[ntp:questions] Sub-millisecond NTP synchronization for local network

Terje Mathisen terje.mathisen at hda.hydro.com
Sun Dec 7 10:44:18 UTC 2008


Maarten Wiltink wrote:
> "Unruh" <unruh-spam at physics.ubc.ca> wrote in message
> news:S5y_k.2887$yK5.1576 at edtnps82...
> [...]
>>> What should I be doing to get 20 us? Buy all new computers with
>>> gigabit Ethernet?
>> I suspect buying better switches. And it looks to me like you should
>> definitely NOT go to gigabit Ethernet if you want good timing.
> 
> Seeing as how gigabit Ethernet uses jumbo frames to reach decent
> throughput, I suspect it's subject to higher jitter at the network
> level. NTP frames are short, but if it's contending with a large
> download, there will be a relatively large unpredictable delay on
> the line.

I wrote a very similar msg, then I started to consider the math:

100 Mbit uses 1.5 kB packets, while Gbit jumbo frames are typically 9 
kB, right?

The thing is that as long as the jumbo frames are less than 10 times the 
size of regular packets, the wire will be busy for a shorter time than 
for a 1.5 K packet on 100 Mbit!

Hybrid interrupt/polled hardware & drivers are a much more likely 
culprit: If the hardware waits a little bit before interrupting, hoping 
to be able to bunch up several packets in a single task, then jitter 
will suffer badly.

Terje
-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"




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