Chrony default NTP client
Make chrony the default NTP client.
- Name: Miroslav Lichvar
- Email: firstname.lastname@example.org
- Targeted release: Fedora 16
- Last updated: 2011-07-22
- Percentage of completion: 100%
Chrony is an NTP client and server designed to work well without permanent network connection. It uses different algorithms to discipline the clock which gives chrony several advantages over ntp:
- works well even with irregular polling of NTP servers, useful for laptops or systems without permanent network connection
- much faster synchronization, it usually needs only minutes instead of hours to minimize the time and frequency error, useful for systems not running 24/7
- larger range for frequency correction (100000 ppm vs 500 ppm), useful for systems running in virtual machines which may have quickly drifting clocks
- after the initial synchronization the clock is never stepped, this is good for applications which need system time to be monotonic
- better response to rapid changes in the clock frequency, useful for systems running in virtual machines
- better stability with temporary asymmetric delays, e.g. when the link is saturated by a large download
An extensive comparison of ntp and chrony done in a simulator is here. Note that current Linux kernels are compiled with SHIFT_PLL=2.
Using chrony over ntp has also other advantages:
- smaller memory footprint (1.3MB vs 6MB resident size)
- no unnecessary process wakeups, this is good for powersaving. The ntpd process normally wakes up every second. We apply a patch which reduces the number of wakeups significantly, but upstream is not interested in fixing it (ntp bug #802).
There are also some disadvantages:
- ntp may still perform better in some conditions, but the differences are quite small (see the simulations)
- chrony corrects the offset by the adjtime() function which means there may be extra short term frequency errors up to 500 ppm
- advanced NTP features like Autokey authentication are not supported
Benefit to Fedora
Better timekeeping on systems not running 24/7 or without permanent internet connection or with low quality/unstable clocks (virtual machines). Once the clock is synchronized, applications are not upset by backward time jumps.
- system-config-date needs to be patched to support both packages and remove the dependency on ntp. The supported options supported by system-config-date are identical in ntp.conf and chrony.conf, so it's just a matter of finding out which one is installed/enabled and select the correct path and service name. A patch is available (bug #616385).
- gnome-settings-daemon needs to be patched to support enabling and disabling chrony service. A patch is available (bug #723212).
- comps needs to be modified to install chrony by default and mark ntp as an optional package
How To Test
- make fresh installation, or install chrony and uninstall ntp
- start NTP in system-config-date and let it run for few minutes
- check "chronyc tracking" output. The Reference ID field should be a valid NTP server (not 0.0.0.0), System time should be close to zero (less than few milliseconds).
- make sure it works also after reboot, suspend and reconnecting to a network.
system-config-date gnome-settings-daemon comps
Switching the optional/default types for the ntp and chrony packages in the comps will revert to installing ntp by default. system-config-date can still support both packages and doesn't need to depend on any of the two packages.
- Fedora now installs chrony as the default NTP client.