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       adjtimex - display or set the kernel time variables


       adjtimex [option]...


       This program gives you raw access to the kernel time variables.  Anyone
       may print out the time variables, but only the superuser may change

       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 may be introduced by either - or --, and unique abbreviations
       may be used.

       Here is a summary of the options, grouped by type.  Explanations

       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]

       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

       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
                    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.)

              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

       -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

       -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.


       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


       adjtimex adjusts only the system clock — the one that runs while the
       computer is powered up.  To set or regulate the CMOS clock, see


       Steven S. Dick <ssd at>, Jim Van Zandt <jrv at>.


       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,

                                March 11, 2009