mirror of
https://github.com/freebsd/freebsd-src
synced 2024-09-06 17:18:32 +00:00
![Brooks Davis](/assets/img/avatar_default.png)
Remove core system call implementations and documentation to lib/libsys and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>. Update paths to allow libc to find them in their new home. Reviewed by: kib, emaste, imp Pull Request: https://github.com/freebsd/freebsd-src/pull/908
176 lines
6.3 KiB
Groff
176 lines
6.3 KiB
Groff
.\" Copyright (c) 2011 The University of Melbourne
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" This documentation was written by Julien Ridoux at the University of
|
|
.\" Melbourne under sponsorship from the FreeBSD Foundation.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.Dd November 21, 2011
|
|
.Dt FFCLOCK 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm ffclock_getcounter ,
|
|
.Nm ffclock_getestimate ,
|
|
.Nm ffclock_setestimate
|
|
.Nd Retrieve feed-forward counter, get and set feed-forward clock estimates
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In sys/timeffc.h
|
|
.Ft int
|
|
.Fn ffclock_getcounter "ffcounter *ffcount"
|
|
.Ft int
|
|
.Fn ffclock_getestimate "struct ffclock_estimate *cest"
|
|
.Ft int
|
|
.Fn ffclock_setestimate "struct ffclock_estimate *cest"
|
|
.Sh DESCRIPTION
|
|
The ffclock is an alternative method to synchronise the system clock.
|
|
The ffclock implements a feed-forward paradigm and decouples the timestamping
|
|
and timekeeping kernel functions.
|
|
This ensures that past clock errors do not affect current timekeeping, an
|
|
approach radically different from the feedback alternative implemented by the
|
|
ntpd daemon when adjusting the system clock.
|
|
The feed-forward approach has demonstrated better performance and higher
|
|
robustness than a feedback approach when synchronising over the network.
|
|
.Pp
|
|
In the feed-forward context, a
|
|
.Em timestamp
|
|
is a cumulative value of the ticks of the timecounter, which can be converted
|
|
into seconds by using the feed-forward
|
|
.Em clock estimates .
|
|
.Pp
|
|
The
|
|
.Fn ffclock_getcounter
|
|
system call allows the calling process to retrieve the current value of the
|
|
feed-forward counter maintained by the kernel.
|
|
.Pp
|
|
The
|
|
.Fn ffclock_getestimate
|
|
and
|
|
.Fn ffclock_setestimate
|
|
system calls allow the caller to get and set the kernel's feed-forward clock
|
|
parameter estimates respectively.
|
|
The
|
|
.Fn ffclock_setestimate
|
|
system call should be invoked by a single instance of a feed-forward
|
|
synchronisation daemon.
|
|
The
|
|
.Fn ffclock_getestimate
|
|
system call can be called by any process to retrieve the feed-forward clock
|
|
estimates.
|
|
.Pp
|
|
The feed-forward approach does not require that the clock estimates be retrieved
|
|
every time a timestamp is to be converted into seconds.
|
|
The number of system calls can therefore be greatly reduced if the calling
|
|
process retrieves the clock estimates from the clock synchronisation daemon
|
|
instead.
|
|
The
|
|
.Fn ffclock_getestimate
|
|
must be used when the feed-forward synchronisation daemon is not running
|
|
.Po see
|
|
.Sx USAGE
|
|
below
|
|
.Pc .
|
|
.Pp
|
|
The clock parameter estimates structure pointed to by
|
|
.Fa cest
|
|
is defined in
|
|
.In sys/timeffc.h
|
|
as:
|
|
.Bd -literal
|
|
struct ffclock_estimate {
|
|
struct bintime update_time; /* Time of last estimates update. */
|
|
ffcounter update_ffcount; /* Counter value at last update. */
|
|
ffcounter leapsec_next; /* Counter value of next leap second. */
|
|
uint64_t period; /* Estimate of counter period. */
|
|
uint32_t errb_abs; /* Bound on absolute clock error [ns]. */
|
|
uint32_t errb_rate; /* Bound on counter rate error [ps/s]. */
|
|
uint32_t status; /* Clock status. */
|
|
int16_t leapsec_total; /* All leap seconds seen so far. */
|
|
int8_t leapsec; /* Next leap second (in {-1,0,1}). */
|
|
};
|
|
.Ed
|
|
.Pp
|
|
Only the super-user may set the feed-forward clock estimates.
|
|
.Sh RETURN VALUES
|
|
.Rv -std
|
|
.Sh ERRORS
|
|
The following error codes may be set in
|
|
.Va errno :
|
|
.Bl -tag -width Er
|
|
.It Bq Er EFAULT
|
|
The
|
|
.Fa ffcount
|
|
or
|
|
.Fa cest
|
|
pointer referenced invalid memory.
|
|
.It Bq Er EPERM
|
|
A user other than the super-user attempted to set the feed-forward clock
|
|
parameter estimates.
|
|
.El
|
|
.Sh USAGE
|
|
The feed-forward paradigm enables the definition of specialised clock functions.
|
|
.Pp
|
|
In its simplest form,
|
|
.Fn ffclock_getcounter
|
|
can be used to establish strict order between events or to measure small time
|
|
intervals very accurately with a minimum performance cost.
|
|
.Pp
|
|
Different methods exist to access absolute time
|
|
.Po or
|
|
.Qq wall-clock time
|
|
.Pc tracked by the ffclock.
|
|
The simplest method uses the ffclock sysctl interface
|
|
.Va kern.ffclock
|
|
to make the system clock return the ffclock time.
|
|
The
|
|
.Xr clock_gettime 2
|
|
system call can then be used to retrieve the current time seen by the
|
|
feed-forward clock.
|
|
Note that this setting affects the entire system and that a feed-forward
|
|
synchronisation daemon should be running.
|
|
.Pp
|
|
A less automated method consists of retrieving the feed-forward counter
|
|
timestamp from the kernel and using the feed-forward clock parameter estimates
|
|
to convert the timestamp into seconds.
|
|
The feed-forward clock parameter estimates can be retrieved from the kernel or
|
|
from the synchronisation daemon directly (preferred).
|
|
This method allows converting timestamps using different clock models as needed
|
|
by the application, while collecting meaningful upper bounds on current clock
|
|
error.
|
|
.Sh SEE ALSO
|
|
.Xr date 1 ,
|
|
.Xr adjtime 2 ,
|
|
.Xr clock_gettime 2 ,
|
|
.Xr ctime 3
|
|
.Sh HISTORY
|
|
Feed-forward clock support first appeared in
|
|
.Fx 10.0 .
|
|
.Sh AUTHORS
|
|
.An -nosplit
|
|
The feed-forward clock support was written by
|
|
.An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
|
|
in collaboration with
|
|
.An Darryl Veitch Aq Mt dveitch@unimelb.edu.au
|
|
at the University of Melbourne under sponsorship from the FreeBSD Foundation.
|