[ntp:questions] time delta between clients
Kevin Oberman
oberman at es.net
Wed Dec 12 00:41:05 UTC 2007
> From: Rick Jones <rick.jones2 at hp.com>
> Date: Wed, 12 Dec 2007 00:21:28 +0000 (UTC)
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
>
>
> 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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>
Netperf is not really the best way to go. The appropriate tool for
one-way latency is OWAMP. http://e2epi.internet2.edu/owamp/
It requires well synchronized time which is where I became seriously
involved in this area. We try to keep our OWAMP server clocks in sync to
<5 microseconds. (And we almost always do.) They are scattered all over
out backbone from coast to coast.
--
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751
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