[ntp:questions] time delta between clients
Rick Jones
rick.jones2 at hp.com
Wed Dec 12 00:21:28 UTC 2007
Some folks have been asking me if it is possible to use netperf to get
the one-way latency between two systems, while sending a
unidirectional stream of data from one to the other. Netperf already
has a TCP_RR test (ping-pong) that will report the average round-trip
latency (and optionally a histogram of the individual RTTs). That
though is not a unidirectional test.
So, I am assuming I need synchronized "clocks." Running ntp on each
of the two systems on which I would run netperf is not a problem.
What I'm curious about, and a topic where I would welcome some gentle
taps with clue bats, is if I can take the difference in offset between
each client and the time server and ass-u-me that is the difference in
time between the two clients. Or do I have to do something ntp-like
in netperf itself between the two systems?
My tests will typically only run for oh, O(60) seconds at a time, so
I'm ass-u-me-ing I can ignore issues of the two client clocks rates
changing much.
For example, here is ntpq peers output from a pair of machines where I
might want to do this:
Client 1:
ntpq> peers
remote refid st t when poll reach delay offset jitter
==============================================================================
+oslowest.raj 15.4.81.61 2 u 118 128 377 0.123 -1.602 2.723
+lart.lart 15.235.160.30 5 u 4 128 377 35.639 4.115 1.508
*shovlhead.nashu .TRUE. 1 u 54 128 377 101.115 -5.273 8.190
Client 2:
ntpq> peers
remote refid st t when poll reach delay offset jitter
==============================================================================
+linger.raj 15.4.81.61 2 u 104 128 377 0.151 0.002 0.976
+lart.lart 15.235.160.30 5 u 97 128 377 35.682 3.556 1.003
*shovlhead.nashu .TRUE. 1 u 33 128 377 105.602 -0.839 3.267
I would take the difference in offset - -5.273 - -0.839 - and take
that to be the difference in time between Client 1 and 2.
Thoughts, suggestions, pointers etc most welcome,
rick jones
BTW, in this case, I disabled the interrupt coalescing on Client 1's
NIC, which I believe is the reason for the difference in delay between
Client 1 and 2 and linger.raj - all three are on the same LAN. It
gets lost in the noise talking to the other two time sources.
--
oxymoron n, commuter in a gas-guzzling luxury SUV with an American flag
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
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