resources such as CPU, memory or I/O bandwidth. There can be
6multiple control group hierarchies, allowing each kind of resource to be
7managed independently.
8
9The command `systemd-cgls` lists all control groups in the `systemd`
10hierarchy, which is what systemd uses to keep track of the processes
11belonging to each service or user session:
12
13```ShellSession
14$ systemd-cgls
15├─user
16│ └─eelco
17│ └─c1
18│ ├─ 2567 -:0
19│ ├─ 2682 kdeinit4: kdeinit4 Running...
20│ ├─ ...
21│ └─10851 sh -c less -R
22└─system
23 ├─httpd.service
24 │ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
25 │ └─...
26 ├─dhcpcd.service
27 │ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
28 └─ ...
29```
30
31Similarly, `systemd-cgls cpu` shows the cgroups in the CPU hierarchy,
32which allows per-cgroup CPU scheduling priorities. By default, every
33systemd service gets its own CPU cgroup, while all user sessions are in
34the top-level CPU cgroup. This ensures, for instance, that a thousand
35run-away processes in the `httpd.service` cgroup cannot starve the CPU
36for one process in the `postgresql.service` cgroup. (By contrast, it
37they were in the same cgroup, then the PostgreSQL process would get
381/1001 of the cgroup's CPU time.) You can limit a service's CPU share in
39`configuration.nix`:
40
41```nix
42{
43 systemd.services.httpd.serviceConfig.CPUShares = 512;
44}
45```
46
47By default, every cgroup has 1024 CPU shares, so this will halve the CPU
48allocation of the `httpd.service` cgroup.
49
50There also is a `memory` hierarchy that controls memory allocation
51limits; by default, all processes are in the top-level cgroup, so any
52service or session can exhaust all available memory. Per-cgroup memory
53limits can be specified in `configuration.nix`; for instance, to limit
54`httpd.service` to 512 MiB of RAM (excluding swap):
55
56```nix
57{
58 systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
59}
60```
61
62The command `systemd-cgtop` shows a continuously updated list of all
63cgroups with their CPU and memory usage.