Miroslav Lichvar wrote:
On Mon, Jan 26, 2015 at 01:03:48PM +0100, Terje Mathisen wrote:
One of the good points about Google's smear is the fact that they use a half
cosine to distribute the offset, which means that they have a time function
which is both continuous and monotonic, as well as having an infinite number
of defined derivatives, i.e. it is maximally smooth.
They could have chosen a better function though. If its second
derivative (wander) started at zero, the NTP clients could adjust
their polling interval if necessary before the error gets large and
the maximum error between the clients could be minimized.
Here is a test showing error between two clients of a server
smearing.a large offset. With the cosine function you can see a large
spike when smearing started.
https://mlichvar.fedorapeople.org/tmp/smear_cos.png
https://mlichvar.fedorapeople.org/tmp/smear_sinx.png
This seems wrong!?!
First of all, you seem to extend the smearing over a million seconds or
so? I.e. 10-15 days?
How large is the adjustment to be smeared out?
The google cosine approach starts with a derivate of zero and ends the
same way, I really can't see how that leads to that huge (more than 128
ms!) spike at the start?
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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