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. 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.
Regards, Ulrich From: Adrian Vovk <adrianv...@gmail.com> Sent: Monday, August 19, 2024 10:45 AM To: Andrei Borzenkov <arvidj...@gmail.com> Cc: Barry <ba...@barrys-emacs.org>; Windl, Ulrich <u.wi...@ukr.de>; Systemd Devel <systemd-devel@lists.freedesktop.org> Subject: [EXT] Re: [systemd-devel] Re: Understanding the effect of AccuracySec= I don't have the initial email for some reason (got caught in spam filter? Idk) so I don't have the full context. On Mon, Aug 19, 2024, 03:55 Andrei Borzenkov <arvidj...@gmail.com<mailto:arvidj...@gmail.com>> wrote: On Mon, Aug 19, 2024 at 10:11 AM Barry <ba...@barrys-emacs.org<mailto:ba...@barrys-emacs.org>> wrote: > > > > On 19 Aug 2024, at 06:55, Windl, Ulrich > <u.wi...@ukr.de<mailto:u.wi...@ukr.de>> wrote: > > > Despite of the fact that my version of systemd does not know that setting, > can you explain what the effect of AccuracySec= really is? > > > The docs https://www.man7.org/linux/man-pages/man5/systemd.timer.5.html say > this > > Note that this setting is primarily a power saving option > that allows coalescing CPU wake-ups. It should not be > confused with RandomizedDelaySec= (see below) which adds a > random value to the time the timer shall elapse next and > whose purpose is the opposite: to stretch elapsing of timer > events over a longer period to reduce workload spikes. For > further details and explanations and how both settings play > together, see below. > > So if you have 5 timers that expire about the same time this allows them to > all run at the sane time thus saving power. It does explain why timers are run at the same time. It does not explain why "daily" + accuracy 6h runs at midnight, not close to 6am When your timer is scheduled for 00:00 but accuracy is 6h, then systemd will try to fire the timer at 00:00 but is allowed to be up to 6 hours "late" to save power. So the timer will fire at any time between 00:00 and 06:00. Let's say you have another timer set to fire at 01:00 + 6hr accuracy. systemd will likely fire both timers at around 01:00. The first timer is allowed to start anywhere between 00:00 and 06:00 and the second between 01:00 and 07:00. These ranges intersect from 01:00 to 06:00, so systemd will fire both timers at some point in that range. It's free to pick exactly when and I'm not looking at the code, but I wouldn't be surprised if it'll pick a time closer to 01:00 to minimize the amount of time it's "late".
