Wandering Thoughts archives

How I think you set up fair share scheduling under systemd

When I started writing this entry, I was going to say that systemd automatically does fair share scheduling between and describe the mechanisms that make that work. However, this turns out to be false as far as I can see; systemd can easily do fair share scheduling, but it doesn't do this by default.

The basic mechanics of fair share scheduling are straightforward. If you put all of each user's processes into a separate cgroup it happens automatically. Well. Sort of. You see, it's not good enough to put each user into a separate cgroup; you have to make it a CPU accounting cgroup, and a memory accounting cgroup, and so on. Systemd normally puts all processes for a single user under a single cgroup, which you can see in eg systemd-cgls output and by looking at /sys/fs/cgroup/systemd/user.slice, but by default it doesn't enable any CPU or memory or IO accounting for them. Without those enabled, the traditional Linux (and Unix) behavior of 'every process for itself' still applies.

(You can still use systemd-run to add your own limits here, but I'm not quite sure how this works out.)

Now, I haven't tested the following, but from reading the documentation it seems that what you need to do to get fair share scheduling for users is to enable DefaultCPUAccounting and DefaultBlockIOAccounting for all user units by creating an appropriate file in /etc/systemd/user.conf.d, as covered in the systemd-user.conf manpage and the systemd.resource-control manpage. You probably don't want to turn this on for system units, or at least I wouldn't.

I don't think there's any point in turning on DefaultMemoryAccounting. As far as I can see there is no kernel control that limits a cgroup's share of RAM, just the total amount of RAM it can use, so cgroups just can't enforce a fair share scheduling of RAM the way you can for CPU time (unless I've overlooked something here). Unfortunately, missing fair share memory allocation definitely hurts the overall usefulness of fair share scheduling; if you want to insure that no user can take an 'unfair' share of the machine, it's often just as important to limit RAM as CPU usage.

(Having discovered this memory limitation, I suspect that we won't bother trying to enable fair share scheduling in our Ubuntu 16.04 installs.)