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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