[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.

-- 
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