[systemd-devel] Power aware units

[Kay Sievers]

On Sun, Nov 3, 2013 at 3:36 PM, Bastien Nocera <hadess at hadess.net> wrote:
> systemd already allows launching specific tasks based on a timer, and
> intervals, and I was wondering whether power awareness was something
> planned for launching and stopping units.
>
> MacOS X 10.9 has some additional metadata for units that allows launchd
> to stop and start particular tasks based on power levels:
> http://arstechnica.com/apple/2013/10/os-x-10-9/16/
>
> We could implement this in 2 ways:
> - systemd itself speaks over D-Bus to UPower (using the new
> DisplayDevice to merge UPS and batteries) and stop/starts the units.
> - systemd ships a set of units that UPower will launch/stop based on
> battery status. This would require UPower to know more about some other
> subsystems as well such as the lock screen status, or the hard drive
> state.
>
> This would be useful for things like backups, housekeeping (emptying old
> files from the trash, old thumbnails, etc.), launching update-db, etc.
> in addition to the simple timer intervals.
>
> I think the first option is the best one, as in addition to UPower, we'd
> need to talk to the kernel/udev (HDD spinning state), and logind (lock
> screen status).

Systemd should not get any direct or indirect dependency on upower for
primary service management tasks. It just doesn't sound right to do
dependencies in this direction.

If systemd needs more information than the current on-battery vs. AC,
we would need to add that to systemd itself, or even add support for a
"virtual/composite battery" to the kernel.

> In addition to that, would it make sense for distributions to start
> porting their cron jobs to use systemd?

This all sounds nice to have, and what we want to have in the longer
run. We could do most of that already today, with just the battery/AC
information, I think.

Kay