[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

David L. Mills mills at udel.edu
Tue Jan 22 14:56:38 UTC 2008


Guys,

I haven't read every word on this thread, but all I can contribute is 
that nothing reported here is anything like my experience here. Our 
servers pogo.udel.edu and rackety.udel.edu are synchronized via GPS and 
PPS. I invite the skeptics to peek at them from time to time. I describe 
their behavior as like cats; most of the time they are quiet and gentle 
at a few microseconds, but once in a whild they show a surge of ten 
microseconds or more, especially after a power failure, which we do get 
from time to time.

There is a persistent report that appears as a low-frequency ringing 
with more or less constant period. This would seem to suggest something 
wrong with the discipline loop transient response. In the past the most 
likely cause has been an ill-advised tinker with the Unix adjtime() 
system call with the dubious purpose of reducing the time to slew the 
clock over some range. This wrecks the transient response and easily 
leads to loop instability. If you are using the kernel time discipline 
and not adjtime() this is not an issue.

There are lots of ways to measure the loop transient response. The 
easiest way is to set the clock some 50-100 ms off from some stable 
source (not necessarily accurate) and watch the loop converge. The 
response should cross zero in about 3000 s and overshoot about 6 percent 
and smoothly amortize over several hours. Be sure to clamp the poll 
interval to 64 s over that period. If it does something else, like show 
an exponentially decreasing ringing. Go looking for trouble.

As for "offset should be much larger than the error", be careful here. 
By error I assume you mean what ntpq rv shows as jitter. The best case 
is when offset is indeed less than jitter; if the error is much larger 
than error, this suggests the frequency has surged and the time 
constant/poll interval needs to be reduced. Watch the poll interval 
behavior in the loopstats data.

Dave

David Woolley wrote:

> In article <eI8lj.17461$yQ1.5617 at edtnps89>,
> Unruh <unruh-spam at physics.ubc.ca> wrote:
> 
> 
>>No, the offset is the value reported in loopstats.
> 
> 
> Same thing.  If chrony is reporting the same measurements, neither set of
> measurements is particularly valid.  You need to measure the actual
> offsets, using something that has a repeatability a couple of orders
> of magnitude better.  Certainly for ntpd, offset should be much larger
> than the error, when locked.  Is the server running ntpd?
> 
> Anyway, as I said, arguing by proxy is difficult and I'm rather hoping that
> Dave Mills will take over.  Certainly it is Dave Mills you have to 
> convince if ntpd is going to change.
> 




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