On 2012-03-15, Ron Frazier (NTP)<timekeepingntpl...@c3energy.com> wrote:
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?
Yes. The poll interval is whatever you ( or ntp sets it to). Ie, it goes
to the externam machine to get data that often. However, ntpd only uses
approximately 20% of the values it gets from that remote machine. Thus
as far as the ntp algorithm on your machine is concerned its effective
poll interval is much longer.
But why in the world are you going to the NIST servers for your time?
Use the pool. A stratum 2 or 3 server can certainly give you the ms
accuracy you apparently want. And if you are using gps, the only purpose
of the remote servers is a) fallback, and b) getting the second right.
None of those require NIST.
You can cut your polling of NIST to infinity without affecting your
time.
Allow me to explain. A long time ago in a galaxy far far away, I wanted
to synchronize the clocks of my Windows machines. This was before I
knew anything about Linux or NTP. However, being a US citizen, I knew
our government provided the National Institute of Standards and
Technology (NIST) agency, and that they had a Time and Frequency
division, just to help handle such things for US citizens and the world.
So, I went here to the Time and Frequency division (the link address
may have changed over time):
http://www.nist.gov/pml/div688/
And then here to the Internet Time Service:
http://www.nist.gov/pml/div688/grp40/its.cfm
And I downloaed their little Windows time program here:
http://www.nist.gov/pml/div688/grp40/upload/nistime-32bit.exe
This thing is designed to poll the NIST servers (and only the NIST
servers) up to every 4 hours and set your clock. That's all it does,
and I don't think it tries to tinker with the clock frequency. I ran
that for many years, and still, sometimes, would get a couple of seconds
drift between polling intervals. There is nothing on the website to
discourage average Joe users from using the system.
A couple of years ago, I learned about and started using Ubuntu. For
timekeeping there, I used NTP. At the time, there was a graphical
interface to it and I picked about 8 random servers by ticking the check
boxes and let it run.
Back around Christmas 2011, I got some atomic wall clocks and an atomic
watch and got fascinated with the idea of getting more accuracy for the
PC's. That led me to doing reserach and that led me to install the
Meinberg port of NTP. Of course, then, I had to figure out how to add
servers and set up ntp.conf.
In answer to your question, I'm using NIST because:
a) My government provides it and, through their website,
encourages the public to use it.
b) I was familiar with it.
c) I thought it would be more accurate.
Having said that, I could discontinue using it if the need arises.
So, I put 4 NIST servers, 4 stratum 2 servers, 4 US pool servers, and
just for good measure, the Ubuntu time server in my ntp.conf. Since all
queries exiting my house show the same IP address, each computer prefers
to use a different NIST server so one server doesn't see as many
apparent requests.
Not long after getting NTP working on Windows, I decided to get into the
whole GPS thing. Hence, all the discussions I've had on this list. My
current goal is to set up my own GPS time server, that doesn't wander,
and use internet servers as a backup, as you said.
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.
Then there is something extremely wrong with those peers. Stop using
them.
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
AGain, that is not usual and indicates something is wrong. It is not use
use of the internet. Something else is very wrong. Using an internet
peer 50ms away I get jitter of less than 100us. (not ms), and I think
that experience is far more typical. If it is your network that is the
problem then it is especially silly to use a source like NIST, since its
is being totally wasted.
I respectfully suggest, that, if you're not in the US, then your
experience is probably not typical of what goes on in the US. Even in
the country, there is very much variation in performance of the
internet. I cannot say whether my network, my ISP, or my country's
internet, or all three cause the poor NTP performance. It is what it
is. It's what I have to deal with. Keep in mind that, hypothetically,
since I'm in the Southeast, if I were to poll a server in the Northwest,
that would be a 7000 mile round trip for the data, probably crossing 15
or more routers. I've given up trying to get great performance out of
internet NTP servers. Even with my crummy (for timekeeping) little USB
only GPS, I'm getting 10 times better performance from it when it works
right than I do from the internet. I'm just going to set up my own time
server and not worry about what the internet servers are doing.
Sincerely,
Ron
--
(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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