Provided by: adjtimex_1.29-7_amd64 
NAME
adjtimex - display or set the kernel time variables
SYNOPSIS
adjtimex [option]...
DESCRIPTION
This program gives you raw access to the kernel time variables. Anyone may print out the
time variables, but only the superuser may change them.
Your computer has two clocks - the "hardware clock" that runs all the time, and the system
clock that runs only while the computer is on. Normally, "hwclock --hctosys" should be
run at startup to initialize the system clock. The system clock has much better precision
(approximately 1 usec), but the hardware clock probably has better long-term stability.
There are three basic strategies for managing these clocks.
For a machine connected to the Internet, or equipped with a precision oscillator or radio
clock, the best way is to regulate the system clock with ntpd(8). The kernel will
automatically update the hardware clock every eleven minutes.
In addition, hwclock(8) can be used to approximately correct for a constant drift in the
hardware clock. In this case, "hwclock --adjust" is run occasionally. hwclock notes how
long it has been since the last adjustment, and nudges the hardware clock forward or back
by the appropriate amount. The user needs to set the time with "hwclock --set" several
times over the course of a few days so hwclock can estimate the drift rate. During that
time, ntpd should not be running, or else hwclock will conclude the hardware clock does
not drift at all. After you have run "hwclock --set" for the last time, it's okay to
start ntpd. Then, "hwclock --systohc" should be run when the machine is shut down. (To
see why, suppose the machine runs for a week with ntpd, is shut down for a day, is
restarted, and "hwclock --adjust" is run by a startup script. It should only correct for
one day's worth of drift. However, it has no way of knowing that ntpd has been adjusting
the hardware clock, so it bases its adjustment on the last time hwclock was run.)
For a standalone or intermittently connected machine, where it's not possible to run ntpd,
you may use adjtimex instead to correct the system clock for systematic drift.
There are several ways to estimate the drift rate. If your computer can be connected to
the net, you might run ntpd for at least several hours and run "adjtimex --print" to learn
what values of tick and freq it settled on. Alternately, you could estimate values using
as a reference the CMOS clock (see the --compare and --adjust switches), another host (see
--host and --review), or some other source of time (see --watch and --review). You could
then add a line to rc.local invoking adjtimex, or configure /etc/init.d/adjtimex or
/etc/default/adjtimex, to set those parameters each time you reboot.
OPTIONS
Options may be introduced by either - or --, and unique abbreviations may be used.
Here is a summary of the options, grouped by type. Explanations follow.
Get/Set Kernel Time Parameters
-p --print -t --tick val -f newfreq --frequency newfreq -o val --offset val -s
adjustment --singleshot adjustment -S status --status status -m val -R --reset
--maxerror val -e val --esterror val -T val --timeconstant val -a[count]
--adjust[=count]
Estimate Systematic Drifts
-c[count] --compare[=count] -i tim --interval tim -l file --logfile file -h
timeserver --host timeserver -w --watch -r[file] --review[=file] -u --utc -d
--directisa -n --nointerrupt
Informative Output
--help -v --version -V --verbose
-p, --print
Print the current values of the kernel time variables. 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
follows:
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 kernels 2.0 through 2.6, 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
-t val, --tick val
Set the number of microseconds that should be added to the system time for each
kernel tick interrupt. For a kernel with USER_HZ=100, 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
900000/USER_HZ...1100000/USER_HZ. If val is rejected by the kernel, adjtimex will
determine the acceptable range through trial and error and print it. (After
completing the search, it will restore the original value.)
-f newfreq, --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. When
USER_HZ=100, the value is scaled such that newfreq = 65536 speeds up the system
clock by about 1 ppm, or .0864 sec/day. Thus, all of these are about the same:
--tick 9995 --frequency 32768000
--tick 10000 --frequency 6553600
--tick 10001 --frequency 0
--tick 10002 --frequency -6553600
--tick 10005 --frequency -32768000
To see the acceptable range for newfreq, use --print and look at "tolerance", or
try an illegal value (e.g. --tick 0).
-s adj, --singleshot adj
Slew the system clock by adj usec. (Its rate is changed temporarily by about 1
part in 2000.)
-o adj, --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.
-S status, --status status
Set kernel system clock status register to value status. Look here above at the
--print switch section for the meaning of status, depending on your kernel.
