unruh wrote:
On 2013-12-19, David Lord <[email protected]> wrote:
unruh wrote:
On 2013-12-19, David Lord <[email protected]> wrote:
Adrian P wrote:
.....
Everywhere is written that FreeBSD is the best OS for time keeper,
since its kernel supports nanosecond accuracies. However, if GPS 18x
LVC PPS signal has only microsecond accuracy, then I suppose there is
no point on using FreeBSD, any linux would be okay as well. So what do
you think, looking from NTP point of view only, is this nanosecond
accuracy the only reason why someone would use FreeBSD, or there are
some other reasons as well? I see you have experience with all kind of
OSes.
I think the combination of hardware used is at least as important
as the OS.
<http://www.febo.com/pages/soekris/> 100ns accuracy
AGreed. But their comparison is strongly cooked. At least on my system,
the serial port interrupt performace is far better than theirs--
something like 2 us, not the 50us they are showing. Certainly 100ns is
better, but more like one order of magnitude, not more than 2.
It might not be very clear but PPS to serial port isn't used
other than as a comparison, ntpd has been modified to use the
Elan Timer with PPS input to the GPIO.
I understand that they do not use pps to serial port, but they compare
to ntpd with serial input to show how much better they are. What I
object to is that that comparison is cooked. ntpd with pps to serial is
a lot better than their graph indicates. They are doing something very
wrongly to get such bad results. And it is unnecessary because they are
better than ntpd with pps to serial.
My loop_summary from Sure GPS + NetBSD-6 + ntp-dev-4.2.7p401
over a 7 day period has offset(us) 7 +/-34 to 21 +/- 52 and
rms offset from 3.9 to 6.1.
If ntpd is throwing away 7 out of 8 polls I'd say their results
could be similar to mine.
Also since from that page "ntp gettime Thu, Oct 19 2006 20:10"
there have been many changes to ntpd.
David
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