[ntp:questions] Win7: ntpd adjusting time backwards

David Taylor david-taylor at blueyonder.co.uk.invalid
Sun Dec 9 14:14:55 UTC 2012

On 09/12/2012 10:17, Jeroen Mostert wrote:
> Drift is currently at -2.3, and no abnormally high/low values have been
> recorded.
> Loopstats collection has been on since the beginning. From the period
> where these big adjustments happen, I get some suspicious data:
> 56269 38100.218 0.000000000 19.486 0.000000238 0.120120 9
> 56269 38105.421 -0.054125528 19.486 0.019136264 0.112362 9
> 56269 38109.421 -0.054433286 19.486 0.017900666 0.105105 9
> 56269 39160.464 -0.072777326 19.415 0.017956687 0.101492 9
> 56269 41770.153 0.000000000 19.415 0.000000238 0.094937 9
> 56269 41775.482 -0.002025903 19.415 0.000716265 0.088805 9
> 56269 41905.412 -0.044579941 19.409 0.015060036 0.083092 9
> 56269 43888.511 -0.066275609 19.287 0.016040319 0.088960 9
> 56269 47068.130 0.000000000 19.287 0.000000238 0.083214 9
> The 0 offsets suggest ntpd regularly thinks we're now in perfect sync,
> something which is certainly not true. I don't know how to properly
> interpret the error and stability values.
>> and use a tool such as Meinberg's NTP Time Server Monitor or my own NTP
>> Plotter to display the results:
>> http://www.meinbergglobal.com/english/sw/ntp.htm
>> http://www.satsignal.eu/software/net.htm#NTPplotter
> Not to look a gift horse in the mouth, but the fact that your plotter
> has drag and drop for input is annoying. This (to me) is one of the
> least convenient mechanisms for providing input. Since the plotter
> cannot meaningfully run without input, you might as well throw an Open
> File dialog up when it starts. And/or use the Windows convention of
> supplying a File -> Open menu. I had to actually look in the readme to
> even realize it used drag and drop at all.
> The lack of feedback doesn't help either. When I drag peerstats.20121209
> it's accepted but nothing happens, but it doesn't tell me why not.
> (Experimentally, the reason is that you must drag all files
> simultaneously, and the set must contain loopstats. Dragging peerstats
> individually never does anything, whether before or after it's already
> seen loopstats.)
> As far as the results go, practically all the graphs look crazy, with
> enormous spikes (as you'd imagine). I'll have to do more research before
> I can meaningfully interpret them.
>> I would recommend moving to the newer pool directive rather than the
>> older
>> multiple pool server lines:
>> http://www.satsignal.eu/ntp/setup.html#pool
>> as it may use more pool servers than you would but, more importantly,
>> it reviews
>> the servers from time to time and will drop badly performing ones
>> (i.e. broken)
>> and replace them with a new one.
> Thanks. For some reason, none of the documentation I've read even
> mentions this directive, and that includes the setup instructions of the
> pool.ntp.org website, and all manpages for ntpd.conf I've come across.
> This is the first time I hear about it.
>> NTP will use interpolation on Windows XP and Windows-8, but normally
>> not in
>> Windows Vista or Windows-7. Yes, it can be worth experimenting with these
>> settings as you later report.
>> Does the poll increase from 64 to higher values as NTP runs, or is it
>> stuck on 64?
> It increases all the way up to 1024.
>> I have seen one issue on Windows-7 where, at boot-up, NTP makes the
>> wrong choice
>> about interpolation because the system clock at that time is running
>> at 15.6 ms,
>> whereas it will later switch to 1 ms (I may have the explanation slightly
>> wrong). For this reason, on both my Windows-7 LAN-synced PCs I have:
>> But I do then see: "using Windows clock directly" in the NTP events.
> Yes, that doesn't appear to be the issue.
>> Please let us know how you get on.
> The "pool" directive had some effect (I now have 9 servers instead of 4)
> and initially the offset stayed under 10 ms, but it seems that as soon
> as the poll interval goes above 64, the offset starts slipping --
> currently at 30 ms. I'll keep things under observation; I get the
> feeling it hasn't quite stabilized yet.

I wonder whether you 0 offset values mean that NTP has restarted itself, 
perhaps after making one the time jumps?

My NTP Plotter program will take an input file or directory from the 
command-line, as documented in the read-me, so you need only set up a 
batch command once and double-click it to see the current data.  I'm 
always open to user suggestions, and so far no-one else has asked for a 
File|Open dialogue!  But you are right that the program expects at least 
a loopstats file (or directory) to be dropped.  It finds the peerstats 

The primary documentation for NTP is the set of HTML pages, not 
"manpages".  I do agree that the pool directive could be more widely 
publicised, and I have suggested that Meinberg incorporate that into 
their default install

If you are still seeing stepping, I would want to investigate that 
further.  It could be that your Internet connection is not so good (any 
Wi-Fi involved?), but it didn't look like that from the stats.  One 
possibility may be to set maxpoll lower than 10 in the pool directive. 
It should be higher than 6 (64 seconds) to avoid being /too/ unfriendly 
to the servers you are using, but perhaps 8 (256 seconds) might prevent 
things for getting so far out that a reset is required.

I don't think we're at the bottom of this yet.  Are you running any 
other software which might attempt to set the time?  The W32time service 
is disabled and stopped?  No fancy audio-visual programs being run? 
Nothing which completely hogs the CPU or saturates the network 
connection?  Just some odd things to think about!
Web: http://www.satsignal.eu

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