| Re: [chrony-dev] Slow bootup with git |
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On Wed, Apr 25, 2012 at 10:18:13AM +0100, Ed W wrote:
> I guess then the question is whether increased accuracy of reading
> is useful beyond 2 samples? Remember that gaussian error bound
> decreases fairly slowly with increasing samples (1 / sqrt(n) - ?).
> Is the reading of 8 samples actually buying us very much?
It doesn't seem so. The actual applied system clock step includes the
RTC drift which over longer time intervals has much larger error than
the measured RTC offset.
BTW, I did some tests with RTC on a Linux 3.3.2 machine with HPET
enabled. When the RTC interrupt is enabled (RTC_UIE_ON) and every
following sample collected, there is visible a periodic sawtooth error
up to 25 ms gaining at 100ppm. When only one sample is collected in
one period with enabled interrupt, the absolute value of the error is
the same, but it seems to be uniformly distributed. It seems the RTC
emulation is somehow reset when the interrupt collection is enabled.
--
Miroslav Lichvar
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