NAME
FFCLOCK —
Feed-forward system clock
SYNOPSIS
options FFCLOCK
DESCRIPTION
The ntpd(8) daemon has been the dominant solution for system clock synchronisation for many years, which has in turn influenced the design of the system clock. The ntpd daemon implements a feedback control algorithm which has been demonstrated to perform poorly in common use cases.Feed-forward clock synchronisation algorithms implemented by an
appropriate daemon, in concert with the FFCLOCK
kernel support, have been shown to provide highly robust and accurate clock
synchronisation. In addition to time keeping, the
FFCLOCK kernel mechanism provides new timestamping
capabilities and the ability to use specialised clocks. Feed-forward
synchronisation is also very well suited for virtualised environments,
reducing the overhead of timekeeping in guests and ensuring continued smooth
operation of the system clock during guest live migration.
The FFCLOCK kernel support provides
feed-forward timestamping functions within the kernel and system calls to
support feed-forward synchronisation daemons (see
ffclock(2)).
Kernel Options
The following kernel configuration options are related to
FFCLOCK:
FFCLOCK- Enable feed-forward clock support.
Configuration
When feed-forward clock support is compiled into the kernel, multiple system clocks become available to choose from. System clock configuration is possible via the kern.sysclock sysctl(8) tree which provides the following variables:
- kern.sysclock.active
- Name of the current active system clock which is serving time. Set to one of the names in kern.sysclock.available in order to change the default active system clock.
- kern.sysclock.available
- Lists the names of available system clocks (read-only).
Feed-forward system clock configuration is possible via the kern.sysclock.ffclock sysctl tree which provides the following variables:
- kern.sysclock.ffclock.version
- Feed-forward clock kernel version (read-only).
- kern.sysclock.ffclock.ffcounter_bypass
- Use reliable hardware timecounter as the feed-forward counter. Will eventually be useful for virtualised environment like xen(4), but currently does nothing.
SEE ALSO
clock_gettime(2), ffclock(2), bpf(4), timecounters(4), sysctl(8)
HISTORY
Feed-forward clock support first appeared in FreeBSD 10.0.
AUTHORS
The feed-forward clock support was written by Julien Ridoux <jridoux@unimelb.edu.au> in collaboration with Darryl Veitch <dveitch@unimelb.edu.au> at the University of Melbourne under sponsorship from the FreeBSD Foundation.
This manual page was written by Julien Ridoux <jridoux@unimelb.edu.au> and Lawrence Stewart <lstewart@FreeBSD.org>.