[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
unruh-spam at physics.ubc.ca
Fri Jan 25 18:00:33 UTC 2008
"David L. Mills" <mills at udel.edu> writes:
>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.
Your evidence that it would drive chrony nuts is what?
>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.
And you tested chrony on this?
>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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