[ntp:questions] Re: NTP4 has 3 different time formats! Namly (32, 64, 128) bits wide
terje.mathisen at hda.hydro.com
Mon Jul 17 06:02:54 UTC 2006
Richard B. Gilbert wrote:
> David L. Mills wrote:
>> I can't claim preconition, as the current NTP timestamp format was
>> invented in 1978 when nominal accuracies were in the 16-ms range.
>> However, the resolution limit of 232 picoseconds is likely to be
>> exceeded when the CPU clock rate approaches 4 GHz, which might not be
>> long off.
> I suppose that even a 2 GHz machine could slice time into 500 picosecond
> increments. But I was thinking in terms of the ability to set a clock
> that accurately. There's no way that I can think of that it could be
> done over a network using today's technology. I'm seeing a ~4us delays
> on my 100 Mb full duplex LAN. I think that means I can't pass time from
> machine A to machine B over my LAN without an uncertainty of ~2us. The
> error is probably less than that but probably is the best we can say.
> So you could get delta time measurments with 232 picosecond resolution
> but getting absolute time accurately with that precsion is not going to
> be easy.
If you can get _repeatable_, at least some of the time, 4 us delays,
then you can use the same statistical methods which NTP is already using
to get absolute accuracy close to an order of magnitude better, i.e.
half a us or so.
The real limiter will be the need for (a) a really good local clock
source, i.e. better than the current 10 cent (?) quartz crystals, and
(b) a hardware method to measure the interrupt latency.
Poul-Henning of FreeBSD and NTP fame did both on his _very_ good NTP
servers, it might be possible that some new motherboards will include a
timing facility to handle (b).
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"
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