178 lines
6.3 KiB
Groff
178 lines
6.3 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 November 21, 2011
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.Dt FFCLOCK 2
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.Os
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.Sh NAME
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.Nm ffclock_getcounter ,
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.Nm ffclock_getestimate ,
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.Nm ffclock_setestimate
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.Nd Retrieve feed-forward counter, get and set feed-forward clock estimates
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/timeffc.h
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.Ft int
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.Fn ffclock_getcounter "ffcounter *ffcount"
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.Ft int
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.Fn ffclock_getestimate "struct ffclock_estimate *cest"
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.Ft int
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.Fn ffclock_setestimate "struct ffclock_estimate *cest"
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.Sh DESCRIPTION
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The ffclock is an alternative method to synchronise the system clock.
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The ffclock implements a feed-forward paradigm and decouples the timestamping
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and timekeeping kernel functions.
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This ensures that past clock errors do not affect current timekeeping, an
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approach radically different from the feedback alternative implemented by the
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ntpd daemon when adjusting the system clock.
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The feed-forward approach has demonstrated better performance and higher
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robustness than a feedback approach when synchronising over the network.
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.Pp
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In the feed-forward context, a
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.Em timestamp
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is a cumulative value of the ticks of the timecounter, which can be converted
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into seconds by using the feed-forward
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.Em clock estimates .
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.Pp
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The
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.Fn ffclock_getcounter
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system call allows the calling process to retrieve the current value of the
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feed-forward counter maintained by the kernel.
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.Pp
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The
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.Fn ffclock_getestimate
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and
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.Fn ffclock_setestimate
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system calls allow the caller to get and set the kernel's feed-forward clock
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parameter estimates respectively.
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The
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.Fn ffclock_setestimate
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system call should be invoked by a single instance of a feed-forward
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synchronisation daemon.
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The
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.Fn ffclock_getestimate
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system call can be called by any process to retrieve the feed-forward clock
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estimates.
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.Pp
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The feed-forward approach does not require that the clock estimates be retrieved
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every time a timestamp is to be converted into seconds.
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The number of system calls can therefore be greatly reduced if the calling
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process retrieves the clock estimates from the clock synchronisation daemon
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instead.
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The
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.Fn ffclock_getestimate
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must be used when the feed-forward synchronisation daemon is not running
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.Po see
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.Sx USAGE
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below
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.Pc .
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.Pp
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The clock parameter estimates structure pointed to by
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.Fa cest
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is defined in
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.In sys/timeffc.h
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as:
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.Bd -literal
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struct ffclock_estimate {
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struct bintime update_time; /* Time of last estimates update. */
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ffcounter update_ffcount; /* Counter value at last update. */
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ffcounter leapsec_next; /* Counter value of next leap second. */
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uint64_t period; /* Estimate of counter period. */
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uint32_t errb_abs; /* Bound on absolute clock error [ns]. */
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uint32_t errb_rate; /* Bound on counter rate error [ps/s]. */
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uint32_t status; /* Clock status. */
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int16_t leapsec_total; /* All leap seconds seen so far. */
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int8_t leapsec; /* Next leap second (in {-1,0,1}). */
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};
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.Ed
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.Pp
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Only the super-user may set the feed-forward clock estimates.
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.Sh RETURN VALUES
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.Rv -std
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.Sh ERRORS
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The following error codes may be set in
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.Va errno :
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.Bl -tag -width Er
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.It Bq Er EFAULT
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The
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.Fa ffcount
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or
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.Fa cest
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pointer referenced invalid memory.
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.It Bq Er EPERM
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A user other than the super-user attempted to set the feed-forward clock
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parameter estimates.
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.El
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.Sh USAGE
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The feed-forward paradigm enables the definition of specialised clock functions.
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.Pp
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In its simplest form,
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.Fn ffclock_getcounter
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can be used to establish strict order between events or to measure small time
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intervals very accurately with a minimum performance cost.
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.Pp
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Different methods exist to access absolute time
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.Po or
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.Qq wall-clock time
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.Pc tracked by the ffclock.
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The simplest method uses the ffclock sysctl interface
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.Va kern.ffclock
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to make the system clock return the ffclock time.
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The
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.Xr clock_gettime 2
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system call can then be used to retrieve the current time seen by the
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feed-forward clock.
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Note that this setting affects the entire system and that a feed-forward
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synchronisation daemon should be running.
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.Pp
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A less automated method consists of retrieving the feed-forward counter
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timestamp from the kernel and using the feed-forward clock parameter estimates
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to convert the timestamp into seconds.
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The feed-forward clock parameter estimates can be retrieved from the kernel or
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from the synchronisation daemon directly (preferred).
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This method allows converting timestamps using different clock models as needed
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by the application, while collecting meaningful upper bounds on current clock
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error.
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.Sh SEE ALSO
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.Xr date 1 ,
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.Xr adjtime 2 ,
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.Xr clock_gettime 2 ,
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.Xr ctime 3
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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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