Re: [gentoo-amd64] Time Drift

[Björn Bredohl]

Richard Freeman schrieb:
> I just noticed that my time was off and after checking the logs I saw
> that ntpd was adjusting the time by 5 minutes several times a day for
> the last month.
>
> Searching around I found some hints that disabling apic might help. This
> is on a K8V deluxe motherboard and running 2.6.20-r7.  Before I disable
> apic, will this have any negative effects on the system?  This is a
> desktop system so I don't really care about power-saving features
> (although I'd like to keep cpudyn working if possible).
>
> I'm also running ivtv (mythtv backend) which apparently can cause clock
> skew due to some kind of DMA error, but I'm not seeing that in the logs.
>
> Any advice?

See "man hwclock"

...
The Adjust Function

The Hardware Clock is usually not very accurate. However, much of its
inaccuracy is completely predictable - it gains or loses the same amount
of time every day. This is called systematic drift. hwclock's "adjust"
function lets you make systematic corrections to correct the systematic
drift.

It works like this: hwclock keeps a file, /etc/adjtime, that keeps some
historical information. This is called the adjtime file.

Suppose you start with no adjtime file. You issue a hwclock --set
command to set the Hardware Clock to the true current time. Hwclock
creates the adjtime file and records in it the current time as the last
time the clock was calibrated. 5 days later, the clock has gained 10
seconds, so you issue another hwclock --set command to set it back 10
seconds. Hwclock updates the adjtime file to show the current time as
the last time the clock was calibrated, and records 2 seconds per day as
the systematic drift rate. 24 hours go by, and then you issue a hwclock
--adjust command. Hwclock consults the adjtime file and sees that the
clock gains 2 seconds per day when left alone and that it has been left
alone for exactly one day. So it subtracts 2 seconds from the Hardware
Clock. It then records the current time as the last time the clock was
adjusted. Another 24 hours goes by and you issue another hwclock
--adjust. Hwclock does the same thing: subtracts 2 seconds and updates
the adjtime file with the current time as the last time the clock was
adjusted.

Every time you calibrate (set) the clock (using --set or --systohc ),
hwclock recalculates the systematic drift rate based on how long it has
been since the last calibration, how long it has been since the last
adjustment, what drift rate was assumed in any intervening adjustments,
and the amount by which the clock is presently off.

A small amount of error creeps in any time hwclock sets the clock, so it
refrains from making an adjustment that would be less than 1 second.
Later on, when you request an adjustment again, the accumulated drift
will be more than a second and hwclock will do the adjustment then.

It is good to do a hwclock --adjust just before the hwclock --hctosys at
system startup time, and maybe periodically while the system is running
via cron.

The adjtime file, while named for its historical purpose of controlling
adjustments only, actually contains other information for use by hwclock
in remembering information from one invocation to the next.

The format of the adjtime file is, in ASCII:

Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in
seconds per day, floating point decimal; 2) Resulting number of seconds
since 1969 UTC of most recent adjustment or calibration, decimal
integer; 3) zero (for compatibility with clock(8)) as a decimal integer.

Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
recent calibration. Zero if there has been no calibration yet or it is
known that any previous calibration is moot (for example, because the
Hardware Clock has been found, since that calibration, not to contain a
valid time). This is a decimal integer.

Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
Coordinated Universal Time or local time. You can always override this
value with options on the hwclock command line.

You can use an adjtime file that was previously used with the clock(8)
program with hwclock.
...