FFCLOCK(2) FreeBSD System Calls Manual FFCLOCK(2)


ffclock_getcounter, ffclock_getestimate, ffclock_setestimateRetrieve feed-forward counter, get and set feed-forward clock estimates


Standard C Library (libc, -lc)


#include < sys/timeffc.h>

ffclock_getcounter( ffcounter *ffcount);

ffclock_getestimate( struct ffclock_estimate *cest);

ffclock_setestimate( struct ffclock_estimate *cest);


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.

In the feed-forward context, a timestamp is a cumulative value of the ticks of the timecounter, which can be converted into seconds by using the feed-forward clock estimates.

The ffclock_getcounter() system call allows the calling process to retrieve the current value of the feed-forward counter maintained by the kernel.

The ffclock_getestimate() and ffclock_setestimate() system calls allow the caller to get and set the kernel's feed-forward clock parameter estimates respectively. The ffclock_setestimate() system call should be invoked by a single instance of a feed-forward synchronisation daemon. The ffclock_getestimate() system call can be called by any process to retrieve the feed-forward clock estimates.

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 ffclock_getestimate() must be used when the feed-forward synchronisation daemon is not running (see USAGE below).

The clock parameter estimates structure pointed to by cest is defined in < sys/timeffc.h> as:

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}). */ 

Only the super-user may set the feed-forward clock estimates.


Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The following error codes may be set in errno:
The ffcount or cest pointer referenced invalid memory.
A user other than the super-user attempted to set the feed-forward clock parameter estimates.


The feed-forward paradigm enables the definition of specialised clock functions.

In its simplest form, ffclock_getcounter() can be used to establish strict order between events or to measure small time intervals very accurately with a minimum performance cost.

Different methods exist to access absolute time (or “wall-clock time”) tracked by the ffclock. The simplest method uses the ffclock sysctl interface kern.ffclock to make the system clock return the ffclock time. The 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.

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.


Feed-forward clock support first appeared in FreeBSD 10.0.


The feed-forward clock support was written by Julien Ridoux <jridoux@unimelb.edu.au> in collaboration with Darryl Veitch <dveitch@unimelb.edu.au> at the University of Melbourne under sponsorship from the FreeBSD Foundation.
November 21, 2011 FreeBSD