Fixes#5663
- Why I did it
It's currently possible for the SNMP timer to conflict with config reload (specifically if the timer triggers while config reload is stopping the SWSS service). config reload triggers SWSS to shutdown, which causes SNMP to shutdown, which conflicts with the SNMP timer causing SNMP to startup. See the linked issue for more details.
- How I did it
Including the After ordering dependency forces the SNMP timer to wait until SWSS finishes stopping, preventing the conflict. If there is an ordering dependency between two units (e.g. one unit is ordered After another), if one unit is shutting down while the other is starting up, the shutdown will always be ordered before the startup. In this case, that means that the SNMP timer is forced to wait for the SWSS shutdown to complete. Only then can the SNMP timer proceed. See here for more details.
It's important to note that the After dependency will not cause SWSS to be started when the SNMP timer fires (assuming that SWSS has not yet been started). The existing Requisite dependency in the SNMP service will also not cause SWSS to be started, instead it will cause the SNMP service to fail if SWSS is not active.
Signed-off-by: Lawrence Lee <lawlee@microsoft.com>
Delay CPU intensive services at boot
- How I did it
Made snmp.timer work and add telemetry.timer.
But this is not enough because it breaks the existing snmp dependency on swss.
So, in this solution snmp timer is a wanted by swss service, but since OnBootSec timer expires only once it will not trigger snmp service, so I added line "OnUnitActiveSec=0 sec" which will start snmp service based on the last time it was active. On boot only OnBootSec will expire, on swss start/restarts only second timer will expire immediately and trigger snmp service.
However, snmp service will not stop after "systemctl stop snmp" because of the second timer which will always expire when snmp service because unavailable.
So there is a conflict which will be handled by systemd if we add "Conflicts=" line to both snmp.service and snmp.timer.
So during boot:
snmp does not start by default
swss starts and starts snmp timer
OnUnitActiveSec=0 does not expire since there is no snmp active
OnBootSec expires and starts snmp service and snmp timer gets stopped
During "systemctl restart swss"
snmp stops because of Requisite on swss
snmp unblocks snmp timer from running
swss starts and starts snmp timer
OnUnitActiveSec=0 expires imidiately and start snmp which stops snmp timer
During "systemctl stop snmp"
stop of snmp service unblocks snmp timer but no one starts the timer so it is not started by "OnUnitActiveSec=0"
