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

[Miroslav L. <mli...@re...>]

On Mon, May 13, 2013 at 08:47:28PM +0200, Richard Cochran wrote:
>   http://linuxptp.sourceforge.net/pi_calc_plots/
> 
> Without discussing the graphs in detail, in general one can say that
> the two sets of constants yield very similar results, with method B
> resulting in lower RMS error.

It seems the cap plots are created from the RMS offsets reported by
ptp4l, which include the jitter. Even if the reported RMS offsets
are similar with both methods, the actual accuracy could be
significantly better with one method. Unfortunately that information
is hidden in the noise. To evaluate the accuracy of the clock
controlled by the servo, more accurate measurements are needed than
the measurements feeded to the servo. This is where simulations are
very useful.

>From the plots I can see that the method B does give better looking
responses to frequency/time steps than the method A, but I'm afraid it
comes at a cost of reduced accuracy when the servo is converged.

> Miroslav, would you mind testing the tweaked constants in your
> clknetsim environment?

Ok, I've run simulations with the two methods with different update
intervals and jitter/wander ratios, and compared the pairs of the RMS
time error and frequency error. The differences are in dB (20*log10).

u\jw |  1e0      |  1e1     |  1e2     |  1e3     |
2^-7 |  -24  -18 | -30  -29 | -35  -40 | -32  -41 |
2^-6 |  -20  -14 | -25  -24 | -31  -35 | -33  -42 |
2^-5 |  -15   -8 | -21  -19 | -26  -29 | -30  -39 |
2^-4 |  -10   -2 | -15  -13 | -20  -23 | -24  -33 |
2^-3 |   -6    3 | -10   -7 | -15  -17 | -20  -28 |
2^-2 |   -3    7 |  -5   -3 | -11  -13 | -16  -24 |
2^-1 |   -2    5 |  -2   -2 |  -7  -13 | -12  -24 |
2^+0 |   -1    0 |  -2   -5 |  -7  -17 | -13  -28 |
2^+1 |    0    0 |  -0   -1 |  -6  -13 | -11  -24 |
2^+2 |    1    0 |   0   -1 |  -3   -8 |  -9  -20 |
2^+3 |    1    0 |   0    0 |  -1   -2 |  -6  -15 |
2^+4 |    1    0 |   0    0 |   0   -1 |  -4  -10 |
2^+5 |    1    0 |   1    0 |   1   -0 |  -1   -4 |
2^+6 |    1    0 |   1    0 |   1    0 |   0   -1 |
2^+7 |    0    0 |   1    0 |   1    0 |   0   -0 |

>From the results it seems there is an area in the first column where
the method B is better in the frequency error, but overall it seems to
perform significantly worse in both time error and frequency error.

Here are some plots of the time and frequency error in time for the
2^-4/1e1 simulation. You can see the red line (method A) is below the
simulated jitter, filtering the noise well, but still fast enough to
keep up with the wander. The green line is within the jitter level,
but doesn't seem very stable.

http://mlichvar.fedorapeople.org/clknetsim/ptp/pi_diff/pi_diff_time.png
http://mlichvar.fedorapeople.org/clknetsim/ptp/pi_diff/pi_diff_freq.png

The scripts are available here:
http://mlichvar.fedorapeople.org/clknetsim/ptp/pi_diff/

Thanks,

-- 
Miroslav Lichvar