Re: [chrony-dev] SW/HW timestamping on Linux

[Denny Page <dennypage@xxxxxx>]

I am now seeing better standard deviations with hardware timestamping than software timestamping. Thank you.

Couple of caveats:
  - I need to disable priority scheduling (-P). With priority scheduling, software stamps still have lower stddev.
  - I can only use a single ethernet interface. With multiple interfaces, software stamps still have lower stddev.

I am still seeing issue strange offset issues.

The view from 192.168.230.2:

210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* 192.168.230.240               1   0   377     0    -20ns[  -21ns] +/-   13us
^+ 192.168.230.244               1   0   377     0    +71ns[  +71ns] +/-   13us
=? 192.168.230.3                 2   0   377     0  +9159ns[+9159ns] +/-   45us
210 Number of sources = 3
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
192.168.230.240            64  38    65     +0.000      0.001    +22ns    41ns
192.168.230.244            64  32    69     +0.000      0.001    -18ns    47ns
192.168.230.3              64  33    65     +0.001      0.001  +8851ns    58ns


The view from 192.168.230.3:

210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* 192.168.230.240               1   0   377     0     +5ns[   +6ns] +/-   13us
^+ 192.168.230.244               1   0   377     0    -32ns[  -32ns] +/-   13us
=? 192.168.230.2                 2   0   377     0    +15us[  +15us] +/-   51us
210 Number of sources = 3
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
192.168.230.240            16   8    15     +0.002      0.009    +24ns    45ns
192.168.230.244            22   9    21     -0.002      0.007    -25ns    48ns
192.168.230.2              16   7    53     -0.004      0.003    +12us    45ns


Both crony instances think the other is off by a large amount. This disagreement is very stable.

Denny



> On Nov 23, 2016, at 01:40, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> 
> On Fri, Nov 18, 2016 at 03:37:07PM +0100, Miroslav Lichvar wrote:
>> On Thu, Nov 17, 2016 at 05:44:23PM -0800, Denny Page wrote:
>>> Although reduced, I’m still seeing spikes with the patch below.
>> 
>> I'm not sure what could be wrong at this point. Maybe it really is a
>> kernel or HW issue. I'm wondering what would be the best way to
>> confirm or reject that idea.
> 
> I think I finally found what was causing the spikes for you. There was
> a typo in the code that was processing raw PHC readings, which
> effectively disabled filtering of delayed readings and added a
> significant error to the PHC sample time. It explains why the results
> I was seeing were not quite as good as I expected. Maybe because I'm
> testing on a machine with a faster CPU, it looked more like noise than
> spikes. 
> 
> It should be now fixed in git. Please test. If you were doing any
> experiments comparing offsets between SW and HW timestamping, you will
> probably need to start again as this bug effectively added an offset
> of maybe 1-3 microseconds. I'm sorry it took so long to figure it out. 


--
To unsubscribe email chrony-dev-request@xxxxxxxxxxxxxxxxxxxx with "unsubscribe" in the subject.
For help email chrony-dev-request@xxxxxxxxxxxxxxxxxxxx with "help" in the subject.
Trouble?  Email listmaster@xxxxxxxxxxxxxxxxxxxx.