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

[Chandra M. <sma...@al...>]

Hi Richard,

Thank you for the details. As soon as I get free from my project, I will share the details with 10Gbps-1588 system.

My understanding is that looking at the offset and its deviation from the rms and the pbb deviation tells us clearly the efficacy of the 1588. Definitely, there is saturation beyond certain number. However, from our experiments, 512 syncs per seconds have looked very promising.

You are absolutely right - 1588 cannot beat SyncE due to the latter's operation at layer 1 as opposed to 1588 packet-based operation at layer 2 & above. However, there is an effort to dispel the myth (if I dare say so) that syntonization HAS TO BE achieved with only SyncE. Many requirements out there are not that stringent.

In any case, I thank you all for your patient and cordial responses.

Thanking you in anticipation,
Regards,
Chandra

(c) : 0175508142
(O): 701.6412

"Knowledge speaks, Wisdom listens"


-----Original Message-----
From: Richard Cochran [mailto:ric...@gm...]
Sent: Friday, March 20, 2015 12:29 AM
To: Chandra Mallela
Cc: Miroslav Lichvar; lin...@li...
Subject: Re: [Linuxptp-users] Expected throughput of the ptp4l

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


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