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doc: improve FAQ section on improving accuracy

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Miroslav Lichvar 2014-12-17 16:28:43 +01:00
parent 395c89cff2
commit 7a90dab8ff
1 changed files with 19 additions and 8 deletions
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chrony.texi.in
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@ -4747,16 +4747,27 @@ they are @code{minpoll}, @code{maxpoll}, @code{polltarget}, @code{maxdelay},
The first three options set the minimum and maximum allowed polling interval,
and how should be the actual interval adjusted in the specified range. Their
default values are suitable for public NTP servers, which normally don't allow
too frequent polling, but if you run your own NTP servers or have permission to
poll the servers frequently, setting the options for shorter polling intervals
may significantly improve the accuracy of the system clock.
default values are 6 (64 seconds) for @code{minpoll}, 10 (1024 seconds) for
@code{maxpoll} and 6 (samples) for @code{polltarget}. The default values
should be used for general servers on the internet. With your own NTP servers
or if have permission to poll some servers more frequently, setting these
options for shorter polling intervals may significantly improve the accuracy of
the system clock.
The optimal polling interval depends on many factors, this includes the ratio
The optimal polling interval depends on many factors, including the ratio
between the wander of the clock and the network jitter (sometimes expressed in
NTP documents as the Allan intercept), the temperature sensitivity of the
crystal oscillator and the maximum rate of change of the temperature. An
example of the directive for a server located in the same LAN could be
crystal oscillator and the maximum rate of change of the temperature.
An example of the directive for an NTP server on the internet that you are
allowed to poll frequently could be
@example
server foo.example.net minpoll 4 maxpoll 6 polltarget 16
@end example
An example using very short polling intervals for a server located in the
same LAN could be
@example
server ntp.local minpoll 2 maxpoll 4 polltarget 30
@ -4765,7 +4776,7 @@ server ntp.local minpoll 2 maxpoll 4 polltarget 30
The maxdelay options are useful to ignore measurements with larger delay (e.g.
due to congestion in the network) and improve the stability of the
synchronisation. The @code{maxdelaydevratio} option could be added to the
previous example
example with local NTP server
@example
server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2