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.

# chronyc -h 127.0.0.1 -p 323 -m sources sourcestats
210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* ntp1.local                    1   0   377     1     +9ns[  +38ns] +/-   26us
210 Number of sources = 1
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
ntp1.local                  8   5     9     +0.000      0.018     +0ns    31ns

# chronyc -h 127.0.0.1 -p 324 -m sources sourcestats
210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^? ntp1.local                    1   0   377     4  +9740ns[+9740ns] +/-   56us
210 Number of sources = 1
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
ntp1.local                 24  14   354     -0.000      0.001  -8542ns   201ns

While the second instance seem to be stable to few hundreds
nanoseconds, the measured offset has an error of about 9 microseconds.
The extra delay of about 40 microseconds is largely asymmetric.

Don't forget to use the xleave option on your clients if their servers
are running chrony. Even with SW timestamping it can help a lot.

-- 
Miroslav Lichvar

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