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发信人: cc (猪仔), 信区: Linux
标  题: The "Inaccurate Clock" Mini-HOWTO
发信站: BBS 水木清华站 (Wed Apr 30 15:49:35 1997)

The "Inaccurate Clock" Mini-HOWTO
=================================
By rbean@execpc.com
last revised Dec. 1996

   The real-time-clock chips used on PC motherboards (and even expensive
workstations) are notoriously inaccurate. Linux provides a simple way to
correct for this in software, making the clock potentially *very* accurate
even without an external time source. But most people don't seem to know
about it, for several reasons:

1: It's not mentioned in most of the general "how to set up linux"
   documentation, and it would be difficult to set up automatically at
   install time (although not impossible in theory, if you have a modem).

2: If you check 'man clock' you'll get clock(3), which is not what you
   want. (try 'man 8 clock').

3: Most people don't seem to care what time it is anyway.

4: Those few who do care often want to use the xntpd package from
   louie.udel.edu to sync to an external time source, such as a
   network time server or radio clock.

   This mini-HOWTO describes the low-tech approach. If you're at all
interested in this sort of thing, I highly recommend that you spend some
time at http://www.eecis.udel.edu/~ntp/ which includes all kinds of
interesting stuff, including complete info on xntpd and links to NIST and
USNO (I have a few more comments on xntpd at the end.)

   NOTE: If you run more than one OS on your machine, you should only let
one of them reset the CMOS clock, so they don't confuse each other.  If
you regularly run both Linux and Windows on the same machine, you may want
to check out some of the shareware clock programs that are available for
Windows instead (follow the links from the URL above).

Using the 'clock' program:
===========================

   Everything you need to know is in the clock(8) man page, but this
mini-HOWTO will walk you through the process.

   NOTE: You must be root to run 'clock', or any other program that
affects either the system time or the CMOS clock.

Checking your installation:
===========================

   Check your system startup files for a command like 'clock -a' or
'clock -ua'. Depending on which distribution you're using, it might be
in /etc/rc.local, or /etc/rc.d/rc.sysinit, or some similar place.

   If it says 'clock -s' or 'clock -us', change the 's' to an 'a', and
then check to see if you have the file /etc/adjtime, which contains a
single line that looks something like this:

0.000000 842214901 0.000000

   These numbers are the correction factor (in seconds per day), the time
the clock was last corrected (in seconds since Jan 1, 1970), and the
partial second that was rounded off last time. If you don't have this
file, login as root and create it, with a single line that looks like this
(all zeros):

0.0 0 0.0

   Then run 'clock -a' or 'clock -ua' manually from the shell to update
the 2nd number (use the 'u' if your clock is set to Universal instead of
local time).

Measuring your clock's drift rate:
==================================

   First, you need to know what time it is :-). Your local time of day
number may or may not be accurate. My favorite method is to call WWV's
voice announcment at (303)499-7111 (this is a toll call). If you have
access to a network time server, you can use the 'ntpdate' program from
the xntpd package (use the -b flag to keep the kernel from messing with
the CMOS clock). Otherwise use 'date -s hh:mm:ss' to set the kernel time
by hand, and then 'clock -w' to set the CMOS clock from the kernel clock.
You'll need to remember when you last set the clock, so write down the
date someplace where you won't lose it. If you used ntpdate, do 'date +%s'
and write down the number of seconds since Jan 1,1970.

   Then come back some days or weeks later and see how far the clock has
drifted. If you're setting the clock by hand, I'd recommend waiting at
least two weeks, and only calculate the drift rate to the nearest .1
sec/day. After several months you could get to the nearest .01 sec/day
(some people claim more accuracy than that, but I'm being conservative
here). If you use 'ntpdate' you shouldn't have to wait that long, but in
any case you can always fine-tune it later.

   You can have cron run 'clock -a' at regular intervals to keep the
system time in line with the (corrected) CMOS time. This command will also
be run from your startup file every time you boot the system, so if you do
that often (as some of us do), that may be enough for your purposes.

   Note that certain programs may complain if the system time jumps by
more than one second at a time, or if it jumps backwards. If you have this
problem, you can use xntpd or ntpdate to correct the time more gradually.

Example:
========

To Set time:
------------
Login as root.
Dial (303)499-7111 (voice), listen for time announcement.
Then type:

date -s hh:mm:ss

but don't press enter until you hear the beep.
[You could use 'ntpdate' here instead of 'date', and skip the phone call]
This sets the "kernel time".
Then type:

clock -w

This sets the CMOS time to match the kernel time.
Then type:

date +%j
[or 'date +%s' if you used 'ntpdate' instead of 'date' above]

and write down the number it gives you for next time.

