We already spotted a perturbing event for the rtpcs interrupts which was the ssh access ! The periodicity was linked to the ssh check from the external Nagios server which, among other things, probes to see if the ssh daemon is capable to handle a session.

The first event was reported here: https://logbook.virgo-gw.eu/virgo/?r=50562  25/01/2021

The second investigation started  around 15/09/2021... we exchanged emails about it with same conclusion: issue doesn't come from ganglia but from ssh nagios monitoring.

The final conclusion is that this jump may be related to the writing to disk of the ssh connection logs since it does not happen on the diskless rtpcs.

Disabling Syslog or journald services has no effect and issue remain.

Emmanuel suggest:

"We have checked more carefully at LAPP, and it seems we also have jumps at each ssh connection, but with a lower amplitude (1 to 2 µs). We are able to produce the same jumps by clearing some caches using:

sudo echo 1 > /proc/sys/vm/drop_caches"

and at the end

"No jump at this time. So no correlation between these caches and the ssh induced jumps."