Limit Application Memory Usage with systemd

Friday, 18 October 2024 | Jin Liu

I saw this question on KDE forum about how to limit memory usage of a specific application in KDE, using systemd specifically. I did some research on that.

Resource control in systemd

man systemd.resource-control lists plenty of options that we can set to a cgroup. E.g., to limit the memory usage of a service, we can add:

MemoryAccounting=yes
MemoryHigh=2G

under the [Service] section of its .service file.

The difference between this and ulimit is that ulimit is per process, while systemd resource control is per cgroup. I.e., the MemoryHigh is accounted to the sum of both the service process, and all sub-processes it spawns, and even detached processes, i.e., daemons.

(That's actually the main point of cgroup: a process tree that a process can't escape via double-forking / daemonizing.)

Apps as systemd services

KDE Plasma launches apps as systemd services. (See this doc and this blog for more details.)

We can find the name of the systemd service of an app like this:

$ systemd-cgls|grep konsole
│ │ │ ├─app-org.kde.konsole@0d82cb37fcd64fe4a8b7cf925d86842f.service
│ │ │ │ ├─35275 /usr/bin/konsole
│ │ │ │ └─35471 grep --color=auto konsole

But the problem is:

  1. The part of the name after @ is a random string, changes every time the app is launched.
  2. The service is generated dynamically:
$ systemctl --user cat app-org.kde.konsole@0d82cb37fcd64fe4a8b7cf925d86842f.service

# /run/user/1000/systemd/transient/app-org.kde.konsole@0d82cb37fcd64fe4a8b7cf925d86842f.service
# This is a transient unit file, created programmatically via the systemd API. Do not edit.
[Service]
Type=simple
...

So if we want to limit the memory usage of Konsole, there's no persistent .service file on disk that we can edit.

Luckily, systemd allows us to create drop-in files to partially modify a service. Also, systemd considers app-org.kde.konsole@0d82cb37fcd64fe4a8b7cf925d86842f.service to be instances of a template named app-org.kde.konsole@.service. (This is how things like getty@tty3.service work.) So we can create a drop-in file named ~/.config/systemd/user/app-org.kde.konsole@.service.d/override.conf with the content:

[Service]
MemoryAccounting=yes
MemoryHigh=2G

and it will apply to all instances of app-org.kde.konsole@.service, even if there's no service file with that name.

(The file doesn't have to be named "override.conf". Any name with .conf works.)

Then we need to reload the systemd user manager: systemctl --user daemon-reload.

Now we can launch Konsole, and check if the memory limit works:

$ systemctl --user show 'app-org.kde.konsole@*.service'|grep MemoryHigh=
EffectiveMemoryHigh=2147483648
MemoryHigh=2147483648
StartupMemoryHigh=infinity

Note: as explained above, the limit applies to the sum of Konsole and all processes it spawns. E.g., if we run kwrite in Konsole, the memory usage of kwrite will be accounted to the limit of Konsole, and the limit we set to KWrite won't apply.

Set defaults for all apps

We can put defaults in ~/.config/systemd/user/app-.service.d/override.conf, and it will match all services whose name starts with app-.

Alternatively, if we run systemd-cgls, we can see that all apps are under a node named app.slice. So we can also put defaults in ~/.config/systemd/user/app.slice.d/override.conf, and all apps will inherit the settings. However, this is different from the previous method, as user services are also under app.slice by default, so they will also inherit the settings.