diff --git a/chrony.texi.in b/chrony.texi.in index a1efb2c..4ea7f96 100644 --- a/chrony.texi.in +++ b/chrony.texi.in @@ -4628,6 +4628,43 @@ On Linux, if @code{chronyd} is compiled with support for Linux capabilities the `-u' option or @code{user} directive in the @file{chrony.conf} file to drop root privileges after start. The configure option @code{--with-user} can be used to drop the privileges by default. + +@subsection How can I improve the accuracy of the system clock with NTP sources? +Select NTP servers that are well synchronised, stable and close to your network. +It's better to use more than one server, three or four is usually recommended as +the minimum, so @code{chronyd} can detect falsetickers and combine measurements +from multiple sources. + +There are also useful options which can be set in the @code{server} directive, +they are @code{minpoll}, @code{maxpoll}, @code{polltarget}, @code{maxdelay}, +@code{maxdelayratio} and @code{maxdelaydevratio}. + +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. + +The optimal polling interval depends on many factors, this includes 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 + +@example +server ntp.local minpoll 2 maxpoll 4 polltarget 30 +@end example + +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 +server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2 +@end example + @c }}} @c {{{ S:Computer is not synchronising @node Computer is not synchronising