[systemd-devel] [EXT] Re: Re: Understanding the effect of AccuracySec=

[Lennart Poettering]

On Mo, 19.08.24 12:15, Windl, Ulrich (u.windl at ukr.de) wrote:

> Thanks to everyone sharing information. Basically that’s what I
> expected, too, except this:
>
> I run about 10 instances of the timer, and all 10 instances are
> started at the same second. My initial expectation wad that systemd
> might spread the instances in the 6 hour windows somehow.

That's what RandomizedDelaySec= is about. It's a difference concept.

https://www.freedesktop.org/software/systemd/man/latest/systemd.timer.html#RandomizedDelaySec=

> Maybe starting the next instance once the previous instance had finished.
> I’m somewhat unsure about the energy saving: Will 10 jobs run
> simultaneously consume less power than the 10 jobs run sequentially?
> My guess is that the timer overhead may be negligible.

Yes, typically it should be more energy efficient to keep a system
idle most of the time, and then pack everything it needs to do into a
brief "spark" of work. Different jobs tend to require resources
differently (i.e. some job might wait for CPU, another for disk IO,
another for network IO and so on), hence if you run them altogether
you maximize the chance to keep your hw busy, and thus removing the
need to power on/power off it continiously. After all, powering
on/powering off (i.e. getting from lower to higher power levels and
back) is time intensive, and wastes resources.

of course, there are differences in hardware, and we only implement a
very generic system here, but by reducing the amount of wakeups we
should generally optimize energy behaviour.

Tools such as powertop for example help you analyzing this, they show
you which parts of the OS cause which wakeups, and help you reducing
them to optimize energy use/battery lifetime. We are just doing our
part in not making things worse for that.

Lennart

--
Lennart Poettering, Berlin