Re: [Linuxptp-devel] Optimal P, I constants

[Keller, J. E <jac...@in...>]

> -----Original Message-----
> From: Miroslav Lichvar [mailto:mli...@re...]
> Sent: Wednesday, April 24, 2013 2:35 AM
> To: Keller, Jacob E
> Cc: lin...@li...
> Subject: Re: [Linuxptp-devel] Optimal P, I constants
> 
> On Tue, Apr 23, 2013 at 06:42:10PM +0000, Keller, Jacob E wrote:
> > I think it might be better (at first) to create a new clock which is based
> on Allan intercept, since most people doing PTP are familiar enough
> with the P/I constants servo.
> 
> I think the options to set P, I directly could be still used, the new
> option would be effective only if the configured P or I was zero, or
> the new option was non-zero.
> 
> > I do think in the long term it definitely makes more sense to put the
> Allan intercept, as that appears to work better, and is easier to measure
> if I am not mistaken? (Jitter * wander in pbb, right?)
> 
> The Allan intercept is approximately
> (jitter / wander_per_second)^(2/3) * 2
> 
> The deviation can be calculated from offsets reported by phc2sys (or
> ptp4l) when free running and the intercept can be estimated from the
> plot. A tool I use is at https://github.com/mlichvar/ppsallan.
> 
> With phc2sys it can be used like this:
> phc2sys -P 1e-50 -I 1e-50 -m | tail -n +10 > log
> awk '{ printf "%.9f\n", $4 * 1e-9 }' < log | ppsallan -B > adev
> plotallan adev
> 
> The offsets should be collected at one second interval, or the result
> needs to be scaled accordingly. To see the +1/2 slope well, it's
> good to have at least 10 hours or more of data.
> 
> > Wouldn't this also have to set the update frequency as well? It
> appears that some update frequencies cause the result to be unstable
> or too slow..
> 
> Yes, the update interval would need to be included in the P, I
> calculation. I think that's the main problem with the current code,
> that the sync interval can change to a longer one in the runtime, but
> P, I don't change and the servo may suddenly be unstable.
> 
> Ideally, the constants would be adjusted automatically according to
> the current conditions. Or try a more sophisticated servo. I have some
> ideas I want to explore, but it's not on my short-term todo list.
> 
> Thanks,
> 
> --
> Miroslav Lichvar

I like the dynamic adjustments idea, but definitely would require work.

- Jake