[systemd-devel] Question about OnUnitInactiveSec= directive

[Mikhail Kasimov]

Hello!

Have a .timer service like:

======

[Unit]
Description=Runs VBA32 Update Hourly
Requires=timers.target

[Timer]
OnBootSec=2min
OnUnitInactiveSec=1h

[Install]
WantedBy=timers.target

======

to run vba32update.service in 1 hour after previous update-session is 
over (OnUnitInactiveSec=1h).

 From man-page: "|OnUnitInactiveSec=| defines a timer relative to when 
the unit the timer is activating was last deactivated."

Ok, here is log-snippet:
======
Июл 08 22:05:00 linux-mk500 systemd[1]: Starting VBA32 Anti-Virus Update 
Service...
Июл 08 22:05:00 linux-mk500 vbacl[14768]: Vba32 console scanner update 
process started
Июл 08 22:05:00 linux-mk500 vbacl[14768]: Reading configuration options 
from ./vbacl.ini
Июл 08 22:05:00 linux-mk500 vbacl[14768]: Using direct connection for update
Июл 08 22:05:02 linux-mk500 vbacl[14768]: Current dir is ./
Июл 08 22:05:02 linux-mk500 vbacl[14768]: Start update from 
http://anti-virus.by/update
Июл 08 22:05:02 linux-mk500 vbacl[14768]: Receiving file list
Июл 08 22:05:02 linux-mk500 vbacl[14768]: File list received
Июл 08 22:05:02 linux-mk500 vbacl[14768]: Update is not needed
Июл 08 22:05:02 linux-mk500 systemd[1]: Started VBA32 Anti-Virus Update 
Service.

Июл 08 23:05:13 linux-mk500 systemd[1]: Starting VBA32 Anti-Virus Update 
Service...
Июл 08 23:05:13 linux-mk500 vbacl[15289]: Vba32 console scanner update 
process started
Июл 08 23:05:13 linux-mk500 vbacl[15289]: Reading configuration options 
from ./vbacl.ini
Июл 08 23:05:13 linux-mk500 vbacl[15289]: Using direct connection for update
Июл 08 23:05:17 linux-mk500 vbacl[15289]: Current dir is ./
Июл 08 23:05:17 linux-mk500 vbacl[15289]: Start update from 
http://anti-virus.by/update
Июл 08 23:05:17 linux-mk500 vbacl[15289]: Receiving file list
Июл 08 23:05:17 linux-mk500 vbacl[15289]: File list received
Июл 08 23:05:17 linux-mk500 vbacl[15289]: Update is not needed
Июл 08 23:05:17 linux-mk500 systemd[1]: Started VBA32 Anti-Virus Update 
Service.

Июл 09 00:05:20 linux-mk500 systemd[1]: Starting VBA32 Anti-Virus Update 
Service...
Июл 09 00:05:20 linux-mk500 vbacl[16050]: Vba32 console scanner update 
process started
Июл 09 00:05:20 linux-mk500 vbacl[16050]: Reading configuration options 
from ./vbacl.ini
Июл 09 00:05:20 linux-mk500 vbacl[16050]: Using direct connection for update
Июл 09 00:05:24 linux-mk500 vbacl[16050]: Current dir is ./
Июл 09 00:05:24 linux-mk500 vbacl[16050]: Start update from 
http://anti-virus.by/update
Июл 09 00:05:24 linux-mk500 vbacl[16050]: Receiving file list
Июл 09 00:05:24 linux-mk500 vbacl[16050]: File list received
Июл 09 00:05:24 linux-mk500 vbacl[16050]: Update is not needed
Июл 09 00:05:24 linux-mk500 systemd[1]: Started VBA32 Anti-Virus Update 
Service.

Июл 09 01:05:50 linux-mk500 systemd[1]: Starting VBA32 Anti-Virus Update 
Service...
Июл 09 01:05:50 linux-mk500 vbacl[16528]: Vba32 console scanner update 
process started
Июл 09 01:05:50 linux-mk500 vbacl[16528]: Reading configuration options 
from ./vbacl.ini
Июл 09 01:05:50 linux-mk500 vbacl[16528]: Using direct connection for update
Июл 09 01:05:55 linux-mk500 vbacl[16528]: Current dir is ./
Июл 09 01:05:55 linux-mk500 vbacl[16528]: Start update from 
http://anti-virus.by/update
Июл 09 01:05:55 linux-mk500 vbacl[16528]: Receiving file list
Июл 09 01:05:55 linux-mk500 vbacl[16528]: File list received
Июл 09 01:05:55 linux-mk500 vbacl[16528]: Update is not needed
Июл 09 01:05:55 linux-mk500 systemd[1]: Started VBA32 Anti-Virus Update 
Service.
======

We see 22:05:02 (end of update-session) --> 23:05:13 (start of next 
update-session) --> 23:05:17 (end of update-session) --> 00:05:20 (start 
of next update-session) --> 00:05:24 (end of update-session) --> 
01:05:50 (start of next update-session) --> 01:05:55 (end of 
update-session).

Question: Why time of new update-session is *not* equal to time of end 
of previous update-session + 1h in section of seconds, e.g. 23:05:17 +1h 
= 00:05:17; 00:05:24 + 1h = 01:05:24 and so on? Is here a way to reach 
this precise coincidence?

Thanks!

Platform: x64, openSUSE Leap 42.1, systemd 210.
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