[ntp:questions] Why does NTPD precision decline when using the HPET ves. the Performance Counter?

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Sep 15 10:59:20 UTC 2014


On 2014-09-04 04:58, Charles Elliott wrote:
> When the platform clock is changed from the
> performance counter (freq: 3.554 MHz) to the
> HPET (freq: 14.318 MHz) the NTP protocol
> precision declines from -22 to -20.  This
> occurs both in versions 4.2.7p442 at 1.2483-o May 09 10:14:35.18
> and 4.2.7p467 at 1.2483-o Aug 28 12:01:29.42.
> The NTPD_USE_INTERP_DANGEROUS=1 environment
> variable is set.  Here are the relevant messages from
> the Event Log:

> 8/23/2014 3:34:59 PM Performance counter frequency 3.554 MHz
> 8/23/2014 3:34:59 PM proto: precision = 0.200 usec (-22)
> 8/23/2014 3:34:59 PM proto: fuzz beneath 0.100 usec

> 8/23/2014 3:52:37 PM Performance counter frequency 14.318 MHz
> 8/23/2014 3:52:38 PM proto: precision = 0.800 usec (-20)
> → → No "proto: fuzz beneath ... " message ← ←

> Is this the way it should be, that the protocol precision
> declines when the clock is more than 4 times faster?

Precision tells you how quickly ntpd can read the system timer,
not the resolution of the system timer.
See http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm
-- 
Take care. Thanks, Brian Inglis


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