NMIS has multiple poll cycles, every 5 minutes an SNMP poll cycle is run, this is also when services are polled.  If fpingd.pl is being used it will poll nodes every ~1 minute by default.  It is recommended to use fpingd.pl when using NMIS, which will use ICMP to poll nodes with higher frequency than the regular NMIS poll cycle.  During the fping poll cycle, the escalation process is also run, this helps to ensure that notifications are sent out with minimal delay, and enables highly granular escalation policies.

NMIS will try to make sure it's own fpingd.pl is running every time a collect or update is run if configured to use it (detailed below).  If fpingd is not enabled or the loss value for the node to ping cannot be found in the fping status file nmis-fping.nmis or nmis-fping.json (on newer systems) then NMIS will run a ping of it's own.  This code can be found in nmis.pl in sub runPing.

fpingd

fpingd is enabled by default, the config setting (in Config.nmis) to control this is:

'daemon_fping_active' => 'true',

fpingd runs a constant cycle of pinging all nodes updating the NMIS event table, sleeping and then doing it all over again. Every 10 poll cycles the Nodes.nmis file is re-read to look for changes.

Nodes are split into groups of size fastping_node_poll and then fping is run on these sets, one after another.  After all the pings are complete fpingd takes the results and updates the NMIS event table, when that is complete NMIS escalations are run and then the daemon will sleep for fastping_sleep + a random value between 0 and 10.

fastping_timeout is an important configuration variable when investigating ping loss, as any ping value higher than this number will be considered loss, 300 ms may be too low if you have a high latency network and are experiencing high ping loss on nodes that appear pingable.

Here is a table of the config variable and what it controls, these are all set in Config.nmis where you can find the default values: