How much do we care about high-load scenarios?
dan-ntp at drown.org
Fri Sep 16 16:11:45 UTC 2016
Quoting Kurt Roeckx <kurt at roeckx.be>:
>> From my own testing with iperf high rate 64 byte UDP packets, max rate
>> before 1% receive packet loss:
>> i3-540 / Intel 82574L nic: ~469kpps
>> Athlon(tm) 64 X2 4400+ / RTL8168 gig nic: ~64kpps
>> Odroid C2: ~62kpps
>> Raspberry Pi 2: ~19kpps
>> Beaglebone Black: ~9kpps
>> Raspberry Pi B+: ~4kpps
> Is there a way to make this ntp packets, or where those ntp
> My guess is that you'll have plenty of CPU to process the maximum
> packets the nic can handle. Once you add crypto this might change
Right, these were iperf tests, so ntp processing will lower those
numbers. Those numbers are "maximum theoretical" which will be higher
than what's actually practical.
Quoting Achim Gratz <Stromeko at nexgo.de>:
> These are throughput numbers, not response time distributions. I think
> it's an established fact from even a back-of-the-envelope calculation
> that NTP doesn't saturate a network link. There's a glimpse of the delay
> distribution getting a fatter tail in the RADclock papers, but that data
> is sadly quite out of date by now.
Correct, my numbers don't include a measure of response time. Proper
measurement of response time (perhaps by using the IEEE1588 timers)
would be nice.
> The average user can't tell the difference since he doesn't know where
> the variability in the remote NTP server comes from. As anecdotal
> evidence, I've had to re-connect my VDSL this past weekend and now one
> of the two PTB servers shows a 100µs difference in offset, while the
> other hasn't changed, for a total of 200µs average difference between
> them. I've moved the PPS offset on the GPS so that on average I'm
> keeping it in the middle of these two. Due to the sequence of events
> it's clear that the network somehow produces this result and not some
> load on a server, but I couldn't know that in the general case.
Yes, network effects when dealing with a WAN can have very significant
effects. From equal-cost multipath routes to other sources of
asymmetric latency, absolute offset numbers get fuzzier as the latency
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