Kernel PLL (from IRC)

Gary E. Miller gem at
Wed Sep 28 19:32:55 UTC 2016

Yo Hal!

On Wed, 28 Sep 2016 12:21:54 -0700
Hal Murray <hmurray at> wrote:

> gemiller:
>   Being the whole point of ntpd, to
>   manage the kernel PLL, this should be
>   obvious.
> No.  The purpose of ntp is to maintain the correct system time.

Sort of.  First, ntpd manages system time by mangaing the kernel PLL, if
one exists.  So we are sying the same thing. ntpd uses the kernel PLL
because the current user space PLL is inferior.  Today's tests easily 
show that.

> ntpd itself is a PLL.

Yes, a user space PLL, which can never be as good as a kernel pbased PLL.

> The purpose of the Kernel PLL is to do the same things with
> PPS pulses but do it better.  It may have been important back
> in the old days when systems were slow and schedulers were crappy.

Uh, no.  The PPS module does not talk to the Kernel PLL module on 
Linux.  It does on some BSD, but not on Linux.

> I don't know how important the Kernel PLL is today.  It's not in most
> distros so it's obviously far from critical.  The only way to test is
> to build a kernel and collect data, then compare that with what we
> can get without it.

Which I have done, the results are immediately obvious.  The user
space PLL is much inferior to the kernel space PLL.

> esr
>   The other is that it kills the
>   TESTFRAME concept, or at least
>    damages its usefulness pretty badly.
>    Logs from PLL systems won't be  ...
> I don't see the problem.  TESTRAME is testing ntpd, not the kernel.
> It records everything that goes in/out of ntpd.  Replay should do the
> same even if run on a different sysem/OS/architecture.

Except an ntpd built on a system with no kernel PLL will not even compile
in that code.  At least as now coded.  Could be fixed.

> (There might be some differences between 32/64 bit systems.)

I would doubt it.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588
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