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