[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

David L. Mills mills at udel.edu
Fri Jan 25 15:50:52 UTC 2008


Well, I have done a market survey of sorts, if you can count my 
consulting clients. There seems general agreement that 1 ms is a good 
target, but there is a wide range of expecttions on how quickly that 
must be achieved. Actually, if the TOY chip is within 1 PPM and the 
downtime is less than 1000 s, convergence is essentially instantaneous. 
My advice to the Aegis crew was to isolate the NTP puppies on the fire 
control Ethernet and allow only a couple of other computers on the wire. 
Crony would work just fine.

Here's another contribution to the market survey. There is a seismic 
network on the sea floor off the Washington state coast. They need a 
millisecond for experiments lasting months, not just 8-hour shifts, and 
that when the experiment boxes get rather warm. Crony might work here as 
well, but it would have to track large swings in temperature.

Here's another one. National Public Radio (NTP) distributes almost all 
program media via IP and digital satellite. They don't need 1 ms, but 
they do need good stability in the face of highly variable transmission 
delays that could drive crony nuts.

And another one. A transatlantic link used by Ford Motor was once a 
statistical multilexor that interleaved terminal keystrokes on a 
demand-assigned basis. Toss NTP packets in that mess and watch the huge 
jitter. That not only drove NTP nuts, it drove the TCP retransmission 
algorithm nuts, too.

Seems like the market is highly fragmented.

I hear you say "100 ms" which I interpret as 100 milliseconds. Even 25 
year old fuzzballs could to much better than that on the congested 
ARPAnet. Did you mean 100 microseconds?


David Woolley wrote:

> David L. Mills wrote:
>> 5. This flap about the speed of convergence has become silly. Most of 
>> us are less concerned about squeezing to the low microseconds in four 
> Have you done the market surveys to confirm this?  I don't have the 
> resources or time to do that, but my impression from the sort of 
> questions that appear on this newsgroup is that most IT managers and 
> turnkey system developers who want better than 100ms clock accuracy want 
> one or both of:
> - fast convergence (small compared with overall bootup time) - a
>   a common case, these days, is that they are not allowed to process
>   financial transactions until convergence is complete;
> - strict monotonicity.
> It may well be that most users don't need better than 100ms, but those 
> users don't care about long term stability, and their long term may be 
> an 8 hour shift.
> (My interest in NTP is more theoretical, as I work in an industry sector 
> that, whilst it deals with timestamped data, those timestamps are often 
> a minute or two out (and are added by equipment that is out of our 
> control), but I do notice the sorts of questions that keep coming up 
> time and time again.)

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