[lttng-dev] Syncing traces with NTP

[jon bird]

Hi,

I'm having some issues trying to correlate timings creating from lttng
with another machine. In essence, I'm trying to perform an end to timing
test which involves sending a UDP packet to the box (which is running
lttng[2.8] to track various events), which then does some work on the
payload and sends it back to the orignator. Prior to transmission and
after reception I print out the start/end times retrieved from
clock_gettime using CLOCK_REALTIME.

Both machines are sync'd up to NTP, I use "ntpq -p" to display the time
offset between the two (I've also used "clockdiff" which confirms this
same offset).

Therefore, my thinking is that I should be able to apply this offset to
the timestamps generated by lttng to have them referenced to the same
clock. The issue I'm finding though is that the timestamps seems wildely
out - in some instances even by a 2-3 seconds (when I'm showing an offset
between the two machines of just a few milliseconds).

So I'm a little puzzled by what I'm seeing here and how to proceed. Any
help appreciated.

Rgs,


Jon.


== mailto:jon at onasticksoftware.co.uk - in real life jon bird
==  http://www.onasticksoftware.co.uk - stuff, on-a-stick
==   "men love women, women love children, children love hamsters"
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