New tour-document section explaining the clock interface and PLL.

Eric S. Raymond esr at
Wed Sep 28 20:32:33 UTC 2016

This is in devel/tour.txt.  Please review, discuss, and patch to clarify
as needed.

== System call interface and the PLL ==

All of ntpd's clock management is done through four system calls:
clock_gettime(2), clock_settime(2), ntp_adjtime(2), and (on some
systems) adjtimex().  The settimeofday(2) call from older BSD
Unuxes (in POSIX but deprecated) is no longer used.

The roles of clock_gettime(2) and clock_settime(2) are simple.
They're used for reading and setting ("stepping", in NTP jargon) the
system clock.  Stepping is avoided whenever possible because it
introduces discontinuities that may confuse applications.  Stepping is
usually done only at ntpd startup (which is typically at boot time)
and only when the skew between system and NTP time is relatively

The sync algorithm prefers slewing to stepping.  Slewing speeds up or
slows down the clock by a very small amount that will, after a
relatively short time, sync the clock to NTP time.  The advantage of
this method is that it doesn't introduce discontinuities that
applications might notice. The slewing variations in clock speed are so
small that they're generally invisible even to soft-realtime

The calls ntp_adjtime(2) and adjtimex(2) are for clock slewing. Both
use a kernel interface to do this. Both use a control technique called
a PLL/FLL (phase-locked loop/frequency-locked loop) to do it. The
difference is that adjtimex(2) adjusts a PLL/FLL implemented in the
kernel, whereas ntp_adjtime(2) implements clock skewing for a PLL
running in userspace (in ntpd itself). The KERNEL_PLL code can produce
much faster convergence from a cold start.

Deep-in-the weeds details about the kernel PLL from Hal Murray follow.
If you can follow these you may be qualified to maintain this code...

Deep inside the kernel, there is code that updates the time by reading the
cycle counter, subtracting off the previous cycle count and multiplying by
the time/cycle.  The actual implementation is complicated mostly to maintain
accuracy.  You need ballpark of 9 digits of accuracy on the time/cycle and
that has to get carried through the calculations.

On PCs, Linux measures the time/cycle at boot time by comparing with another
clock with a known frequency.  If you are building for a specific hardware
platform, you could compile it in as a constant.
You see things like this in syslog:

tsc: Refined TSC clocksource calibration: 1993.548 MHz

You can grep for "MHz" to find these.

(Side note.  1993 MHz is probably 2000 MHz rounded down slightly by
the clock fuzzing to smear the EMI over a broader band to comply with
FCC rules.  It rounds down to make sure the CPU isn't overclocked.)

There is an API call to adjust the time/cycle.  That adjustment is ntpd's
drift.  That covers manufacturing errors and temperature changes and such.
The manufacturing error part is typically under 50 PPM.  I have a few systems
off by over 100.  The temperature part varies by ballpark of 1 PPM / C.

There is another error source which is errors in the calibration code and/or
time keeping code.  If your timekeeping code rounds down occasionally, you
can correct for that by tweaking the time/cycle.

There is another API that says "slew the clock by X seconds".  That is
implemented by tweaking the time/cycle slightly, waiting until the correct
adjustment has happened, then restoring the correct time/cycle.  The "slight"
is 500 PPM.  It takes a long time to make major corrections.

That slewing has nothing (directly) to do with a PLL.  It could be
implemented in user code with reduced accuracy.

There is a PLL kernel option to track a PPS.  It's not compiled into most
Linux kernels.  (It doesn't work with tickless.)  There is an API to turn it
on.  Then ntpd basically sits off to the side and watches.

RFC 1589 covers the above timekeeping and slewing and kernel PLL.

RFC 2783 covers the API for reading a time stamp the kernel grabs when a PPS

		<a href="">Eric S. Raymond</a>

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