2.2 KiB
Control Groups
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:
$ 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:
{
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):
{
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
}
The command systemd-cgtop shows a continuously updated list of all
cgroups with their CPU and memory usage.