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>.

Debian                                          December 1, 2011                                      FFCLOCK(4)