[systemd-devel] systemd-timesync and journalctl questions

[Mantas Mikulėnas]

Well, systemd itself uses Linux timerfd to receive "clock changed" events.
See time_change_fd() in src/basic/time-util.c, and the large comment in
clock_state_update() in src/time-wait-sync/time-wait-sync.c in systemd's
source tree.

On Sat, Sep 15, 2018 at 4:15 PM D.S. Ljungmark <ljungmark at modio.se> wrote:

> That could work, but then it’s still needing a signal to figure it out in
> order to get it right. Thanks. I’ll see about implementing that.
>
> On Sat, 15 Sep 2018 at 14:28, Mantas Mikulėnas <grawity at gmail.com> wrote:
>
>> On Sat, Sep 15, 2018 at 3:02 PM D.S. Ljungmark <ljungmark at modio.se>
>> wrote:
>>
>>> I’ve got a follow up here. We want code to run both before and after
>>> time is synchronized the first time. But we -also- need to know how big the
>>> first time jump is.
>>> It’s lokel to be in the matter of weeks or months in our application,
>>> and we’d want to know how much it was lagging.
>>> We start logging measurements before things are connected, and tag them
>>> as unknown, later when we get an application sync on time we can
>>> recalculate to proper dates,( provided we weren’t losing power  for many
>>> times before we get a network sync)
>>>
>>
>> AFAIK you could look at the difference between real time and
>> CLOCK_BOOTTIME (or MONOTONIC). Whenever you notice the clock jumping,
>> compare the new difference with the previously seen one, and you'll know
>> how much to offset the old records by. That doesn't depend on any external
>> software and works just as well with timesyncd as it does with ntpdate or
>> manual adjustments.
>>
>> (For example, every journald entry includes the real-time *and* monotonic
>> timestamps.)
>>
>> --
>> Mantas Mikulėnas
>>
>

-- 
Mantas Mikulėnas
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