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FFCLOCK(4) FreeBSD Kernel Interfaces Manual FFCLOCK(4)
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.
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>.
FreeBSD 14.0-RELEASE-p11 December 1, 2011 FreeBSD 14.0-RELEASE-p11