ADJTIMEX(8)
NAME
adjtimex - display or set the kernel time variables
SYNOPSIS
adjtimex [--print] [--tick val] [--frequency newfreq]
[--compare[=count]] [--adjust[=count]] [--interval tim]
[--offset val] [--singleshot adjustment] [--maxerror val]
[--esterror val] [--timeconstant val] [--help] [--version]
DESCRIPTION
This program gives you raw access to the kernel time vari-
ables. For a machine connected to the Internet, or
equipped with a precision oscillator or radio clock, the
best way to regulate the system clock is with xntpd(8).
For a standalone or intermittently connected machine, you
may use adjtimex instead to at least correct for system-
atic drift.
Anyone may print out the time variables, but only the
superuser may change them.
If your computer can be connected to the net, you might
run xntpd for at least several hours and use adjtimex
--print to learn what values of tick and freq it settled
on. Alternately, you could estimate values using the CMOS
clock as a reference (see the --compare and --adjust
switches). You could then add a line to rc.local invoking
adjtimex to set those parameters each time you reboot.
OPTIONS
Options may be introduced by either - or --, and unique
abbreviations may be used.
--print
Print the current values of the kernel time vari-
ables. NOTE: The time is "raw", and may be off by
up to one timer tick (10 msec). "status" gives the
value of the time_status variable in the kernel.
For Linux 1.0 and 1.2 kernels, the value is as fol-
lows:
0 clock is synchronized (so the kernel should
periodically set the CMOS clock to match the
system clock)
1 inserting a leap second at midnight
2 deleting a leap second at midnight
3 leap second in progress
4 leap second has occurred
5 clock not externally synchronized (so the
kernel should leave the CMOS clock alone)
For Linux 2.0 kernels, the value is a sum of these:
1 PLL updates enabled
2 PPS freq discipline enabled
4 PPS time discipline enabled
8 frequency-lock mode enabled
16 inserting leap second
32 deleting leap second
64 clock unsynchronized
128 holding frequency
256 PPS signal present
512 PPS signal jitter exceeded
1024 PPS signal wander exceeded
2048 PPS signal calibration error
4096 clock hardware fault
--tick val
Set the number of microseconds that should be added
to the system time for each kernel tick interrupt.
There are supposed to be 100 ticks per second, so
val should be close to 10000. Increasing val by 1
speeds up the system clock by about 100 ppm, or
8.64 sec/day. tick must be in the range
9000...11000.
--frequency newfreq
Set the system clock frequency offset to newfreq.
newfreq can be negative or positive, and gives a
much finer adjustment than the --tick switch. The
value is scaled such that newfreq = 1<<16 speeds up
the system clock by about 1 ppm, or .0864 sec/day.
Thus, --tick 10000 --frequency 6553600 is about the
same as --tick 10001 --frequency 0. newfreq must
be in the range -6553600...6553600, allowing maxi-
mum adjustments of plus or minus 100 ppm.
--compare[=count]
Periodically compare the system clock with the CMOS
clock. After the first two calls, print values for
tick and frequency offset that would bring the sys-
tem clock into approximate agreement with the CMOS
clock. CMOS clock readings are adjusted for sys-
tematic drift using using the correction in
/etc/adjtime -- see clock(8). The interval between
comparisons is 10 seconds, unless changed by the
--interval switch. The optional argument is the
number of comparisons. (If the argument is sup-
plied, the "=" is required.)
--adjust[=count]
Same as --compare, except the recommended values
are actually installed after every other compari-
son.
--interval tim
Set the interval in seconds between clock compar-
isons for the --compare and --adjust options.
--singleshot adj
Slew the system clock by adj usec. (Its rate is
changed temporarily by about 1 part in 2000.)
--offset adj
Add a time offset of adj usec. The kernel code
adjusts the time gradually by adj, notes how long
it has been since the last time offset, and then
adjusts the frequency offset to correct for the
apparent drift. adj must be in the range
-512000...512000.
--maxerror val
Set maximum error (usec).
--esterror val
Set estimated error (usec). The maximum and esti-
mated error are not used by the kernel. They are
merely made available to user processes via the
adjtimex(2) system call.
--timeconstant val
Set phase locked loop (PLL) time constant. val
determines the bandwidth or "stiffness" of the PLL.
The effective PLL time constant will be a multiple
of (1 << val). For room-temperature quartz oscil-
lators, David Mills recommends the value 2, which
corresponds to a PLL time constant of about 900 sec
and a maximum update interval of about 64 sec. The
maximum update interval scales directly with the
time constant, so that at the maximum time constant
of 6, the update interval can be as large as 1024
sec.
Values of val between zero and 2 give quick conver-
gence; values between 2 and 6 can be used to reduce
network load, but at a modest cost in accuracy.
--help Print the program options.
--version
Print the program version.
EXAMPLES
If your system clock gained 8 seconds in 24 hours, you
could set the tick to 9999, and then it would lose 0.64
seconds a day (that is, 1 tick unit = 8.64 seconds per
day). To correct the rest of the error, you could set the
frequency offset to (1<<16)*0.64/.0864 = 485452. Thus,
putting the following in rc.local would approximately cor-
rect the system clock:
adjtimex --tick 9999 --freq 485452
NOTES
adjtimex deals only with the system clock -- the one that
runs while the computer is powered up. To set or regulate
the CMOS clock, see clock(8).
AUTHORS
Steven S. Dick lt;ssd@nevets.oau.org, Jim Van Zandt
lt;jrv@vanzandt.mv.com.
SEE ALSO
date(1L) gettimeofday(2) settimeofday(2) clock(8)
xntpd(8) /usr/src/linux/include/linux/timex.h,
/usr/src/linux/include/linux/sched.h, /usr/src/linux/ker-
nel/time.c, /usr/src/linux/kernel/sched.c