-R, --reset
Reset clock status after setting a clock parameter. For early Linux kernels, using
the adjtimex(2) system call to set any time parameter the kernel think the clock is
synchronized with an external time source, so it sets the kernel variable
time_status to TIME_OK. Thereafter, at 11 minute intervals, it will adjust the
CMOS clock to match. We prevent this "eleven minute mode" by setting the clock,
because that has the side effect of resetting time_status to TIME_BAD. We try not
to actually change the clock setting. Kernel versions 2.0.40 and later apparently
don't need this. If your kernel does require it, use this option with: -t -T -t -e
-m -f -s -o -c -r.
-m val, --maxerror val
Set maximum error (usec).
-e val, --esterror val
Set estimated error (usec). The maximum and estimated error are not used by the
kernel. They are merely made available to user processes via the adjtimex(2)
system call.
-T val, --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
(2^val). For room-temperature quartz oscillators, 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 convergence; values between 2 and 6 can
be used to reduce network load, but at a modest cost in accuracy.
-c[count], --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 system clock
into approximate agreement with the CMOS clock. CMOS clock readings are adjusted
for systematic drift using using the correction in /etc/adjtime — see hwclock(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
supplied, the "=" is required.) If the CMOS clock and the system clock differ by
more than six minutes, adjtimex will try shifting the time from the CMOS clock by
some multiple of one hour, up to plus or minus 13 hours in all. This should allow
correct operation, including logging, if the --utc switch was used when the CMOS
clock is set to local time (or vice-versa), or if summer time has started or
stopped since the CMOS clock was last set.
-a[count], --adjust[=count]
By itself, same as --compare, except the recommended values are actually installed
after every third comparison. With --review, the tick and frequency are set to the
least-squares estimates. (In the latter case, any count value is ignored.)
--force-adjust
Override the sanity check that prevents changing the clock rate by more than 500
ppm.
-i tim, --interval tim
Set the interval in seconds between clock comparisons for the --compare and
--adjust options.
-u, --utc
The CMOS clock is set to UTC (universal time) rather than local time.
-d, --directisa
To read the CMOS clock accurately, adjtimex usually accesses the clock via the
/dev/rtc device driver of the kernel, and makes use of its CMOS update-ended
interrupt to detect the beginning of seconds. It will also try /dev/rtc0 (for
udev), /dev/misc/rtc (for the obsolete devfs) and possibly others. When the
/dev/rtc driver is absent, or when the interrupt is not available, adjtimex can
sometimes automatically fallback to a direct access method. This method detects the
start of seconds by polling the update-in-progress (UIP) flag of the CMOS clock.
You can force this direct access to the CMOS chip with the --directisa switch.
Note that the /dev/rtc interrupt method is more accurate, less sensible to
perturbations due to system load, cleaner, cheaper, and is generally better than
the direct access method. It is advisable to not use the --directisa switch, unless
the CMOS chip or the motherboard don't properly provide the necessary interrupt.
-n, --nointerrupt
Force immediate use of busywait access method, without first waiting for the
interrupt timeout.
-l[file], --log[=file]
Save the current values of the system and CMOS clocks, and optionally a reference
time, to file (default /var/log/clocks.log). The reference time is taken from a
network timeserver (see the --host switch) or supplied by the user (see the --watch
switch).
-h timeserver, --host timeserver
Use ntpdate to query the given timeserver for the current time. This will fail if
timeserver is not running a Network Time Protocol (NTP) server, or if that server
is not synchronized. Implies --log.
-w, --watch
Ask for a keypress when the user knows the time, then ask what that time was, and
its approximate accuracy. Implies --log.
-r[file], --review[=file]
Review the clock log file (default /var/log/clocks.log) and estimate, if possible,
the rates of the CMOS and system clocks. Calculate least-squares rates using all
suitable log entries. Suggest corrections to adjust for systematic drift. With
--adjust, the frequency and tick are set to the suggested values. (The CMOS clock
correction is not changed.)
-V, --verbose
Increase verbosity.
--help Print the program options.
-v, --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 (2^16)*0.64/.0864 =
485452. Thus, putting the following in rc.local would approximately correct the system
clock:
adjtimex --tick 9999 --freq 485452
NOTES
adjtimex adjusts only the system clock — the one that runs while the computer is powered
up. To set or regulate the CMOS clock, see hwclock(8).
AUTHORS
Steven S. Dick <ssd at nevets.oau.org>, Jim Van Zandt <jrv at comcast.net>.
SEE ALSO
date(1L), gettimeofday(2), settimeofday(2), hwclock(8), ntpdate(8), ntpd(8),
/usr/src/linux/include/linux/timex.h, /usr/src/linux/include/linux/sched.h,
/usr/src/linux/kernel/time.c, /usr/src/linux/kernel/sched.c
March 11, 2009 ADJTIMEX(8)