Re: [chrony-users] chronyc tracking question

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Tue, 25 Apr 2017, Miroslav Lichvar wrote:

> On Mon, Apr 24, 2017 at 03:54:25PM -0400, Chris Perl wrote:
> > 1.  If there is asymmetry, its unlikely it is constant for the entire
> > life of the chrony process, assuming you're running chrony for a
> > reasonable period and have a reasonably designed network and your time
> > sources are located reasonably close ("reasonable" can obviously be
> > different for different people).
>
> A major source of constant asymmetry is the timestamping. Unless you
> are using HW timestamping, there can easily be an asymmetry of few
> tens of microseconds due to interrupt coalescing and other delays in
> the kernel, driver and HW. If both ends had the same asymmetry, it
> would cancel out, but at least in my experience that's unusual. Even
> if both machines had the same HW and SW, there may be a difference due
> to the timing of the packets (i.e. server sends a response
> immediatelly after receiving a request).
>
> For example, here is a client with an Intel i210 card running two
> chronyd instances using the same server. One is using HW timestamping
> and controlling the clock, the other is using SW timestamping and just
> monitoring the server with the noselect option.

Running two instances of chrony means that one has to wait with its interrupt
while the other finishes. That can give a large delay. I once tried that (not
with one doing hardware timestamping however) and found a large delay (about
10us if I remember correctly) of the second waiting for the first.

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