[LinuxPPS] convergence patch

[Hal V. Engel]

On Thursday 07 May 2009 11:16:35 am Bernhard Schiffner wrote:
> I run machines which have little uptime because they are switched on and
> off regulary. If they can't sync their time to below videoframe-precision
> within minutes (and hold it there!)

I am not sure what "videoframe-precision" is.  Does this mean +-1 millisecond 
or +-100 microsecinds or +-10 mincroseconds or +-1 microsecond or something 
else?  Lots of video work is done on Windows machines and the time API there 
only has a precision of +- 15.6 milliseconds (IE. 1/64th of a second).  So 
just about any *nix machine that is using ntp to sync to public time servers 
on the Internet should do better and machines using PPS type reference clocks 
should be 3 to 4 orders of magnitude better.  

The kernel convergence patch should help with startup convergence issues for 
current linux kernels and also help keep offsets low after startup.  In 
addition, I use the --panicgate parm when starting the ntp daemon which will 
cause ntp to sync the system clock to the reference clock at start up.  The 
clock will jump once instead of being slewed if the system clock is off by 
more than 128 milliseconds.   This will be the case on most machines that have 
been powered off for more than a few minutes.  What I find is my starting 
offset is typically under 50 microseconds once ntp causes the time to sync 
with the ref clock when I do this and that ntp without the convergence patch 
will keep offsets below +-100 microseconds as the oscillator temperature comes 
up to normal operating temperature (perhaps 15 minutes).  I am hoping that ntp 
will do better during this warm up period with the convergence patch in place.

By the way this:

# ntptime
ntp_gettime() returns code 0 (OK)
  time cdadc497.bcf77680  Thu, May  7 2009 13:35:35.738, (.738151202),
  maximum error 1004 us, estimated error 0 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 0.008 us, frequency -38.976 ppm, interval 1 s,
  maximum error 1004 us, estimated error 0 us,
  status 0x2001 (PLL,NANO),
  time constant 4, precision 0.001 us, tolerance 500 ppm,

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================
*GPS_ONCORE(0)   .GPS.            0 l   11   16  377    0.000    0.000   0.001

is how my machine is doing with the convergence patch a few hours after 
restarting the kernel and ntp.  I think I am going to keep the patch in place 
for now.

Hal