Re: ✘--disable-kernel-pll

[Daniel Franke]

On 9/28/16, Gary E. Miller <gem at rellim.com> wrote:
> The knerel PLL and the ntpd userspace PLL are supposed to be using the
> same algorithm and constants.  Might be worth checking if the constants
> are still the same.

They aren't. In ntp_loopfilter.c, we have

#define CLOCK_PLL       16.     /* PLL loop gain (log2) */

but that "(log2)" is a lie; check everywhere CLOCK_PLL is used and
it's in multiplication and division, not bitshifts.

The kernel PLL's equivalent variable is called SHIFT_PLL, and really
is log2. So if SHIFT_PLL were 4, that would be equivalent. But since
Linux kernel 2.6.31, it's been changed from 4 to 2, which means it's
less damped than the userspace PLL. Last I checked, FreeBSD still uses
SHIFT_PLL=4.

The Linux kernel PLL has had a *lot* of refactoring that the NTP
codebase hasn't. So there are probably a lot of other differences I
haven't noticed and it's possible that comparing constants doesn't
even make sense any more.