On 07/11/2013 22:15, Brian Inglis wrote:
From the attached extract from my ntp log the current performance
counter appears to have much higher drift ~25PPM than my hardware clock;
my own calibration, timings, and loopstats over the last couple of years
show ~0.9PPM.
A possible reason is that currently when calibrating and using PCC only
a bare rdtsc instruction is used.
From discussions in various places, summarized well in the article
linked below, rdtsc may be executed out of order, adding jitter.
These discussions recommend rdtsc be preceded by mfence (as it works on
all PCs that support rdtsc) to avoid out of order execution during
calibration loops.
The calibration also needs to be wired to a single cpu, results from the
first call to the calibration function discarded to eliminate cache and
pipeline fill delays, and all results significantly higher than average
discarded because the hardware can switch into System Management Mode
BIOS at random, mainly to handle USB devices like mice, keyboards, drives.
See
http://lists.freebsd.org/pipermail/freebsd-amd64/2012-July/014756.html,
links from that, and similar articles on the LKML and MSDN.
Sounds like one for the NTP Bugzilla
http://bugs.ntp.org/index.cgi
--
Cheers,
David
Web: http://www.satsignal.eu
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