Miroslav,
No, it not expected, unless you are referring to broadcast mode when
started with +-100 ms initial offset. That has been corrected as per
your bug report.
For record, a hold timer is started when the first update is received
after startup and ends when the residual offset is less than 0.5 ms or
after a timeout of 600 s. During the hold interval the PLL loop time
constant is set very low and the frequency discipline is disabled. With
this arrangement, the offset typical converges within 600 s, even with
initial offsets up to +-100 ms, and much less if the initial offset is
in the 10-50 ms range. If you see different behavior, either with
client/server or broadcast modes, please report.
Note that, if the initial frequency error is significant, there may
still be a surge correction. If the frequency file is not present at
startup, the frequency will be measured, typically within +-1 PPM,
within 600 s, following with the above scheme will be in effect. Under
worst case conditions, there still could be a wobble following startup
not exceeding 1 ms. If somebody finds an extraordinarily unlikely y set
of circumstances leading to, say 2 ms, I'm not going to lose sleep over
that.
Dave
The Miroslav Lichvar wrote:
On Fri, Oct 22, 2010 at 11:39:47AM +0100, David J Taylor wrote:
Thanks, Dave. I may be missing something here, but it seems to me
that 4.2.7p58 still takes a number of hours to reach the accuracy
limits where thermal effects dominate. It's that which matters to
me, rather than something in the first few minutes. I agree the
graphs would not show such short time-scale initial disturbances.
Did the clock frequency change before you started the new version?
I played with the latest ntp-dev a bit and there indeed is a
improvement on start, mainly when the initial offset is around
0.01-0.05s. But the frequency error has to be very small to make a
difference, see these plots:
http://mlichvar.fedorapeople.org/tmp/ntp_start_offset.png
http://mlichvar.fedorapeople.org/tmp/ntp_start_freq.png
Also, I've noticed when ntpd is started without driftfile and the
initial offset is over 0.05 second, the overshoot can easily reach 100
percent, is this expected?
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