MAN page from Fedora 21 adjtimex-1.29-9.fc21.x86_64.rpm
Section: Maintenance Commands (8)
Updated: March 11, 2009Index
adjtimex - display or set the kernel time variables
This program gives you raw access to the kernel time variables. Anyone may print out the time variables, but only the superusermay change them.
Your computer has two clocks - the "hardware clock" that runs all thetime, and the system clock that runs only while the computer is on.Normally, "hwclock --hctosys" should be runat startup to initialize the system clock. The system clock has much better precision (approximately 1 usec), butthe hardware clock probably has better long-term stability. There arethree basic strategies for managing these clocks.
For a machine connected to the Internet, or equipped with a precisionoscillator or radio clock, the best way is to regulate the system clockwith ntpd(8). The kernel willautomatically update the hardware clock every eleven minutes.
In addition, hwclock(8) can be used to approximately correct for aconstant drift in the hardware clock. In this case, "hwclock--adjust" is run occasionally. hwclock notes how long it hasbeen since the last adjustment, and nudges the hardware clock forwardor back by the appropriate amount. The user needs to set the timewith "hwclock --set" several times over the course of a few days sohwclock can estimate the drift rate. During that time,ntpd should not be running, or else hwclock will concludethe 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. (Tosee why, suppose the machine runs for a week with ntpd, is shutdown for a day, is restarted, and "hwclock --adjust" is run by astartup script. It should only correct for one day's worth of drift.However, it has no way of knowing that ntpd has been adjustingthe hardware clock, so it bases its adjustment on the last timehwclock was run.)
For a standalone or intermittently connected machine, where it's notpossible to run ntpd, you may use adjtimex instead tocorrect 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 ntpdfor at least several hours and run "adjtimex --print" to learnwhat values of tick and freq it settled on. Alternately, you couldestimate 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 torc.local invoking adjtimex, or configure/etc/init.d/adjtimex or /etc/default/adjtimex, to setthose parameters each time you reboot.
Options may be introduced by either -
, and uniqueabbreviations may be used.
Here is a summary of the options, grouped by type. Explanationsfollow.
- 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--log file-h timeserver--host timeserver-w--watch-r[file]--review[=file]-u--utc-d--directisa-n--nointerrupt
- Informative Output
- -p, --print
- Print the current values of the kernel time variables. NOTE: The timeis "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. ForLinux 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 timefor each kernel tick interrupt. For a kernel with USER_HZ=100, thereare supposed to be 100 ticks per second, so val should be closeto 10000. Increasing val by 1 speeds up the system clock byabout 100 ppm, or 8.64 sec/day. tick must be in the range900000/USER_HZ...1100000/USER_HZ. If val is rejected by thekernel, adjtimex will determine the acceptable range throughtrial and error and print it. (After completing the search, it willrestore the original value.)
- -f newfreq, --frequency newfreq
- Set the system clock frequency offset to newfreq. newfreqcan be negative or positive, and gives a much finer adjustment thanthe --tick switch. When USER_HZ=100, the value is scaled suchthat 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 -32768000To 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 hereabove at the --print switch section for the meaning ofstatus, depending on your kernel.
- -R, --reset
- Reset clock status after setting a clock parameter. For early Linuxkernels, using the adjtimex(2) system call to set any time parameterthe kernel think the clock is synchronized with an external timesource, so it sets the kernel variable time_status to TIME_OK.Thereafter, at 11 minute intervals, it will adjust the CMOS clock tomatch. 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 versions2.0.40 and later apparently don't need this. If your kernel doesrequire 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 quartzoscillators, David Mills recommends the value 2,which correspondsto a PLL time constant of about 900 sec and a maximum update intervalof about 64 sec. The maximum update interval scales directly with thetime constant, so that at the maximum time constant of 6, theupdate interval can be as large as 1024 sec.
Values of val between zero and 2 give quick convergence; valuesbetween 2 and 6 can be used to reduce network load, but at a modest costin accuracy.
- -c[count], --compare[=count]
- Periodically compare the system clock with the CMOS clock. After thefirst two calls, print values for tick and frequency offset that wouldbring the system clock into approximate agreement with the CMOS clock.CMOS clock readings are adjusted for systematic drift using using thecorrection in /etc/adjtime --- see hwclock(8). Theinterval between comparisons is 10 seconds, unless changed by the--interval switch. The optional argument is the number ofcomparisons. (If the argument is supplied, the "=" isrequired.) If the CMOS clock and the system clock differ by more thansix minutes, adjtimex will try shifting the time from the CMOSclock by some multiple of one hour, up to plus or minus 13 hours inall. This should allow correct operation, including logging, if the--utc switch was used when the CMOS clock is set to local time (orvice-versa), or if summer time has started or stopped since the CMOSclock was last set.
- -a[count], --adjust[=count]
- By itself, same as --compare, except the recommended values areactually installed after every third comparison. With --review,the tick and frequency are set to the least-squares estimates. (Inthe latter case, any count value is ignored.)
- Override the sanity check that prevents changing the clock rate bymore 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 theclock via the /dev/rtc device driver of the kernel, and makes use of itsCMOS update-ended interrupt to detect the beginning of seconds. Itwill also try /dev/rtc0 (for udev), /dev/misc/rtc (for the obsoletedevfs) 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 accessmethod. This method detects the start of seconds by polling theupdate-in-progress (UIP) flag of the CMOS clock. You can force thisdirect access to the CMOS chip with the --directisa switch.
Note that the /dev/rtc interrupt method is more accurate, less sensibleto perturbations due to system load, cleaner, cheaper, and is generallybetter than the direct access method. It is advisable to not use the--directisa switch, unless the CMOS chip or the motherboarddon't properly provide the necessary interrupt.
- -n, --nointerrupt
- Force immediate use of busywait access method, without first waitingfor the interrupt timeout.
- -l[file], --log[=file]
- Save the current values of the system and CMOS clocks, and optionallya 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 --watchswitch).
- -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 TimeProtocol (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 thattime 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. Suggestcorrections to adjust for systematic drift. With --adjust, thefrequency and tick are set to the suggested values. (The CMOS clockcorrection is not changed.)
- -V, --verbose
- Increase verbosity.
- Print the program options.
- -v, --version
- Print the program version.
If your system clock gained 8 seconds in 24 hours, youcould 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 followingin rc.local would approximately correct the system clock:
adjtimex --tick 9999 --frequency 485452
adjusts only the system clock --- the one that runswhile the computer is powered up. To set or regulate the CMOS clock,see hwclock
Steven S. Dick <ssd at nevets.oau.org>, Jim Van Zandt <jrv at comcast.net>.
- SEE ALSO
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