64 lines
2.2 KiB
Markdown
64 lines
2.2 KiB
Markdown
|
# Control Groups {#sec-cgroups}
|
||
|
|
|
||
|
|
To keep track of the processes in a running system, systemd uses
|
||
|
|
*control groups* (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 `systemd-cgls` lists all control groups in the `systemd`
|
||
|
|
hierarchy, which is what systemd uses to keep track of the processes
|
||
|
|
belonging to each service or user session:
|
||
|
|
|
||
|
|
```ShellSession
|
||
|
|
$ systemd-cgls
|
||
|
|
├─user
|
||
|
|
│ └─eelco
|
||
|
|
│ └─c1
|
||
|
|
│ ├─ 2567 -:0
|
||
|
|
│ ├─ 2682 kdeinit4: kdeinit4 Running...
|
||
|
|
│ ├─ ...
|
||
|
|
│ └─10851 sh -c less -R
|
||
|
|
└─system
|
||
|
|
├─httpd.service
|
||
|
|
│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
|
||
|
|
│ └─...
|
||
|
|
├─dhcpcd.service
|
||
|
|
│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
|
||
|
|
└─ ...
|
||
|
|
```
|
||
|
|
|
||
|
|
Similarly, `systemd-cgls cpu` 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 `httpd.service` cgroup cannot starve the CPU
|
||
|
|
for one process in the `postgresql.service` 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
|
||
|
|
`configuration.nix`:
|
||
|
|
|
||
|
|
```nix
|
||
|
|
{
|
||
|
|
systemd.services.httpd.serviceConfig.CPUShares = 512;
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
By default, every cgroup has 1024 CPU shares, so this will halve the CPU
|
||
|
|
allocation of the `httpd.service` cgroup.
|
||
|
|
|
||
|
|
There also is a `memory` 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 `configuration.nix`; for instance, to limit
|
||
|
|
`httpd.service` to 512 MiB of RAM (excluding swap):
|
||
|
|
|
||
|
|
```nix
|
||
|
|
{
|
||
|
|
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
The command `systemd-cgtop` shows a continuously updated list of all
|
||
|
|
cgroups with their CPU and memory usage.
|