[ntp:questions] What prevents continuous time within an operating system ?

Leon McCalla leonmccalla at hotmail.com
Mon Jan 18 14:05:01 UTC 2016


The mother board ypu are using may hava a very cheap oscillator that is very temperature sensitive. You may find that. As the air temperature in the room changes 4 or 5 degrees, the cpu frequency may change +/- 0.1% or more. 

It doesn't seem like a big deal but 0.1% is 3 seconds per hour. NTPd could be working non stop trying to compensate for this.

Leon.

Sent from my android device.

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Sent: Mon, 18 Jan 2016 8:22
Subject: questions Digest, Vol 135, Issue 8

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Today's Topics:

   1. Re: What prevents continuous time within an operating system
      ? (Brian Inglis)
   2. Re: Fwd: Re: Poll interval stuck (Brian Inglis)
   3. Re: Fwd: Re: Poll interval stuck (Geoff Down)


----------------------------------------------------------------------

Message: 1
Date: Sun, 17 Jan 2016 16:42:32 -0700
From: Brian Inglis <Brian.Inglis at SystematicSw.ab.ca>
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] What prevents continuous time within an
	operating system ?
Message-ID: <569C26E8.6060004 at SystematicSw.ab.ca>
Content-Type: text/plain; charset=utf-8; format=flowed

On 2016-01-16 08:26, MAYER Hans wrote:
> I connected a GPS based 1PPS signal to a Banana Pi as stratum 0 source.
> I watched the offset in the peerstats file and it goes up and down sometimes.
> So I assumed this happens due to the fading of the signal from the GPS satellites.
> Later on I installed a second 1PPS signal based on a rubidium frequency standard.
> So I realized that both time-stamps have the same issue. See here
> https://farm2.staticflickr.com/1628/24296211776_a7e56c71b4_b.jpg
> https://farm2.staticflickr.com/1642/24214179542_89e2b28d21_b.jpg
>
> 127.127.22.1 is rubidium, 127.127.22.0 is GPS.
> 127.127.22.1 is the time source, 127.127.22.0 is configured "noselect".
> As you can see 127.127.22.1 offset is typically between +/- 5 micro seconds
> but currently at around 18:00 h the offset is 40 micro seconds.
> the same behavior with the GPS signal.
> Both fit very well. So it's not the time source, it must be the operating system.
> And of course this doesn't happen always at the same time, it's different.
>
> I am not sure if this is related to the Banana Pi and it's operating system.
> OS is Bananian OS which is actually a Debian OS with kernel  3.4.108+
> It has 1GB of RAM where almost all is free. Nothing else as NTPD and GPSD
> is running.

If those are peerstats offsets, plot those two against loopstats offset and
frequency drift, and you may see a correlation between system clock frequency
drift and PPS timing.

Watch ntpq -p -c rv -c cv values and check what states the daemon, references,
and other sources show.

Post the plot and ntpq output here for more help.

> I also wonder why NTP needs almost 2 hours to find back to the "normal" offset
> if it does read the time stamp each 32 seconds ?

Other suggestions to improve stability:

Set min/maxpoll for local references to 4.
Set a preferred source to label the PPS ticks e.g. server ... prefer.
Add good LAN or nearby internet sources.

Post server entries in ntp.conf here for more help.

Have you set affinity for ntpd or is it pinned to a non-zero CPU
to reduce contention?
Have or can you disable any power management features?

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada


------------------------------

Message: 2
Date: Sun, 17 Jan 2016 18:54:31 -0700
From: Brian Inglis <Brian.Inglis at SystematicSw.ab.ca>
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] Fwd: Re: Poll interval stuck
Message-ID: <569C45D7.5030708 at SystematicSw.ab.ca>
Content-Type: text/plain; charset=utf-8; format=flowed

>> On Fri, Jan 15, 2016 at 7:54 AM, Geoff Down <geoffdown at fastmail.net>
>> wrote:
>>>    (using the default config from Macports, so no minpoll set).
On 2016-01-16 17:39, Geoff Down wrote:
> Thanks both, its a PowerMac PPC (so not recent....)
>       remote           refid      st t when poll reach   delay   offset
>       jitter
> ==============================================================================
> *ussjc2-ntp-002. .GPSs.           1 u   63   64  377  153.056    1.119
> 0.276

Instead of the default config e.g.
	server time.apple.com
which appears to have selected a server on the other side of a continent or
ocean from you (153ms delay!) - Cupertino, California, USA
you might want to change that to e.g.
	pool time.apple.com  iburst preempt
	pool pool.ntp.org    iburst preempt
or even
	pool CC.pool.ntp.org iburst preempt
where CC is your two character ISO country code e.g. us...
in the hopes that ntpd can pick some better server(s) closer to you.

> On Fri, Jan 15, 2016, at 06:06 PM, Paul wrote:
>> Macports suggests OS X.  If it's of recent vintage did you disable
>> pacemaker?

If you are running on a laptop, and/or over USB or wifi, the best you get
may be 1ms (default units from ntpq), even if you disable all power saving.
Run ntpq -p -c rv and see how stable those servers and numbers are over time.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada


------------------------------

Message: 3
Date: Mon, 18 Jan 2016 10:07:38 +0000
From: Geoff Down <geoffdown at fastmail.net>
To: Brian Inglis <Brian.Inglis at SystematicSw.ab.ca>,
	questions at lists.ntp.org
Subject: Re: [ntp:questions] Fwd: Re: Poll interval stuck
Message-ID:
	<1453111658.1089302.495057218.010CB331 at webmail.messagingengine.com>
Content-Type: text/plain



On Mon, Jan 18, 2016, at 01:54 AM, Brian Inglis wrote:
> >> On Fri, Jan 15, 2016 at 7:54 AM, Geoff Down <geoffdown at fastmail.net>
> >> wrote:
> >>>    (using the default config from Macports, so no minpoll set).
> On 2016-01-16 17:39, Geoff Down wrote:
> > Thanks both, its a PowerMac PPC (so not recent....)
> >       remote           refid      st t when poll reach   delay   offset
> >       jitter
> > ==============================================================================
> > *ussjc2-ntp-002. .GPSs.           1 u   63   64  377  153.056    1.119
> > 0.276
> 
> Instead of the default config e.g.
> 	server time.apple.com
> which appears to have selected a server on the other side of a continent
> or
> ocean from you (153ms delay!) - Cupertino, California, USA
> you might want to change that to e.g.
> 	pool time.apple.com  iburst preempt
> 	pool pool.ntp.org    iburst preempt
> or even
> 	pool CC.pool.ntp.org iburst preempt
> where CC is your two character ISO country code e.g. us...
> in the hopes that ntpd can pick some better server(s) closer to you.
> 

Thank you. Once I've edited my config, is there a way to reload it
without stopping ntpd altogether?

-- 
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