When implementing a container runtime, you often need to interact with D-Bus. The most common example is cgroup operations. I tend to forget the details, so I'm writing this note for future reference.

SetUnitProperties(in s name, in b runtime, in a(sv) properties)

You can modify unit properties at runtime using properties. cgroup resource limits (e.g., MemoryMax) can also be set through this interface.

Trying it out with busctl

1. Create a test Unit

   /etc/systemd/system/memhog.service

   [Unit]
   Description=memhog demo service (for SetUnitProperties example)
   
   [Service]
   Type=simple
   ExecStart=/usr/bin/python3 -c "import time; buf=bytearray(150*1024*1024); print('allocated', len(buf)); time.sleep(3600)"
   Restart=no
   
   [Install]
   WantedBy=multi-user.target

   Reload and start the service.

   $ sudo systemctl daemon-reload
   $ sudo systemctl start memhog.service
   $ sudo systemctl status memhog.service
   

2. Get the Unit object path on D-Bus Retrieve the object path using GetUnit.

   $ busctl --system call \
   org.freedesktop.systemd1 /org/freedesktop/systemd1 \
   org.freedesktop.systemd1.Manager GetUnit \
   s "memhog.service"
   o "/org/freedesktop/systemd1/unit/memhog_2eservice"
   

3. Check the type of MemoryMax using introspect

   Since SetUnitProperties takes a variant value, we need to know the type. Let's check it with introspect.

   $ busctl --system introspect \
   org.freedesktop.systemd1 /org/freedesktop/systemd1/unit/memhog_2eservice \
   | grep MemoryMax
   .MemoryMax  property  t  18446744073709551615  -
   

   t is uint64

4. Limit memory usage with D-Bus SetUnitProperties Finally, call SetUnitProperties

   $ sudo busctl --system call \
   org.freedesktop.systemd1 /org/freedesktop/systemd1 \
   org.freedesktop.systemd1.Manager SetUnitProperties \
   "sba(sv)" "memhog.service" true \
   1 \
   "MemoryMax" t 104857600
   

   Arguments:

   • true means runtime (until restart)
   • The leading 1 in a(sv) is the count of (sv) pairs
   • 104857600 = 100MiB

5. Verify the setting (D-Bus / systemctl) Verify via D-Bus

   $ busctl --system get-property \
   org.freedesktop.systemd1 /org/freedesktop/systemd1/unit/memhog_2eservice \
   org.freedesktop.systemd1.Service MemoryMax
   t 104857600
   

   Verify via systemctl

   $ systemctl show memhog.service -p MemoryMax
   MemoryMax=104857600
   

6. Cleanup

   $ sudo systemctl stop memhog.service
   $ sudo rm /etc/systemd/system/memhog.service
   $ sudo systemctl daemon-reload