On 3/15/2012 11:42 AM, unruh wrote:
On 2012-03-14, Ron Frazier (NTP)<timekeepingntpl...@c3energy.com> wrote:
On 3/14/2012 5:04 PM, Ron Frazier (NTP) wrote:
On 3/14/2012 4:00 PM, David J Taylor wrote:
Hi David T,
NOW .... you understand.
<snip>
PS to my prior message.
I don't think the problem so much is the delay to the internet servers,
or even to get out of my house. NTPD is supposed to take care of that
as long as it's pretty much symmetrical. I think the problem is that
the Windows clock is like a wild tiger that doesn't want to be tamed and
which is running every which way. For whatever reason, cpu load, heat,
cosmic vibrations, whatever, the intrinsic frequency of the windows
clock is always changing. In order to avoid beating up on the internet
servers too much, I have to poll them at least every 4 minutes apart.
If you let it, NTPD will extend that out to 16 minutes or more. So,
Actually, the effective NTPD poll interval is abotu 8 times the stated
interval. The clock filter throws away about 7 out of 8 poll results in
an attempt to get rid of assmetric polls. Ie, it assumes that the
shortest round trip interval out of the past 8 is the best estimate of
the symmetric roundtrip and throws away the rest. Thus if you have
polling every 4 min (poll interval 8) the effective interval is about
every half hour.
That is fine if the clock is an even half way reasonable clock (Ie rate does
not change by more than say 2PPM over that time)
You're saying the effective polling interval is 8x what minpoll is.
However, if the access policy for a NIST server is no more than 20
times per hour or every 3 minutes, and I set minpoll to 6 or
approximately every minute, even if the clock algorithm throws away 7 of
8 samples; am I not still sampling? Am I not still "hitting" the NIST
server every minute and are they not going to ban me from accessing it
if that continues?
when the clock source is polled, say the PC clock is too fast, so NTPD
slows it down. Then, when you poll the clock source again, say the PC
clock is too slow, so NTPD speeds it up. Because of the varying
intrinsic frequency of the clock, you can never find a clock speed that
just works, because then the system goes and changes, by changes in the
oscillator, how much time passes at those particular settings. It's a
battle you cannot win. By polling my GPS every 8 seconds, I can keep
the clock under control based on it's current needs which are varying
second by second. Of course, when discussing internet servers, 30 ms of
What are you talking about. There is no evicence either in your data or
in any reports by anyone of 30ms variation is network offsets.
Even on ADSL, it is in the microsecond range, not millisecond.
I'm not sure exactly what you're asking. If you're referring to my
comment about internet peer jitter, I occasionally see jitter numbers
for internet peers on the Meinberg Time server monitor screen in the 20
- 30 ms range and more frequently see numbers in the 10 - 20 ms range
for jitter. Here is a recent screen shot:
http://dl.dropbox.com/u/9879631/internet%20jitter%20example.jpg
Note that there are three peers with jitter in the 10 - 20 ms range.
If you were asking about the offsets my computers experience using the
internet as a time source, my TAZ computer polls the internet
exclusively and it's offsets routinely fluctuate + / - 50 ms.
http://dl.dropbox.com/u/9879631/TAZ%20loopstats%202012-03-07%20to%202012-03-14.jpg
http://dl.dropbox.com/u/9879631/ntp.conf-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120313-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120314-TAZ
Sincerely,
Ron
jitter doesn't help any.
<snip>
--
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
Ron Frazier
timekeepingdude AT c3energy.com
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