Systemd runs things in cgroups by default, so installing the postgres package from your systemd distro's official repos and ensuring that is enabled and running

Systemd runs things in cgroups by default, so installing the postgres package from your systemd distro's official repos and ensuring that is enabled and running will get you postgres in cgroups to start with. From there you can check the systemd.resource-control(5) man page for some potentially interesting tunables.