[ntp:questions] Red Hat vote for chrony

Paul tik-tok at bodosom.net
Fri Dec 5 22:55:12 UTC 2014


On Fri, Dec 5, 2014 at 3:35 PM, Charles Swiger <cswiger at mac.com> wrote:

> Well, we do have time enthusiasts around who like to achieve the best
> precision they can, regardless of whether there is a specific business
> justification or not.  :-)
>

Sure but that doesn't help someone that just wants a simple (-minded)
configuration to keep a few clocks in sync.


> > If all your external clocks fail I suspect the typical user can depend
> > on the disciplined virtual clock for days.
>
> For real hardware, sure-- once the intrinsic frequency drift has been
> setup,
> you can free-run for days into weeks without drifting too far.  Cell phone
> towers (especially CDMA) are a decent example of such fault-tolerant
> systems.
>
c

While perhaps strictly correct I was talking about the common crystal in a
typical computer not the  Cesium, Rubidum or OCXO in a cell site and
there's really no basis for comparison.


> What confusion?  Certainly it's a decent paper to read....
>

I misquoted.  This "Without measuring the local clock against some other
clock or oscillator"

suggests comparing a clock to a better frequency reference but BIPM creates
a virtual clock (with better Allen deviation) and everyone steers toward
that.  Perhaps you meant something else.

Yes, you need to compare timestamps using purpose-built systems like a
> TCXO, Cesium, or Rubidium clock
>

That wasn't my point.  You need a purpose built NTP server to expose

its virtual clock for comparison to an external frequency reference.  Of
course you need a "purpose" built reference but only in the sense that
you'd use "real" counter rather than one in a voltmeter.

> Even back in 2002 with very inexpensive commodity hardware, FreeBSD was
> able to
> achieve accuracy measured to ~260 nanoseconds:
>

Hmmmm.  So phk uses a $1,500 rubidium standard as a system oscillator and
you call it inexpensive and commodity.  He also ran a particular install of
BSD and a non-standard NTP.  All of those are what I was referring to when
I said purpose built system to measure the variance of an NTP disciplined
virtual oscillator.  By the way high resolution low-latency counters in
computers have become commodity items.  The software to use them -- not so
much.

It might be nice to conduct a similar experiment with Chrony but it's all
pointless.  As Bill suggests you want to measure typical performance in
typical environments.  That's the bit I said was interesting and I don't
think the published numbers make it clear what's better.

Frankly I suspect even that is pointless.  If someone asked me how to do
NTP on the cheap I'd say buy or build some number of Laureline-like PLLs in
a box with a NTP packet emulator or some NTP servers off Ebay and run
SNTP/OpenNTP on the clients.   If you have a larger budget then buy
Meinberg, Microsemi et. al. new.


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