Re: [chrony-users] Large ppm clock slew rate

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

t to add: If you look at the man page
man 8 adjtimex
(you need to install the adjtimex package) there are two adjustments.
One is the tick adjustment. Each incriment changes the tick time ( the time
the computer assumes a tick of the clock is worth ) by about 100PPM.

 -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
The other is the frequency
-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).

This is the one that is about plus or minus 500PPM So, chrony uses both tick
and frequency to adjust the rate of the clock, frequency for fine adjustment,
tick for coarse.
I believe ntpd only uses the frequency adjustment.


On Mon, 18 Jan 2021, Steven Sommars wrote:

> I have a Linux installation which may require clock slew rate > 500 ppm, which exceeds normal
> adjustimex limits.  Chrony lists a 100000 ppm maximum slew rate.  How is that done?
>
> Is there a document that describes chrony's clock discipline algorithm?