Kernel PLL, from IRC

Gary E. Miller gem at
Wed Sep 28 19:01:29 UTC 2016

Yo Hal!

On Wed, 28 Sep 2016 11:14:49 -0700
Hal Murray <hmurray at> wrote:

> gemiller
>   I have a hard time believing a PLL
>   running at user timer interrupt time
>   can be anywhere as good as a kerenl
>   PLL running at TICK intervals.
> There are two parts to this discussion.  There is the PPS time stamp
> and the PLL.

No.  PPS is unrelated to the kernel PPL.  It is a totally separate
kernel module.

> The PPS time stamp gives you the data to work with.


> The work can be
> done in user space or in the kernel.

Yes, but not as well in user space.

>  New samples arrive every
> second.

No.  Twice a second.  But now you are confusing the PPS with the PLL.

>  Suppose I use the same algorithm.  Running the code in user
> space adds a few sub-ms kernel calls.

And a LOT more jitter.

>  (I'm assuming the scheduler
> cooperates.)

Bad assumption.

>  I don't think that minor delay will be significant.

Agreed, but the jitter is.  Also the frequency of updates is
different by orders of magnitude.

> The kernel PLL is probably using different parameters than ntpd.

As I have just seen.  Very dufferent results.

> gemiller
>   yes, best to measure
> Check your clockstats on your minpoll-0 runs and see if it really was
> running at 0.

Yes, it was.  That is the recent bug fixes.

>  On my quick tries, it was actually running at 1 (2
> samples per batch) I didn't track it down.

Until Sep 26, then it got fixed.  But once again, unrelated to PPS or

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