"Ron Frazier (NTP)" <timekeepingntpl...@c3energy.com> wrote in message
news:4f61168b.8030...@c3energy.com...
[]
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.
Windows is not designed as a real-time OS, and its timekeeping is not as
good as other systems, but with one good local server you can sync Windows
PCs to that server with something like: minpoll 5 maxpoll 5. Obviously,
you would not do that over the Internet without permission.
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, 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 jitter doesn't help any.
Windows itself doesn't vary the clock frequency unless you enable the
power-saving frequency of the chipset. What I /have/ seen on Windows is
poorly written third-party drivers which hold off interrupts for too long
or are otherwise badly behaved, and chipsets or motherboards which are
equally unfriendly. I have one application which can ruin timekeeping,
given half a chance! Note the scale on PC Gemini which has an AMD
processor on an ASUS A8N SLI motherboard. +/- 100 ms, not +/- 3 ms. I
see large offset steps, which NTP tries its best to keep up with.
[]
The point is, that it's a continually moving target. The windows clock
is the same way. It never runs at the same frequency from minute to
minute. Even if you get it running right one minute, it's wrong the
next.
All PCs are a continually moving target! Some move more than others, but
give a good PPS source, the major variation on even Windows PCs is due to
temperature fluctuations.
Let's take, for example, the TAZ computer I mentioned earlier. Forget
GPS for the moment. With the default settings, NTPD will eventually be
polling the internet server every 16 minutes. The problem is not
exclusively that there is jitter in the time retrieved from the time
server. Let's also forget that for the moment. Say we poll the
internet server and it says the time is exactly 12:00:00. Rounding to
the seconds level just for simplicity, say my clock says 12:00:02, so
I'm 2 seconds fast. So, we slow down the clock by tweaking its
parameters. Then, we wait 16 more minutes. Now the time server says
12:16:00. Say my clock says 12:15:57. Now, I'm 3 seconds slow, so we
speed the clock back up. Now, theoretically, just like my pendulum
clock, I should be able to get the parameters dialed in so the clock
keeps time. However, behind the scenes, the intrinsic operational speed
changed. While I'm sure I'm butchering the internal technicalities,
let's say the clock has a speed knob, and if we set the speed knob to a
value of 100, the the clock will count exactly 1 second while exactly 1
second passes. If this stayed true, NTPD would eventually set the speed
knob at 100 and everybody would be happy. But the intrinsic speed of
the oscillator changed, so that now setting that knob at 100 now makes
the clock think 1.03 seconds has passed when, in actuality, 1 second has
passed. So the clock is running fast. So, NTPD dials the speed knob
back to 97. But now, the oscillator has changed again so that a setting
of 97 now makes the clock think that .95 seconds have passed, when, in
actuality, 1 second has passed. This is why I'm getting such wild
oscillation in the graph. No matter what NTPD does to tweak the clock
speed, and no matter how accurate that is at the time, that adjustment
never has the same effect the next time the time server is polled. Just
as setting the screw on the pendulum of my mechanical clock doesn't have
the same effect tomorrow as it does today. There will never be a
setting that just works.
I don't think it's a game you can win on a standard windows computer.
So, I'm not even inclined to play the internet NTP server game. So, by
polling my GPS every 8 seconds, the computer has much less time to
drift, no matter what the oscillator is doing, and it needs much less
correction. Then, NTPD can adapt dynamically and almost instantly to
put in whatever adjustments are needed at that moment in time. The
overall net effect, assuming the GPS is accurate, is that the computer's
clock is never more than 10 ms away from GPS time, rather than being 50
ms away from an internet time server's time. With PPS, I could get it
much better still.
Sincerely,
Ron
Basically you have a trade-off between frequency error and time offset.
IIRC, polling quickly will reduce the offset, but you get greater
frequency variations. Once you have one good PPS-synced server, you will
be able to sync other PCs to that (within the limits of Wi-Fi). There are
folk here who may even pipe PPS signals to many PCs to sync them up. I
have a very minor version of that idea as my GPS 18 LVC send output is
connected to two PCs in parallel - both with RS-232 ports. They are PCs
Pixie and Bacchus on the graphs here:
http://www.satsignal.eu/mrtg/performance_ntp.php
Note how high CPU causes a temperature shift on Bacchus, and hence up to 1
ms offset shift. That's from a poorly written disk defragmentation
facility, but the newer free ones mostly don't work on Windows 2000
Server!
http://www.satsignal.eu/mrtg/performance_bacchus.php
Cheers,
David
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