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130 lines
4.4 KiB
Groff
130 lines
4.4 KiB
Groff
.\" Copyright (c) 2011 The University of Melbourne
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.\" All rights reserved.
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.\"
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.\" This documentation was written by Julien Ridoux at the University of
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.\" Melbourne under sponsorship from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 1, 2011
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.Dt FFCLOCK 4
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.Os
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.Sh NAME
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.Nm FFCLOCK
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.Nd Feed-forward system clock
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.Sh SYNOPSIS
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.Cd options FFCLOCK
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.Sh DESCRIPTION
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The
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.Xr ntpd 8
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daemon has been the dominant solution for system clock synchronisation for many
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years, which has in turn influenced the design of the system clock.
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The ntpd daemon implements a feedback control algorithm which has been
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demonstrated to perform poorly in common use cases.
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.Pp
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Feed-forward clock synchronisation algorithms implemented by an appropriate
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daemon, in concert with the
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.Nm
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kernel support, have been shown to provide highly robust and accurate clock
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synchronisation.
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In addition to time keeping, the
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.Nm
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kernel mechanism provides new timestamping capabilities and the ability to
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use specialised clocks.
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Feed-forward synchronisation is also very well suited for virtualised
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environments, reducing the overhead of timekeeping in guests and ensuring
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continued smooth operation of the system clock during guest live migration.
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.Pp
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The
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.Nm
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kernel support provides feed-forward timestamping functions within the kernel
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and system calls to support feed-forward synchronisation daemons
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.Po see
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.Xr ffclock 2
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.Pc .
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.Ss Kernel Options
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The following kernel configuration options are related to
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.Nm :
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.Pp
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.Bl -tag -width ".Dv FFCLOCK" -compact
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.It Dv FFCLOCK
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Enable feed-forward clock support.
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.El
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.Ss Configuration
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When feed-forward clock support is compiled into the kernel, multiple system
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clocks become available to choose from.
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System clock configuration is possible via the
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.Va kern.sysclock
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.Xr sysctl 8
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tree which provides the following variables:
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.Bl -tag -width " " -offset indent
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.It Va kern.sysclock.active
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Name of the current active system clock which is serving time.
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Set to one of the names in
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.Va kern.sysclock.available
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in order to change the default active system clock.
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.It Va kern.sysclock.available
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Lists the names of available system clocks
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.Po
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read only
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.Pc .
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.El
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.Pp
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Feed-forward system clock configuration is possible via the
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.Va kern.sysclock.ffclock
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sysctl tree which provides the following variables:
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.Bl -tag -width " " -offset indent
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.It Va kern.sysclock.ffclock.version
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Feed-forward clock kernel version
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.Po
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read only
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.Pc .
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.It Va kern.sysclock.ffclock.ffcounter_bypass
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Use reliable hardware timecounter as the feed-forward counter.
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Will eventually be useful for virtualised environment like
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.Xr xen 4 ,
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but currently does nothing.
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.El
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.Sh SEE ALSO
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.Xr clock_gettime 2 ,
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.Xr ffclock 2 ,
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.Xr bpf 4 ,
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.Xr timecounters 4 ,
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.Xr sysctl 8
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.Sh HISTORY
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Feed-forward clock support first appeared in
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.Fx 10.0 .
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.Sh AUTHORS
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.An -nosplit
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The feed-forward clock support was written by
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.An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
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in collaboration with
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.An Darryl Veitch Aq Mt dveitch@unimelb.edu.au
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at the University of Melbourne under sponsorship from the FreeBSD Foundation.
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.Pp
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This manual page was written by
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.An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
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and
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.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
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