Re: [chrony-users] Time offset on versions 3+ without hw timestamping

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

It has been a while now and I do not entirely remember. I wrote my own
interrupt handler module, which would grab the time immediately that it was
called by the kernel to service the interrupt, so it was effectively the
kernel that was servicing the timestamping. If I recall correctly I had two
modules attached to two interrupts, but of course one would be called before
theother, and that would give a delay in the servicing.


On Thu, 31 Aug 2017, Miroslav Lichvar wrote:

> On Wed, Aug 30, 2017 at 12:14:05PM -0700, Bill Unruh wrote:
> > > You could run a separate server instance of chronyd on a different
> > > port with HW timestamping for the monitoring client. It needs to
> > > support the interleaved mode to be able to get the server's HW
> > > transmit timestamps.
> >
> > The problem is interrupt contention, and clock reading contention. I once
> > tried to have two different readers (chrony and ntpd, or two versions of
> > chrony) reading the interrupt at the same time and one of them was about 8us
> > out because it did not get the interrupt until the first one had finished.
> > Ie, in monitoring you do not want to do it "on the second" since it might
> > interfere with the other one.
>
> Was that with timestamping of PPS in userspace using ioctl(TIOCMIWAIT)?
>
> I don't think that applies to timestamping of NTP packets. A receive
> timestamp is made by the kernel, not by the application, and separate
> instances of chronyd are not receiving both the same packet. They
> won't share the same UDP port.
>
> When running multiple instances of chronyd, it's important than only
> one of them is adjusting the clock. Also, they should be configured to
> use different "port", "cmdport", "bindcmdaddress", and "pidfile".
>
> --
> Miroslav Lichvar
>
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