ADJTIMEX

Section: Maintenance Commands (8)
Updated: March 2, 1995
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NAME

adjtimex - display or set the kernel time variables

SYNOPSIS

adjtimex [-offset val] [-singleshot adjustment] [-frequency newfreq] [-maxerror val] [-esterror val] [-timeconstant val] [-tick val] [-status val] [-print] [-help]

DESCRIPTION

This program gives you raw access to the kernel time variables. For a machine connected to the Internet, or equipped with a precision oscillator or radio clock, the best way to keep the system clock correct is with xntpd(8). For a standalone or intermittently connected machine, you may use adjtimex instead to at least correct for systematic drift.

Anyone may print out the time variables, but only the superuser may change them.

If your computer is connected to the net, but you prefer not to run xntpd due to its size, you might still run it for a day or two and use adjtimex -print to learn what values of tick and freq it settled on. You could then kill xntpd, and add a line to rc.local invoking adjtimex to set those parameters each time you reboot.

OPTIONS

-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.

-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 -newfreq 6553600 is about the same as -tick 10001 -newfreq 0. newfreq must be in the range -6553600...6553600 (-100<<16...100<<16).

-singleshot adjustment

Slew the system clock by adjustment usec. (Its rate is changed temporarily by about 1 part in 2000.)

-offset val

Add a time offset of val usec. The kernel code adjusts the time gradually by val, notes how long it has been since the last time offset, and then adjusts the frequency offset to correct for the apparent drift. val must be in the range -131000...131000.

-maxerror val

Set maximum error (usec).

-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.

-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 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.

-status val

Set time_status variable in the kernel:

      0   the kernel should periodically set the 
          CMOS clock to match the system clock
      1   insert a leap second at midnight
      2   delete a leap second at midnight
      3   leap second in progress
      4   the kernel should leave the CMOS clock alone

Using any of the above switches automatically resets the status to 0. Do not use -status with -singleshot. In that case, setting the time_status back to 4 must be done in a separate operation. This is a race condition. The kernel must be patched to permit using -status with -singleshot as an atomic operation.

-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).

-help

Print the program options.

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 correct 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 read or set the CMOS clock, see clock(8).

AUTHOR

Steven S. Dick <ssd@nevets.oau.org>.

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