[ntp:questions] adj_systime frequency adjustment calculations
Kemal Oral CANSIZLAR
ocansizlar at comcast.net
Wed Mar 30 01:10:11 UTC 2005
I have an implementation question regarding time adjustment (clock slewing)
on Windows XP. It is about the function
under "nt_clockstuff.c" source file. I am in need of explanation on what the
parameter "now" stands for. As far as I understood, it is the offset between
reference time and the system time. However, is it the offset of system time
relative to reference time or the other way around? I believe the value
"now" is in seconds and is determined to be the offset/difference between
two clocks over one second interval.
Also, can somebody explain what exactly is going on inside adj_systime()
routine? I am basically trying to adapt the frequency adjustment
calculations to the high resolution performance counter, instead of the
system clock. Performance counter has a very high frequency (in my system,
it is the same as my CPU; 3.2 GHz) relative to low frequency system clock...
I am having problems on the calculations. I guess there is some
misinterpretation of some parameters in my code, since my performance
counter readings (which I use to simulate the system clock) tend to deviate
largely after about 15 minutes, and causing NTP to give the error "frequency
error X PPM exceeds tolerance 500 PPM". So, how can I go about calculating
the frequency adjustment needed for any general counter (performance counter
in my case) given the values "now" and the frequency the counter is
operating at. My calculations were as below:
In my case, I am only holding a double value for frequency, and treat the
performance counter as a continuous, utopic time provider. So, in Time
Freq(offset) = Delta_T / T.
Delta_T is the offset parameter passed by NTP to adj_systime() routine. T is
1 second in our case. Having Freq(offset) computed, we also have in
Freq(offset) = [ Freq(measured) - Freq(current) ] / Freq(current).
>From here, we can find Freq(measured) which is the actual frequency our
counter should operate at to compensate for the offset. We know
Freq(current) - it is the freq that our counter operates at. So, in our
Freq(offset) = Delta_T because T = 1,
and then we should reach the desired frequency by
Freq(new) = (1+Delta_T)*Freq(current)
And this is not working... I guess I am misinterpreting some internal
variables that NTP provides me, or my calculations are nonsense...?
I would greatly appreciate any input.
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