[ntp:questions] frequency adjusting only

Ryan Malayter malayter at gmail.com
Tue Apr 22 20:10:37 UTC 2008


On Tue, Apr 22, 2008 at 9:39 AM, Richard B. Gilbert
<rgilbert88 at comcast.net> wrote:
> That's going to be very difficult to do!  Getting the whole herd to the
> point where the difference between any two nodes is less than 50 usec is
> damned near impossible using NTP on a LAN.  You might want to consider
> using a Pulse Per Second (PPS) signal delivered to each node, with NTP
> used only to timestamp the rising edge.  If you compensate for cable
> delays you can probably get to within 50-100 usec.

It likely depends on the LAN. What types of switches are in use? What
NICs? What topology? What traffic? The more "enterprisey" brands,
Cisco, Extreme, Foundry, Force10, HP, etc. typically have much better
queuing behavior and much better latency characteristics. In network
switches, you often actually get what you pay for in terms of
performance and reliability. We use NETGEAR, Linksys, Dell, and other
"value brand" switches for workstation access, and they have erratic
performance characteristics, to put it mildly. Even though they are
marketed as "enterprise-class" gear.

If all the boxes in question had quality Intel NICs and were connected
to the same lightly loaded Cisco 4948, I would guess that 50 µs
average offset might be achievable using NTP.

-- 
RPM



More information about the questions mailing list