Adjusting Linux OOM killer priorities with systemd

Use systemd's OOMScoreAdjust= setting to influence which services the Linux kernel kills when the system runs out of memory.

• Lower values make a process less likely to be killed.
• Higher values make a process more likely to be killed.
• The valid range is -1000 to 1000.

Suggested priorities

These are relative preferences rather than a guarantee: the kernel also considers factors such as each process's memory usage.

Configure the services

Create a systemd drop-in for MariaDB:

sudo systemctl edit mariadb.service

[Service]
OOMScoreAdjust=-500

Repeat for the remaining services with their corresponding values:

sudo systemctl edit varnish.service
sudo systemctl edit solr.service
sudo systemctl edit clamav-daemon.service

Apply the changes:

sudo systemctl daemon-reload
sudo systemctl restart mariadb varnish solr clamav-daemon

Verify the settings

systemctl show mariadb varnish solr clamav-daemon \
  --property=OOMScoreAdjust

To inspect the kernel setting for a service's main process directly:

pid=$(systemctl show --property=MainPID --value mariadb.service)
cat "/proc/${pid}/oom_score_adj"

Find distribution-specific unit names

Service names vary between distributions. For example, ClamAV might use clamd@scan.service instead of clamav-daemon.service.

systemctl list-unit-files | grep -E 'maria|varnish|solr|clam'

Caveats

Avoid OOMScoreAdjust=-1000 unless a service must effectively never be killed. Making essential memory consumers immune can leave the kernel without a safe way to recover.

OOMScoreAdjust= controls the kernel OOM killer. If the journal says systemd-oomd selected the service, configure systemd-oomd's cgroup policy separately—for example, investigate ManagedOOMPreference=avoid.

Links

• [[Notes/Tech/Server memory usage]]
• [[Notes/Tech/Varnish varnishadm quick reference]]