Latency and jitter inconsistent

Latency and jitter inconsistent

[Jonathan Weinert]

Hello,

I made various measurements with a ls1021a-twr and a
rt-preemption-patch kernel. I was able to capture the latency with
cyclictest which can be seen in picture 1.

In addtion I captured the periodic jitter of a realtime-application
which outputs a square wave which can be seen in picture 2. This
Applikation was initialised with a priority of 99 as well as the
arguments of cyclictest.

In short I could measure these times with a system without load:

Latency with the command: cyclictest -l 3600000 -m -n -p 99 -i 1000 -> 47 us
Periodic jitter with  prio 99, mlockall, clock_nanosleep and
monotic_clock -> 17.04 us

How is this possible? I know there is the possibility that a
oscilloscope can have blind times but not with that amount of cycles.
(21600000 with a cycle time of 1ms)

Picture 1: http://imgur.com/a/Kaaje
Picture 2: http://imgur.com/a/PNxbo

I would be grateful for an answer!

Best regards

J. Weinert