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 <arvidjaar@xxxxxxxxx> wrote:
On Mon, Aug 19, 2024 at 10:11 AM Barry <barry@xxxxxxxxxxxxxxxx> wrote:
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> On 19 Aug 2024, at 06:55, Windl, Ulrich <u.windl@xxxxxx> wrote:
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> Despite of the fact that my version of systemd does not know that setting, can you explain what the effect of AccuracySec= really is?
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> The docs https://www.man7.org/linux/man-pages/man5/systemd.timer.5.html say this
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> 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.
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> 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".
