freebsd/lib/libc/sys/ffclock.2

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.\" This documentation was written by Julien Ridoux at the University of
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.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.