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Diffstat (limited to 'lib/libc/sys/ffclock.2')
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diff --git a/lib/libc/sys/ffclock.2 b/lib/libc/sys/ffclock.2 new file mode 100644 index 0000000..0e8f09b --- /dev/null +++ b/lib/libc/sys/ffclock.2 @@ -0,0 +1,177 @@ +.\" 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. +.\" +.\" $FreeBSD$ +.\" +.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 jridoux@unimelb.edu.au +in collaboration with +.An Darryl Veitch Aq dveitch@unimelb.edu.au +at the University of Melbourne under sponsorship from the FreeBSD Foundation. |