Yo Kurt! On Wed, 25 Jan 2017 00:01:43 +0100 Kurt Roeckx <[email protected]> wrote:
> On Tue, Jan 24, 2017 at 10:38:50PM +0100, Achim Gratz wrote:
> > sys_fuzz (which whitens the quantization noise on the clock reading)
> > doesn't make a difference or you might not. But presumably Dave
> > Mills didn't put it in there just because he was trying to add
> > useless code.
>
> So it seems to me that this is the wrong order, you first add the
> noise and then do the quantization, not the other way around.
The quantization noise is already in the time read from the system..
clock_gettime() has a resolution of one nano Second, but depending on the
host, it increments in steps that are a lot bigger. ntpd tries
to determine that minimum increment and store it in sys_tick.
Annoying there was no way to read that variable that I could find.
Now there is a way, just pushed to git head, new option:
ntpq -c 'rv 0 tick'
Results are in milli Seconds. Here is what I get on a Xeon:
spidey ntpsec # ntpq -c 'rv 0 tick fuzz'
fuzz=0.000202, tick=0.000202
Tick seems to equal fuzz. About 202 nano Seconds.
Table time:
Xeon: fuzz=0.000202, tick=0.000202
skylake: fuzz=0.000109, tick=0.000109
RasPi: fuzz=0.003, tick=0.003
RasPi2: fuzz=0.001823, tick=0.001823
RasPi3: fuzz=0.000937, tick=0.000937
mac mini: fuzz=0.0005, tick=0.001
Pretty much sys_fuzz == sys_tick. Except that mac!
Looks like ntpd is built to handle a lot of complexity that no longer exists
in my test platforms. Except that mac...
looks like sys_tick can be less than sys_fuzz. Anyone got the reverse
case?
RGDS
GARY
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