>>> In article <[EMAIL PROTECTED]>, Spoon <[EMAIL PROTECTED]> writes:
Spoon> Here's why I ask: I'm working with a standard that deals with 27 MHz Spoon> clocks, and that standard states that the frequency must not change Spoon> faster than 75 mHz per second. Harlan> If ntpd has the correct drift adjustment, you should be in good Harlan> shape. Spoon> drift or skew? The value in the driftfile. If you run 'ntpq -c rv' it will be the 'frequency=...' value. Harlan> Under what exact conditions must the "don't change faster than Harlan> 75mHz/sec" be met? Spoon> Consider two systems A and B. At initialization, B's clock is set to Spoon> A's clock (offset = 0 initially). After 30-60 seconds, it is You mean for the first 30-60 seconds, right? Spoon> acceptable to make a large correction to compensate for the average Spoon> skew. From then on, B's skew correction parameter should not be Spoon> changed faster than 75 mHz per second. OK. Spoon> I'll try and think aloud, in case someone can see through my Spoon> confusion. Consider H : a 26,999,900 Hz clock. Thus H has an offset Spoon> of 100 Hz = 3.7 ppm Spoon> I am talking about a frequency offset, i.e. what NTP calls skew Spoon> AFAIU. OK, I think we're on the same page now. Spoon> In other words, H "misses" 100 ticks every second. Harlan> If left uncorrected, yes. Spoon> So far, so good. Spoon> I'd have to add 100 / 27e6 = 3.7 µs every second to keep H from Spoon> drifting away from the correct time. Harlan> Once ntpd has achieved "state 4", ntpd will calculate the drift and Harlan> will automatically handle this. Spoon> According to the definition's page you've mentionned, drift is the Spoon> variation in skew, I think you meant ntpd will calculate the skew? I'll have to reread the definitions page again. OK, it's directly neither skew nor drift. It's the frequency adjustment needed to keep the clock accurate, and it accomodates the actual frequency of the clock crystal and the value of HZ. Yes, ntpd will calculate it and adjust for it. Harlan> Are you comfortable that the specs of the machines you are using Harlan> will keep time well enough that their clocks can be kept in-sync by Harlan> limiting adjustment to 75mHz/sec? Spoon> I'm still testing this. Basically, the two machines will remain at Spoon> constant temperature. I'm hoping that clock drift on either machine Spoon> will be very small, i.e. skew between the two clocks should remain Spoon> almost constant. Thus, once ntpd has computed the correct skew, Spoon> everything should be OK. Harlan> And have you read about the problems you can cause by having HZ at a Harlan> value other than 100? Particularly with Linux kernels? Spoon> No. Where is that? I just looked and I thought I saw somebody post something about this here, although it might have been on [EMAIL PROTECTED] I just looked, and I could not find it. It was a discussion about how one had to be careful to make sure that the HZ value was "properly" divisible, and also not so fast that interrupts were lost. Have you just tried this yet? H _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
