60 lines
2.2 KiB
Markdown
60 lines
2.2 KiB
Markdown
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# Control Groups {#sec-cgroups}
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To keep track of the processes in a running system, systemd uses
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*control groups* (cgroups). A control group is a set of processes used
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to allocate resources such as CPU, memory or I/O bandwidth. There can be
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multiple control group hierarchies, allowing each kind of resource to be
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managed independently.
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The command `systemd-cgls` lists all control groups in the `systemd`
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hierarchy, which is what systemd uses to keep track of the processes
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belonging to each service or user session:
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```ShellSession
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$ systemd-cgls
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├─user
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│ └─eelco
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│ └─c1
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│ ├─ 2567 -:0
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│ ├─ 2682 kdeinit4: kdeinit4 Running...
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│ ├─ ...
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│ └─10851 sh -c less -R
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└─system
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├─httpd.service
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│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
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│ └─...
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├─dhcpcd.service
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│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
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└─ ...
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```
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Similarly, `systemd-cgls cpu` shows the cgroups in the CPU hierarchy,
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which allows per-cgroup CPU scheduling priorities. By default, every
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systemd service gets its own CPU cgroup, while all user sessions are in
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the top-level CPU cgroup. This ensures, for instance, that a thousand
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run-away processes in the `httpd.service` cgroup cannot starve the CPU
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for one process in the `postgresql.service` cgroup. (By contrast, it
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they were in the same cgroup, then the PostgreSQL process would get
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1/1001 of the cgroup's CPU time.) You can limit a service's CPU share in
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`configuration.nix`:
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```nix
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systemd.services.httpd.serviceConfig.CPUShares = 512;
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```
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By default, every cgroup has 1024 CPU shares, so this will halve the CPU
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allocation of the `httpd.service` cgroup.
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There also is a `memory` hierarchy that controls memory allocation
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limits; by default, all processes are in the top-level cgroup, so any
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service or session can exhaust all available memory. Per-cgroup memory
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limits can be specified in `configuration.nix`; for instance, to limit
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`httpd.service` to 512 MiB of RAM (excluding swap):
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```nix
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systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
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```
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The command `systemd-cgtop` shows a continuously updated list of all
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cgroups with their CPU and memory usage.
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