Kernel PPS processing

Gary E. Miller gem at
Wed Jun 29 20:58:11 UTC 2016

Yo Matthew!

On Wed, 29 Jun 2016 16:38:40 -0400
Matthew Selsky <Matthew.Selsky at> wrote:

> > Can you quantify the individual effects?  And is that kernel PPS,
> > KPPS in ntpsec, or just PPS in ntpsec?  
> We're using a GPS PCIe card with OCXO HQ with a daemon that writes to
> SHM and then ntpd reads from the SHM driver.

Very interesting.  The list has been wondering if there were SHM drivers
other than gpsd.

I'm sure Eric would like to document what you are using.  Can you be more
specific?  Like is your SHM driver open source?

> Measuring on this server via ntpq -p we were seeing offsets of +/-
> 3us without either of these two kernel parameters.  With nohz=off,
> the offsets were +/- 1us.  With both kernel parameters we see offsets
> too small for ntpq -p to measure.

Very interesting.  Many people will reboot tonight.  :-)

ntpd does keep better stats than ntpq -p shows.  Look in the ntpd
log files.

BTW, are you seeing the sideral day oscillation?

> This was all measured while running ntpd Classic.

The PLL code has not changed, yet.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 473 bytes
Desc: OpenPGP digital signature
URL: <>

More information about the devel mailing list