To Reset Time and Check Drift Rate:
----------------------------------

Find the date you wrote down last time.
Login as root
Then type:

clock -a

This sets the kernel time to match the current CMOS time.
Dial (303)499-7111 (voice), listen for announcement.
Then type:

date

and press enter when you hear the beep, but while you're waiting, write
down the time they announce, and don't hang up yet. This tells you what
time your machine thought it was, when it should have been exactly on the
minute.  Now type in

date hh:mm:00

using the minute *after* the one that was just announced, and press enter
when you hear the beep again (now you can hang up). For hh use the local
hour. This sets the "kernel time".
Then type:

clock -w

which writes the new (correct) time to the CMOS clock.
Now type:

date +%j
[or 'date +%s' if that's what you used before]

You now have three numbers (two dates and a time) that will allow you to
calculate the drift rate.

Calculating the Correction Factor:
----------------------------------

When you ran 'date' on the minute, was your machine slow or fast? If it
was fast, you'll have to subtract some number of seconds, so write it down
as a negative number. If it was slow, you have to add some seconds, so
write it down as positive.

Now subtract the two dates. If you used 'date +%j', the numbers represent
the day-of-year (1-365, or 1-366 in leap years). If you've passed Jan 1
since you last set the clock you'll have to add 365 (or 366) to the 2nd
number. If you used 'date +%s' then your number is in seconds, and you'll
have to divide it by 86400 to get days.

If you already had a correction factor in /etc/adjtime, you'll have to
account for the number of seconds you've already corrected. If you've
overcorrected, this number will have the opposite sign of the one you just
measured; if you've undercorrected it will have the same sign. Multiply
the old correction factor by the number of days, and then add the new
number of seconds (signed addition-- if the two numbers have the same
sign, you'll get a larger number, if they have opposite signs you'll get a
smaller number).

Then divide the total number of seconds by the number of days to get the
new correction factor, and put it in /etc/adjtime in place of the old one.
Write down the new date (in seconds or days) for next time.

Here's what my /etc/adjtime looks like:

-9.600000 845082716 -0.250655

[note 9.6 seconds per day is nearly five minutes per month!]

A few words about xntpd:
========================

   Your system actually has two clocks-- the battery powered "real time
clock" that keeps track of time when the system is turned off (also known
as the "CMOS clock", "Hardware clock", or "RTC") and the "kernel time"
(sometimes called the "software clock" or "system clock") which is based
on the timer interrupt and is initialized from the CMOS clock at boot
time.  The two will drift at different rates, so they will gradually drift
apart from each other, and also away from the "real" time.

   All references to "the clock" in the xntpd documentation refer to the
"kernel clock". When you run xntpd or timed (or any other program that
uses the 'adjtimex' system call), the linux kernel assumes that the kernel
clock is more accurate than the CMOS clock, and resets the CMOS time every
11 minutes from then on (until you reboot the machine). This means that
'clock' no longer knows when the CMOS clock was last reset, so you can't
use the correction factor in /etc/adjtime. You can use 'ntpdate' in your
startup file to initially set the clock from a timeserver before starting
xntpd.  If you don't always have access to an accurate time source when
you boot the machine, this can be a bit awkward-- xntpd isn't really
designed to be used in situations like that.

   Xntpd includes drivers for many radio clocks, and can also be set to
call NIST's dial-up time service at regular intervals (be sure to
calculate the effect on your phone bill when setting the interval between
calls). It can also apply a correction factor to the kernel clock if it
loses contact with its other sources for an extended period of time.

   Most radio clocks cost $3-4K, but you can get plans for an inexpensive
'gadget box' (actually a 300 baud modem) that sits between your computer
and any shortwave radio tuned to Canada's CHU time station (see
ftp://ftp.udel.edu/pub/ntp/gadget.tar.Z). The Heathkit WWV receiver (the
"Most Accurate Clock") is also still available (although not as a kit),
and costs around $4-500. GPS signals also contain time information, and
some GPS recievers can connect to a serial port. This may become the low
cost solution in the near future.

   In theory, someone could write a program to use NIST's dial up time
service to calculate the drift rate of both the CMOS clock and the kernel
clock automatically. I am not aware of any stand-alone program to do this,
but most of the code could probably be borrowed from xntpd.

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