Re: [Linuxptp-users] Expected throughput of the ptp4l

[Richard C. <ric...@gm...>]

On Wed, Mar 18, 2015 at 09:29:02AM +0100, Richard Cochran wrote:
> I have seen good results with 1, 2, 4, and 8, packets per second on a
> low end embedded system.  With 128, some time stamps are dropped due
> to hardware/driver constraints.

Here is a random metric from the boards on my desk.  The CPUs are the
TI AM335x, but using the DP83640 PHY as the PTP Hardware Clock.  The
slave is directly connected to the master with 1 meter cable over a
100 Mbit link.

Measuring the edges of a 1 kHz output at various Sync rates, and with
a DelayReq rate of 1 Hz, I see the following differences.

  Rate   Offset
  ----------------
  2^0    +/- 75 ns
  2^-3   +/- 25 ns
  2^-4   +/- 20 ns
  2^-5   +/- 20 ns

As expected, increasing the DelayReq rate to 2^-5 makes no difference.

So with this hardware, I have already reached the limit of
synchronization performance.  Increasing the Sync rate to 512 frames
per second would not improve the picture.

In contrast, using SyncE on the exact same hardware immediately yields
an offset of +/- 1 nanosecond. (Actually, it probably is smaller, but
I can't measure it with my lousy scope.)

HTH,
Richard