[ntp:questions] how to have offset < 1ms

Miroslav Lichvar mlichvar at redhat.com
Thu Apr 15 08:55:53 UTC 2010

On Wed, Apr 14, 2010 at 11:55:54PM +0000, unruh wrote:
> On 2010-04-14, lhommedumatch <ludocluba at yahoo.com> wrote:
> > The ntpgmtaceb (this clock loses 1.10e-11second every second => ie 8.6
> > 10e-7s each days) is providing zda+pps, I have worked on using PPS
> > with LinuxPPS.
> > But I need to patch the kernel. I'm using CentOS 5.4 => linux2.6.18.
> Why do you need to patch the kernel? Just use shmpps or gpsd to deliver
> the PPS time to ntpd to discipline the clock. Now, that may only give
> you 1usec accuracy, instead of 1/2usec, but since discussion was about
> ms, that should not be the problem

Userspace timestamps may decrease the accuracy by more than just 0.5
us. When I compare kernel timestamps and timestamps from gpsd, there
is a 20-40us difference, even when the gpsd process has real time
priority and the machine is otherwise idle.

Of course, for 1ms requirement this doesn't matter.

Miroslav Lichvar

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