[ntp:hackers] Weird initial frequency calculations.

Brian Utterback brian.utterback at sun.com
Thu Jul 31 16:33:50 UTC 2008


While further investigating the initial frequency calculations, I got 
some weird results.

What I did was take a system that had a known drift of approximately 
-28 ppm. I then made a script that ran the p120 ntpd, after first 
setting the system frequency and offset to 0 using ntptime. I then 
traced the calls to ntp_adjtime, looking for the call that set the 
kernel frequency after the initial frequency calculation. I used the 
same four servers each time, all reporting time to within less than a 
millisecond or so of each other. I did not configure iburst.

What I found was that ntpd set the frequency in 3 to 7 minutes, or 18 
to 26 minutes. The frequency set in the 3 to 7 minute cases were 
terrible approximations while those in the 18 to 26 mintue range were 
pretty good, or at least in the ballpark.

So, I don't why there is a difference, but something weird is going on 
in how ntpd is doing this initial frequency calculation.

Here are the data I have collected. The first column is the time since 
ntpd started up, and the second column is the frequency the kernel was 
being set to:

03:23  freq=615
03:28  freq=-751
03:31  freq=-8245
03:32  freq=-4135
03:37  freq=-3478
03:40  freq=-6324
03:41  freq=-5407
03:42  freq=-3551
03:43  freq=-4969
03:43  freq=2062
03:45  freq=-7290
03:47  freq=-4073
03:48  freq=-3894
03:49  freq=-6067
03:52  freq=-3325
03:53  freq=-1238
03:56  freq=-1556
03:56  freq=-2672
03:59  freq=-838
04:00  freq=-2594
04:49  freq=-6159
04:53  freq=-2309
05:36  freq=1578
05:44  freq=-7642
05:45  freq=1642
07:06  freq=-4963
18:42  freq=-22073
18:45  freq=-26337
18:48  freq=-24193
19:11  freq=-22233
19:46  freq=-25569
19:52  freq=-23362
20:15  freq=-26172
20:52  freq=-22816
21:02  freq=-26722
21:23  freq=-19315
23:17  freq=-27761
26:34  freq=-27200

-- 
blu

There are two rules in life:
Rule 1- Don't tell people everything you know
----------------------------------------------------------------------
Brian Utterback - Solaris RPE, Sun Microsystems, Inc.
Ph:877-259-7345, Em:brian.utterback-at-ess-you-enn-dot-kom


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