[ntp:questions] Red Hat vote for chrony

Charles Swiger cswiger at mac.com
Sun Dec 7 19:42:26 UTC 2014

> On Dec 6, 2014, at 11:46 AM, Paul <tik-tok at bodosom.net> wrote:
>> On Sat, Dec 6, 2014 at 7:27 AM, Charles Swiger <cswiger at mac.com> wrote:
>> Um, certainly?  Are you concerned about the quality of the XO which shipped with a Soekris board?
>> That'd be important if you were running a stratum-2+ timeserver.
> If you don't do the reading you won't understand the problems.

I would agree with this point if it were applicable.

> Here's a salient quote from Ackerman
[ ... ]
> This isn't about modifying a 4501 for testing or measurement.  It's about modifying it for high resolution production service.  By the way in the current era the 4501 is poor choice.  Common SoC based systems like the BeagleBone Black have everything you need (and more), are cheaper and no un-soldering is required.

The thread was originally about Red Hat adopting chrony, and how one
might measure its timekeeping with sub-microsecond accuracy.  You then claimed:

"But to try and stay on point:  how do you measure the performance of a typical disciplined "system" (virtual) clock in a general purpose computer -- ideally without specialized hardware and software but those are okay if the results can be meaningfully generalized.  Given those measures you can decide if you prefer NTP, Chrony, SNTP or something else."

...so the details of using SoC based systems rather than a Net4501 nowadays shouldn't
be relevant to what you asked for.  Heck, even today's modern smartphone has an
AGPS chip and more horsepower than a ~2002-era Soekris with an embedded CPU.

>> You setup a high quality clock somewhere and pull timestamps from the
>> general purpose computer you want to measure. 
> Perhaps you're unfamiliar with the problem PTP solves.  That is one of things you have to deal with if you're going to use external measurements to assess the quality of your clock discipline.

If you want to use PTP for timestamping, rather than a PPS signal over a
GPIO pin, parallel/serial port, fine.  What PTP time source did you have in mind?

If a $1500 PRS-10 is out-of-bounds even for measurement purposes against a
~$250 embedded system + GPS puck, surely a Symmetricom XLi or Meinberg LANTIME
PTP grandmaster clock would also fall outside of your chosen realm of
"without specialized hardware".

> Maybe you're also unfamiliar with what NTP is doing:  It disciplines a virtual (system) clock which is derived from the on-board (usually uncompensated) crystal.

I've been providing NTP stratum-1 or -2 timeservice to the public since the late 80's.
I can recall switching to ntpd from timed about the timeframe that physical networks
were moving from coax to RJ-45 plugs, anyway.

Nevertheless, you're welcome to patronize me about my limited knowledge of NTP if you
at least have a productive point to make by doing so.

> That's why NTP can act as a thermometer.  The big problem is temperature induced wander and that's something (back to RedHat and Chrony) Chrony claims to do better.

Yes, ntpd and chrony will notice temperature changes causing a frequency drift, at least
when a system is using cheap AT-cut XOs, which commonly show a temperature-dependency
frequency variation in the 10s of PPM range for a 10 C change around room temperature.
A more expensive SC-cut XO will likely remain stable to a few PPM for such a range,
and a calibrated TCXO or OCXO will do better still.

I suspect most people are a bit more likely to use air conditioning to control ambient
temperature changes then they are to desolder and swap out their crystals in the
hopes of obtaining more precise timekeeping....


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